TDS / EC / PPM Converter (500 & 700 Scale)
Convert between EC, and PPM on the 500 and 700 scales instantly.
Result
Conversions use standard factors; individual meters vary. Calibrate your meter regularly and always note which scale you are reading.
History
What it is
This converter translates nutrient strength between EC (electrical conductivity in mS/cm) and the two common PPM/TDS scales: the 500 scale (NaCl, used by Hanna and most US meters) and the 700 scale (442, used by Truncheon and many EU meters). It clears up the confusion when a feed chart and your meter use different units.
Who should use it
Hydroponic and soil-less growers comparing feed charts, meters or guides that use different units, and anyone whose meter reads PPM but whose nutrient schedule is given in EC (or vice versa).
How to use it
- Enter the number your meter or feed chart shows.
- Choose which unit that number is in — EC, PPM 500, or PPM 700.
- Read the equivalent value on all three scales at once.
Example calculation
Worked example
An EC of 1.4 mS/cm equals 700 PPM on the 500 scale (1.4 × 500) and 980 PPM on the 700 scale (1.4 × 700). So a chart asking for '700 ppm' on a US meter is the same strength as '980 ppm' on a Truncheon — both are EC 1.4.
Formulas used
PPM₅₀₀ = EC · 500 • PPM₇₀₀ = EC · 700 (EC in mS/cm)
How to read your result
Always note which scale a number uses. The same solution can read 700 or 980 PPM depending on the meter. EC is the most universal unit, so many growers standardise on EC to avoid mistakes.
Recommended ranges
| EC (mS/cm) | PPM 500 | PPM 700 |
|---|---|---|
| 0.8 | 400 | 560 |
| 1.2 | 600 | 840 |
| 1.6 | 800 | 1,120 |
| 2.0 | 1,000 | 1,400 |
| 2.4 | 1,200 | 1,680 |
Common mistakes to avoid
- Comparing PPM numbers without knowing whether they are 500- or 700-scale.
- Assuming all 'PPM' meters agree — they can differ by 40%.
- Forgetting that EC/PPM measures total dissolved salts, not which nutrients are present.
- Not accounting for the EC already in your source water before adding nutrients.
Frequently asked questions
What is the difference between the 500 and 700 PPM scales?
They use different conversion factors from EC: PPM 500 = EC × 500, PPM 700 = EC × 700. The same solution reads higher on the 700 scale.
How do I convert EC to PPM?
Multiply EC (in mS/cm) by 500 for the 500 scale or by 700 for the 700 scale. To go from PPM to EC, divide by the same factor.
Which scale should I use?
Use whichever matches your meter and feed chart, and stay consistent. EC is the least ambiguous unit and is preferred by many growers.
Does EC tell me my nutrient balance?
No. EC and PPM only measure total dissolved salts, not the ratio of individual nutrients. Use a feed schedule for balance.
Should I subtract my source water EC?
Yes. Measure your tap or RO water first; the nutrients you add are the difference between final and starting EC.