PPM to EC Converter
Convert between PPM, EC, and TDS instantly — works with all three conversion scales (500, 640, 700). Built for growers who need to translate between meters, nutrient charts, and grow forums without second-guessing the math.
Convert PPM, EC & TDS
Pro tip
Most meter manufacturers use one scale by default but allow switching. Check your meter's manual — assuming the wrong scale can put your feed off by 40%.
At this reading
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Enter a value to see stage guidance.
Common PPM / EC mistakes
- Mixing scales — a "1000 PPM" chart on the 500 scale is NOT the same as 1000 PPM on your 700-scale meter. Always match scales before feeding.
- Confusing μS/cm with mS/cm — 1000 μS/cm equals 1.0 mS/cm. Entering 1000 as mS/cm gives meaningless results (that's seawater-level).
- Ignoring source water baseline — tap water at 200 PPM means your feed strength is 200 higher than the chart says. Start with RO water for full control.
- Skipping meter calibration — probes drift. Calibrate monthly with 1413 μS/cm solution. A reading 5%+ off means recalibrate or replace.
Your reading equals
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EC (mS/cm)
PPM 500
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PPM 640
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PPM 700
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EC μS/cm
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Three steps to an accurate conversion
Check your meter's scale
Before you enter anything, find out which scale your meter uses. Pull up the manual or check the unit display — most pens show μS/cm (microsiemens) or mS/cm (millisiemens). TDS meters show PPM, but which PPM scale matters: 500 (Hanna, Truncheon, most US meters), 700 (Eutech, Milwaukee), or 640 (most European meters).
Enter your reading and pick the matching unit
Type the number from your meter into the input field, then select the unit that matches your meter's display. The converter shows all five equivalent values at once — so whatever scale your nutrient chart, forum post, or meter uses, you'll have the number you need.
Compare to your feed chart
Take the converted value that matches your nutrient manufacturer's chart. Most feed schedules list ranges like "800–1000 PPM (500 scale)" or "EC 1.6–2.0." Use the row that matches the units on your chart — not whatever unit your meter happens to display.
Common mistake: mixing scales
A nutrient chart that says "1000 PPM" without specifying a scale could mean EC 1.4, 1.6, or 2.0 — a 40% difference. If your plants are burning on a "weak" feed, scale confusion is the most likely culprit. When in doubt, feed to EC — it's scale-independent and universal.
Why PPM scales exist (and why it matters)
PPM (parts per million) and EC (electrical conductivity) measure the same thing from different angles. EC is a direct measurement — it reads how well your nutrient solution conducts electricity, which scales with dissolved salt content. PPM is a derived value: the meter measures EC, then multiplies by a conversion factor to estimate total dissolved solids. That conversion factor is where the three scales come from.
500
NaCl scale
Also called the "Hanna scale" or "US scale." Assumes the dissolved solids behave like sodium chloride. Most common on consumer meters sold in the US.
700
442 scale
Based on a 40% sodium sulfate / 40% sodium bicarbonate / 20% sodium chloride mixture. Closer to the real composition of many fertilizers. Standard on Eutech and Milwaukee meters.
640
KCl scale
European standard, based on potassium chloride as the reference salt. Used on most European-manufactured meters and in horticultural research.
The conversion formula
The math is simple: PPM = EC × scale factor. A solution at EC 1.0 mS/cm reads as 500 PPM on a 500-scale meter, 700 PPM on a 700-scale meter, or 640 PPM on a 640-scale meter — all describing the exact same water. The liquid didn't change. The meter's assumption about what's dissolved in it changed.
| EC (mS/cm) | PPM 500 | PPM 700 | PPM 640 | Typical use |
|---|---|---|---|---|
| 0.4 | 200 | 280 | 256 | Seedlings, clones |
| 0.8 | 400 | 560 | 512 | Early veg |
| 1.2 | 600 | 840 | 768 | Late veg |
| 1.6 | 800 | 1120 | 1024 | Early flower |
| 2.0 | 1000 | 1400 | 1280 | Peak flower |
| 2.4 | 1200 | 1680 | 1536 | Heavy feeders |
Why EC is the honest unit
Professional cultivators and research papers almost always use EC, not PPM. EC is a raw measurement — no assumption about salt composition, no conversion factor, no scale ambiguity. When a nutrient manufacturer's feed chart lists "EC 1.6," that number means the same thing everywhere in the world. When a chart lists "1120 PPM" without specifying a scale, it's effectively ambiguous: it could mean EC 1.6 (700 scale), EC 2.24 (500 scale), or EC 1.75 (640 scale).
If you're dialing in a new feed schedule or switching nutrient brands, work in EC and convert to PPM only when a chart forces you to. Your meter reads both — just toggle the display.
Accuracy limits worth knowing
PPM conversions are estimates, not measurements. The actual total dissolved solids in a real nutrient solution rarely match any of the three reference salts perfectly — real nutrients are mixtures of nitrates, phosphates, sulfates, and chelated micros. The 500 scale typically underestimates real TDS by 10–30%; the 700 scale is usually within 5–10% of true TDS for standard mineral nutrients. For practical growing, this doesn't matter — consistency matters more than absolute accuracy. Pick a scale, stick with it, and dial your feed to your plants, not your number.
PPM, EC & TDS — your questions answered
Which PPM scale should I use?
Use whichever scale your nutrient manufacturer's feed chart uses. If your chart doesn't specify, or you want to avoid scale confusion entirely, work in EC instead. EC is scale-independent — an EC of 1.6 means the same thing whether you're in Michigan, Manchester, or Melbourne. Most professional growers and research papers use EC for this reason.
Why does my meter show different numbers than my nutrient chart?
Almost always, your meter and your chart are using different PPM scales. A feed chart listing "1000 PPM" on a 700 scale equals 1400 PPM on a 500-scale meter — a 40% apparent difference in the same solution. Find out what scale your meter uses (check the manual or look for a scale selector button), convert your chart's numbers to match, or switch to EC and skip the conversion entirely.
What's the difference between μS/cm and mS/cm?
They're the same unit at different scales. 1 mS/cm (millisiemens per centimeter) equals 1000 μS/cm (microsiemens per centimeter). Most grow meters default to μS/cm because grow-range readings fall in the 500–3000 μS/cm range, which is easier to read than 0.5–3.0 mS/cm. If you enter an EC value like "988" assuming mS/cm, you'll get nonsense results — that number is almost certainly μS/cm, or about 1.0 mS/cm.
Can I trust PPM readings from tap water?
Tap water PPM readings tell you how much dissolved solid is in your source water — useful baseline information, but not necessarily useful for feeding. Tap water can be 100–400 PPM of calcium, magnesium, chlorine, sodium, and other minerals that aren't the nutrients your plants want. Always factor your tap water's baseline PPM into your total feed strength, or start with RO (reverse osmosis) water for full control.
What EC should I feed cannabis at each stage?
Typical ranges for cannabis in soil or coco: seedlings and clones 0.2–0.5 EC, early veg 0.6–1.0 EC, late veg 1.0–1.4 EC, early flower 1.4–1.8 EC, peak flower 1.8–2.2 EC. Hydroponic systems generally run 10–20% lower. These are starting points — read your plants. Leaf tip burn suggests too high, pale new growth suggests too low. Different strains tolerate different ranges.
Why does runoff PPM usually read higher than input PPM?
Plants uptake water faster than they uptake nutrients, which concentrates the remaining salts in your medium. If you feed at 1000 PPM and runoff reads 1400 PPM, your medium is building up salts — time to flush or reduce feed strength. If runoff reads 800 PPM, your plants are eating heavily and may need more. Runoff EC is one of the most useful diagnostic numbers in a grow.
Do I need a separate PPM meter and EC meter?
No. PPM and EC meters use the same sensor — they measure conductivity, then either display it as EC directly or multiply by a conversion factor to display PPM. Any decent combo pen (Apera, Bluelab, Hanna Groline) lets you toggle between units. Save your money for a quality sensor with a good calibration solution instead of buying two devices.
How often should I calibrate my PPM/EC meter?
At least monthly for hobby grows, weekly for serious growers, and before any critical measurement for commercial operations. Calibration drifts as the probe's platinum contacts age. Use a standard calibration solution (1413 μS/cm is the universal reference). A meter that reads 1413 on a 1413 solution is accurate; anything more than 5% off needs calibration or probe replacement.
Gear to measure & manage your nutrient solution
Accurate conversions are only useful if your meter is accurate. Start with a quality PPM/EC pen, add a pH meter to round out your water diagnostics, then dial in the rest of your feed system.
pH & EC Meters
Combo pens that read both PPM/EC and pH — the two numbers every grower needs before every feed.
Nutrients & Additives
Complete nutrient lines with published EC and PPM feed charts — no guesswork, no scale conversion.
pH Up & pH Down
Food-grade solutions to adjust your feed water. Essential alongside any EC management routine.
RO & Water Filtration
Start from zero PPM with reverse osmosis water. The cleanest baseline for precise feed control.
Cal-Mag & Supplements
Calcium and magnesium supplements — critical when feeding RO water or running heavy bloom schedules.
Complete Nutrient Systems
Full feed programs from brands like FoxFarm with detailed PPM and EC targets for every week of the grow.