Use the EC to PPM converter below to translate any electrical conductivity reading into parts per million on the 500, 640/650, or 700 scale — whichever matches your meter and feeding chart. Enter your EC value, select your scale, and get an instant conversion. The guide that follows explains what these numbers mean, how to find your meter’s scale, and how to stop chasing PPM figures that don’t match your targets.
These principles apply whether you’re dialing in a single tent reservoir or standardizing measurements across a multi-room commercial facility.
Grower Tools
EC to PPM Converter
Convert EC to PPM across all three scales — 500, 640, and 700 — simultaneously. Switch between EC→PPM and PPM→EC, identify your meter's scale, and check whether your reading is on target for your growth stage. EC is the universal standard; this tool bridges the gap when your feeding chart speaks a different language. Questions about your meter? Talk to a Grow Expert.
EC ↔ PPM Converter
Simple mode converts EC to PPM on all three scales. Switch to PPM to EC to work backwards from a PPM reading.
How to use this converter
- EC → PPM: Enter your EC reading — the converter instantly shows the equivalent PPM on all three scales. Select your growth stage to see a target range check.
- PPM → EC: Switch the toggle, enter your PPM value, and select your meter's scale. The converter returns the EC and shows the same PPM value expressed on all other scales.
- Not sure which scale your meter uses? See the meter brand table below, or mix a 1.0 EC solution and compare your meter's PPM reading — 500 = 500 scale, 700 = 700 scale.
- Always anchor decisions to EC — it's the consistent, universal measurement. Use PPM conversions only when cross-referencing feeding charts.
Formulas: PPM₅₀₀ = EC × 500 · PPM₆₄₀ = EC × 640 · PPM₇₀₀ = EC × 700 · EC = PPM ÷ scale
Key insight: A 1.4 EC solution reads 700 PPM (500 scale), 896 PPM (640 scale), or 980 PPM (700 scale). Same nutrient strength — three completely different numbers. This confusion is the #1 reason growers accidentally over- or underfeed.
📊 Three-Scale Comparison Chart Chart ▾
Run the converter above to see your EC expressed across all three PPM scales.
EC to PPM Conversion Table — All Three Scales
| EC (mS/cm) | PPM 500 (Hanna/US) | PPM 640 (European) | PPM 700 (Bluelab) | Typical Stage |
|---|---|---|---|---|
| 0.4 | 200 | 256 | 280 | Seedling / RO water |
| 0.6 | 300 | 384 | 420 | Seedling / Clone |
| 0.8 | 400 | 512 | 560 | Seedling transition |
| 1.0 | 500 | 640 | 700 | Early Veg |
| 1.2 | 600 | 768 | 840 | Veg |
| 1.4 | 700 | 896 | 980 | Veg / Transition |
| 1.6 | 800 | 1,024 | 1,120 | Transition / Early Flower |
| 1.8 | 900 | 1,152 | 1,260 | Early Flower |
| 2.0 | 1,000 | 1,280 | 1,400 | Flower |
| 2.2 | 1,100 | 1,408 | 1,540 | Peak Flower |
| 2.4 | 1,200 | 1,536 | 1,680 | Peak Flower (heavy feeders) |
| 2.6 | 1,300 | 1,664 | 1,820 | Max recommended |
| 3.0 | 1,500 | 1,920 | 2,100 | Commercial / salt-tolerant only |
Which Scale Does My Meter Use?
| Meter Brand / Model | PPM Scale | Common Region | Notes |
|---|---|---|---|
| Hanna Instruments (most models) | 500 | North America | HI98318, HI9813, combo pens |
| Milwaukee MW301, MW302 | 500 | North America | Budget combo meters |
| Most budget TDS pens ($10–25) | 500 | North America | eBay/Amazon pens, unbranded |
| General Hydroponics TDS meters | 500 | North America | Sold at most hydro shops |
| Apera Instruments | 500 | North America | PC60, EC60 series |
| Bluelab Truncheon | 700 | Global / ANZ | Also displays CF and EC |
| Bluelab Combo Pen | 700 | Global | User-selectable on some models |
| Eutech / Oakton | 700 | Professional | Lab-grade instruments |
| Truncheon (original) | 700 | UK / Australia | Analog float meter |
| Most European meters | 640 | Europe | Based on KCl standard |
Get a meter you can trust.
Shop Bluelab, Hanna, Apera, and Oakton EC & PPM meters.Why PPM Readings Don't Match (And Why That's a Math Problem, Not a Nutrient Problem)
When two growers report different PPM values for the same reservoir, the solution itself usually hasn’t changed — only the math has. Every EC/PPM meter physically measures electrical conductivity (EC) and then multiplies that number by a fixed factor to display a PPM figure. There are three common factors: 500, 640–650, and 700. Because manufacturers choose different factors, the same nutrient solution produces three completely different PPM numbers while its EC remains identical.
This is why EC is treated as the universal standard. It’s the actual measurement your meter takes — no conversion, no interpretation. A reading of 1.5 mS/cm means exactly the same thing on every properly functioning conductivity meter on the planet. PPM does not
The conversion formula is simple:
PPM = EC (mS/cm) × Scale Factor
| EC (mS/cm) | 500 Scale (PPM) | 650 Scale (PPM) | 700 Scale (PPM) |
|---|---|---|---|
| 0.5 | 250 | 325 | 350 |
| 1.0 | 500 | 650 | 700 |
| 1.5 | 750 | 975 | 1,050 |
| 2.0 | 1,000 | 1,300 | 1,400 |
| 2.4 | 1,200 | 1,560 | 1,680 |
| 2.8 | 1,400 | 1,820 | 1,960 |
At EC 2.0, one grower reads 1,000 PPM and another reads 1,400 PPM — a 400‑point gap on the exact same reservoir. If one is following a 500‑scale chart while measuring on a 700‑scale meter, they’ve been running their crop at roughly 40% over‑target, not 28%, without knowing it (1,400 ÷ 1,000 = 1.4).
How to Identify Your Meter's Scale
Your meter’s scale determines which PPM number it shows you. Most modern meters let you check or switch this setting in a few seconds.
Step 1 — Check the settings menu. Look for a toggle labeled “500/700,” “NaCl/KCl,” or “0.5/0.7” in your device’s settings. The 500 scale typically uses a sodium chloride (NaCl) reference; the 700 scale typically uses a potassium chloride (KCl) reference.
Step 2 — Look up your brand’s default. If you can’t find a settings menu, use this reference table as your starting point. Confirm the actual setting in your device — many meters have been switched from their factory default without the owner realizing it.
| Meter Brand | Default Scale | Notes |
|---|---|---|
| Bluelab OnePen | 700 (switchable) | Displays EC, CF, PPM 500, PPM 700, and mS/cm; scale selected in menu. |
| Bluelab Guardian Monitor | 700 (switchable) | Wall-mount or inline continuous monitor; EC‑based with selectable ppm output. |
| Bluelab Truncheon | Shows all scales | Printed EC, CF, 500, and 700 on barrel; no user scale setting required. |
| Hanna GroLine (HI9814) | 500 or 700 (selectable) | Common in US horticulture; factor is user-selectable between 0.5 (500) and 0.7 (700). |
| Milwaukee CD611 / EC60 | 500 (NaCl-based) | Fixed scale; displays µS/cm or ppm with 0.5 conversion factor. |
| HM Digital | 500 | NaCl-based; widely sold budget meters using 0.5 TDS factor. |
| Oakton | 700 | Often KCl-based conversion with 0.7 factor on many EC/TDS models. |
| Inexpensive imports | Varies / often unlabeled | Verify in fine print or default to EC when scale is undocumented. |
Step 3 — Verify against a known solution. Place your meter in a 1.41 mS/cm calibration solution. If the PPM display reads approximately 700, you’re on the 700 scale. If it reads approximately 500, you’re on the 500 scale. That’s all it takes to confirm.
If your meter is switchable — as most Bluelab OnePen and Hanna GroLine models are — the cleanest long‑term fix is to set it to display EC directly and skip the scale question entirely.
What Scale Does Your Nutrient Brand Use?
This is the table most growers never find. When a feeding chart lists a PPM target without naming a scale, you need to know which conversion the brand used when they wrote those numbers. Using the wrong scale against a chart means you’re running your crop at the wrong concentration — potentially by 30–40% or more.
| Nutrient Brand | PPM Scale |
|---|---|
| Advanced Nutrients | 700 (for charts expressed in ppm/TDS) |
| Botanicare | 700 (on legacy ppm‑based charts; many newer charts emphasize EC) |
| Cutting Edge Solutions | 500 |
| Dutch Master | 500 |
| Dyna-Gro | 500 |
| FoxFarm | 700 |
| General Hydroponics | 500 (RO water assumed on many charts) |
| General Organics | 500 (RO water assumed) |
| House & Garden | 700 |
| Humboldt Nutrients | 500 |
| Nectar for the Gods | 700 |
| Rock Nutrients | 700 |
| Roots Organics | 500 |
| RxGreen | 500 (distilled water assumed) |
| Technaflora | 500 |
No PPM number listed? Several professional nutrient lines — including HGV Nutrients and many modern commercial programs — publish feeding parameters exclusively in EC. If your feeding chart is EC‑only, use the calculator above to translate to PPM if needed, but consider staying in EC. The chart writers are telling you something: they want you standardized on EC.
If your nutrient brand isn’t on this list, contact the manufacturer and ask: “Do your feeding charts use the 500 or 700 PPM scale, and what water quality are the targets based on?” Most will confirm within one email.
EC Targets by Growth Stage
EC targets rise steadily as plants mature, from roughly 0.4–0.8 mS/cm for seedlings up to about 2.0–2.8 mS/cm during peak flower, depending on cultivar, medium, and system type. The table below anchors each stage to EC first, then converts both common scales for reference. Use these as starting ranges — actual targets vary with strain, environment, and your source water.
| Growth Stage | EC (mS/cm) | 500 Scale (PPM) | 700 Scale (PPM) |
|---|---|---|---|
| Seedlings / Clones | 0.4–0.8 | 200–400 | 280–560 |
| Early Vegetative | 1.0–1.5 | 500–750 | 700–1,050 |
| Late Vegetative | 1.5–2.0 | 750–1,000 | 1,050–1,400 |
| Early Flower | 1.8–2.4 | 900–1,200 | 1,260–1,680 |
| Peak Flower | 2.0–2.8 | 1,000–1,400 | 1,400–1,960 |
| Late Flower / Flush | 0.0–1.0 | 0–500 | 0–700 |
Because these targets are expressed in EC first, the right column always comes free — convert to whichever PPM scale your chart uses and you’re done. Start at the lower end of each range when introducing plants to a new feed strength, and increase gradually while monitoring runoff EC for salt accumulation
Your Source Water Has EC Too — And It Changes Your Targets
Before you add a single drop of nutrient, your water already contributes to the EC of your final solution. Most municipal tap water reads roughly 0.2–0.5 mS/cm from dissolved minerals; well water can push even higher; only true RO water typically drops below 0.1 mS/cm.
This matters because many nutrient manufacturers write their PPM or EC targets assuming low‑mineral starting water. If your tap reads 0.4 mS/cm and you mix to hit EC 2.0, your nutrients are only contributing EC 1.6 — not 2.0. Over time, the minerals in your source water accumulate in the root zone, and EC drift occurs even when you haven’t changed your feed rate.
The calculation: Add your source water EC to your nutrient‑only EC to get total reservoir EC
Total EC = Source Water EC + Nutrient EC
If your tap reads 0.4 mS/cm and your target for late veg is EC 1.8, you’re targeting an EC contribution of roughly 1.4 from nutrients. Mix to that, not to 1.8, and then measure the finished reservoir.
For any source water reading above about 0.4 mS/cm — especially with calcium‑ or bicarbonate‑heavy city tap — consider running water through an RO filter before mixing nutrients. Starting from clean water makes your EC readings honest and your feeding charts accurate.
How to Switch from PPM to EC (Step-by-Step)
When you’ve been running a PPM‑based program and want to standardize on EC — or when you replace a meter and notice your readings have changed — follow this four‑step process to reconcile your targets and avoid feeding errors.
Step 1 — Find the EC value of your current target. If your target is a PPM number, divide it by its scale factor. A target of 1,050 PPM on the 700 scale equals 1,050 ÷ 700 = 1.5 mS/cm EC.
Step 2 — Confirm your new meter’s scale. Check the settings menu or spec sheet for the 500/700 (or NaCl/KCl) toggle, using the brand table above as your guide.
Step 3 — Convert if you need a PPM reference. On a 500‑scale meter, that 1.5 mS/cm target displays as 1.5 × 500 = 750 PPM. On a 700‑scale meter, 1.5 × 700 = 1,050 PPM.
Step 4 — Standardize on EC going forward. Set your meter to display EC in mS/cm. Write your targets in mS/cm on your feeding chart. From this point, any meter you pick up reports the same number — no scale to worry about.
Use the converter at the top of this page to run any remaining PPM targets through the math once and record the EC equivalent. You’ll only need to do this once.
Four Mistakes That Lead to Wrong Readings
Knowing the scales exist is half the battle. Here’s where growers still trip up after they understand the concept.
Mistake 1: Assuming a chart’s scale without checking. Feeding charts from nutrient companies frequently omit the scale label. A chart that lists “800 PPM” for seedlings could be 800 on the 500 scale (EC 1.6) or 800 on the 700 scale (EC ~1.14) — two very different feed strengths for the most vulnerable stage of your crop. Always confirm with the brand directly or cross‑reference with the table above.
Mistake 2: Comparing notes across different meters. “My EC is 1.8, but my buddy says he runs 1,400 PPM” lands in the same territory — different scales make peer comparison meaningless unless everyone specifies the factor. This is the core reason professional programs standardize on EC.
Mistake 3: Not accounting for source water. Running tap water without checking its EC leads to creeping over‑fertilization in recirculating systems. Measure your source water before adding nutrients and factor that into your target, as described above.
Mistake 4: Calibrating with the wrong calibration solution. Calibration solutions are often scale‑specific when expressed in ppm. A 1,382 ppm calibration solution designed for a 700‑scale meter will miscalibrate a 500‑scale meter if you rely on the ppm value instead of EC. Check your meter’s manual for the correct calibration standard — or better yet, calibrate in mS/cm using a conductivity standard and bypass the scale question entirely.
For a comprehensive rundown on where feeding programs go wrong, see nutrient mistakes growers make in the Learning Center.
Meters That Make EC Simple
The right meter removes the scale guessing game entirely. The best setups display EC directly, let you switch PPM scales to match any chart, and stay calibrated over months of daily use.
For All Growers: A Combination Meter That Does Everything
The Bluelab OnePen pH/EC/Temp Meter covers all three of your core reservoir metrics — pH, EC, and temperature — in a single waterproof probe. It displays conductivity as EC, CF, PPM 500, or PPM 700, plus mS/cm, so you can match any feeding chart or lock into EC‑first permanently. The Bluetooth connection to the Edenic app lets you log and share data in real time, and the replaceable probe extends the tool’s useful life. If you manage a reservoir daily and want one instrument in your pocket rather than two or three, this is a strong option.
A dedicated EC‑only option for growers who already have a pH meter: the Milwaukee CD611 Digital Conductivity Pen is a compact 500‑scale pen with automatic temperature compensation, 300‑hour battery life, and straightforward single‑point calibration. It’s the right choice when you need reliable EC readings without complexity, at a fraction of the cost of a combo meter.
For growers who prefer the Hanna ecosystem: the Hanna Instruments GroLine Portable pH/EC/TDS Meter combines pH, EC, TDS, and temperature in an IP67‑rated waterproof body. The 3‑in‑1 probe uses a renewable cloth junction for the pH electrode and an integrated temperature sensor for automatic compensation — a reliable workhorse for daily hydroponic reservoir testing. The instrument allows selection between 0.5 and 0.7 TDS conversion factors, so you can align with either 500 or 700 scale.
For Commercial Operations: Standardizing EC Across Multiple Rooms
At facility scale, scale confusion shifts from an occasional annoyance to a source of systematic, repeatable crop variation. When multiple operators, rooms, and staff rotations are involved, a PPM‑based program creates a vulnerability every time someone reaches for a meter set to a different scale. Standardizing on EC removes that variable entirely.
A practical rollout for a multi‑room facility:
First, pull every meter in the building and verify its settings. Set all meters to display EC in mS/cm and record the setting in your equipment log. Where meters are switchable, remove or de‑emphasize the PPM display option if the interface allows.
Next, rewrite all feeding charts, batch sheets, and SOPs in EC. Eliminate unlabeled PPM numbers from every document the team touches — a chart that says “1,200 PPM” is ambiguous; a chart that says “2.4 mS/cm” is not.
Set a calibration schedule. Monthly is the baseline for high‑use meters; twice‑monthly is appropriate in continuously recirculating systems. Probe drift is the next source of variance once scale mismatch is eliminated, and it compounds silently.
For recirculating systems, log runoff EC against input EC at every irrigation event. An input‑to‑runoff gap of roughly 0.2–0.5 mS/cm indicates healthy uptake. A widening gap signals salt accumulation that needs to be addressed before it affects the root zone.
The TrolMaster Aqua-X EC/Temp Sensor is built for facilities that need continuous inline EC tracking — it integrates with the TrolMaster Aqua‑X system to log reservoir EC around the clock, so drift is caught between manual checks rather than at the next canopy inspection.
For facilities running automated fertigation, see fertigation 101: automated nutrient delivery for a complete workflow on integrating EC monitoring with proportional dosing systems.
Complete Your Nutrient Management Setup
EC and PPM are only half of reservoir management — pH determines whether those nutrients are actually available to your plants. For a full picture of how EC and pH interact in a feeding program, see our guides on pH for plants and nutrients and pH. For correct mixing order when preparing your reservoir, start with how to mix plant nutrients — sequence matters, and EC readings taken at the wrong mixing stage will mislead you.
EC vs PPM: FAQs
What is the formula for converting EC to PPM?
Multiply your EC reading in mS/cm by your meter’s scale factor: 500, 640/650, or 700. An EC of 1.5 mS/cm equals 750 PPM on the 500 scale, 960–975 on the 640/650 scale, or 1,050 PPM on the 700 scale. To convert PPM back to EC, divide by the same factor. The converter at the top of this page applies the math automatically for any scale.
Expanded: The formula is universal — the only variable is confirming which scale your meter and feeding chart each use. Mismatched scales produce errors of roughly 15–40%, large enough to cause nutrient deficiency or burn at the extremes.
Commercial: At facility scale, standardizing on EC in mS/cm eliminates the conversion entirely. SOPs written in EC transfer to any meter without a scale verification step.
How do I know what scale my PPM meter uses?
Check your meter’s settings menu for a “500/700” or “NaCl/KCl” toggle. Most Bluelab OnePen and Hanna GroLine models are switchable. Bluelab conductivity devices commonly support both 500 and 700 scales; Hanna, Milwaukee, and many US budget meters default to a 0.5 factor (500) unless changed. If you’re unsure, place the meter in a 1.41 mS/cm calibration solution — a display near 700 means 700 scale, near 500 means 500 scale.
Expanded: The brand reference table in the guide above covers the most common meters. For inexpensive imports with no labeling, divide the PPM reading by a known EC value — if EC 1.0 shows 700 PPM, you’re on the 700 scale.
Commercial: When onboarding new staff or new meters, add scale verification to your equipment commissioning checklist. One misconfigured meter running against a 500‑scale SOP is enough to compromise an entire room’s program.
What is a good EC level for hydroponics?
EC targets range from 0.4–0.8 mS/cm for seedlings up to around 2.0–2.8 mS/cm during peak flower, depending on cultivar, medium, and system type. Vegetative growth typically runs 1.0–2.0 mS/cm. Coco and rockwool programs sometimes run slightly different numbers than deep water culture or NFT. Always account for your source water EC — most tap water contributes roughly 0.2–0.5 mS/cm before nutrients are added.
Expanded: EC targets in feeding charts are starting points, not fixed prescriptions. Monitor runoff EC alongside input EC: a runoff that climbs significantly above input EC signals salt accumulation that requires a reservoir flush or water correction
Commercial: Input‑to‑runoff EC differential within about 0.2–0.5 mS/cm is a standard benchmark for healthy uptake in recirculating and high‑frequency drip systems.
Why does my PPM reading not match my nutrient chart?
Almost always, your meter and chart are using different PPM scales. A chart written on the 700 scale lists higher numbers than a 500‑scale meter displays for the same solution — a gap of up to ~40% at moderate EC values. Match your meter’s scale to the chart’s scale, or convert both to EC to remove the discrepancy entirely
Expanded: Nutrient brands don’t always label their charts with a scale. The brand‑specific scale reference table in this guide covers the most commonly used nutrient lines. If yours isn’t listed, contact the manufacturer directly.
Is EC the same as PPM or TDS?
EC and PPM/TDS relate to the same property — dissolved ionic concentration — but express it differently. EC is the direct measurement your meter takes, in units like mS/cm or µS/cm. PPM on most hydroponic meters is a derived number calculated from that EC using a scale factor. TDS (total dissolved solids) is technically a weight‑based measure of dissolved matter, but in practice most handheld TDS meters display a 500‑ or 700‑scale PPM conversion, not a laboratory‑grade TDS measurement. EC is the only one of the three that stays consistent across all meters and references.
Can I use PPM instead of EC for my grow?
Yes, PPM works reliably when every meter, feeding chart, and team member uses the same scale. The problems start at the edges: switching meters, consulting a feeding chart from another source, or comparing notes with another grower who uses a different scale. EC removes all of those edge cases because it carries no conversion factor — the reading is the reading.
What is the difference between the 500 and 700 scale?
The 500 scale (NaCl‑based, often labeled TDS) multiplies EC by 500 and is common in the United States. The 700 scale (often KCl‑based) multiplies by 700 and is common in the UK, Europe, and Australia. Neither is more “accurate” in an absolute sense — they’re different reference standards. The 500 scale tends to produce smaller numbers for the same nutrient strength; the 700 scale produces larger ones. The 640/650 scale is a middle‑ground conversion used by some European meters and a handful of North American brands.
Does temperature affect my EC reading?
Yes. Electrical conductivity increases as temperature rises — a solution at 30°C will read higher than the same solution at 15°C. Quality meters include automatic temperature compensation (ATC) that normalizes readings to a standard temperature (typically 25°C), so you get consistent results regardless of when you dip the probe. Budget meters without ATC can produce misleading readings at temperature extremes. Always test near your normal feeding temperature and keep meters calibrated.
Which meter should I buy for measuring EC in hydroponics?
The right choice depends on what you already have. If you don’t own a pH meter, start with a combination instrument that reads pH, EC, and temperature in one probe — this covers your core reservoir metrics without carrying multiple tools. If you already have a dedicated pH meter, an EC‑only pen is all you need and costs significantly less. Look for automatic temperature compensation (ATC), a switchable 500/700 display or at least clear scale documentation, and a replaceable probe when possible. See our guide on pH testers for soil and hydroponics for side‑by‑side comparisons.
How do I convert EC to PPM if I don't know my scale?
Start by assuming the 500 scale if you’re in the US and your meter has no labeling — many North American budget meters default to 500. Then verify: place your meter in a 1.41 mS/cm calibration solution and check the PPM display. About 700 = 700 scale; about 500 = 500 scale. If you don’t have calibration solution, divide the PPM your meter shows by the EC reading to estimate the factor — around 500 suggests the 500 scale; around 700 suggests the 700 scale.