EC to PPM Converter

What Is EC to PPM Conversion, and Why Hydroponic Growers Need It

Electrical conductivity, EC, is the raw physical measurement a nutrient meter actually takes, how well a solution conducts electricity, which rises directly with dissolved nutrient salt content. PPM (parts per million) is a derived estimate built from that reading, since a conductivity meter has no way to directly count dissolved particles, it infers a concentration figure using an assumed relationship between conductivity and dissolved solids. Grow With Hydroponics' EC and PPM guide notes that growers lean on PPM specifically because nutrient dosing charts and product labels are usually written in PPM, even though the meter itself only ever measures EC.

This calculator takes an EC reading and converts it to PPM across the three scales actually in use, plus a crop target comparison and the conductivity factor reading some meter brands display instead.

The Three PPM Scales: 500, 640, and 700, and Why They Disagree

There is no single, universal EC-to-PPM formula, three different conversion standards are in active use, and they don't agree with each other. The 500 scale, based on a sodium chloride reference standard and common on US hydroponics meters, multiplies EC in mS/cm by 500. The 700 scale, based on a potassium chloride reference and common on Australian meters like Truncheon and Bluelab, multiplies by 700. A middle 640 (sometimes cited as 650) scale sees use internationally as well.

The gap between scales is substantial, not a rounding difference: HydroGrowLab's comparison of the 500 and 700 scales points out that an identical 1.4 mS/cm reading works out to 700 ppm on the 500 scale but 980 ppm on the 700 scale, a 40% spread for water that hasn't changed at all, purely a function of which scale the meter or the person reading it assumes.

EC to PPM Formula and Worked Example

The conversion itself is simple multiplication once the scale is chosen: PPM = EC (in mS/cm) × scale factor, or if the meter reads in µS/cm, divide by 1000 first to get mS/cm before multiplying. For a reading of 1.4 mS/cm: 500 scale gives 700 ppm, 640 scale gives 896 ppm, and 700 scale gives 980 ppm, three genuinely different numbers from the exact same physical measurement.

Because EC itself doesn't change between scales, only the derived PPM figure does, comparing EC readings directly between two meters is always more reliable than comparing PPM numbers unless both meters are confirmed to use the same scale, a point Hydrobuilder's EC to PPM reference makes explicitly for growers troubleshooting mismatched readings between two different meters.

Which Meter Brands Use Which Scale

Meter brand doesn't map perfectly to scale, and models within the same brand can differ, but general regional patterns hold reasonably well. A widely referenced grower forum thread compiling manufacturer scale information notes that US-market meters like Hanna and Milwaukee models most often default to the 500 scale, while Australian brands like Truncheon and Bluelab typically use the 700 scale, and European meters vary, sometimes defaulting to 640.

Because this varies by specific model, checking the meter's own manual rather than assuming based on brand alone is the only fully reliable way to know which scale a given device is using.

Target EC and PPM Ranges by Crop

Different crops tolerate, and often need, meaningfully different nutrient strength. Urban Harvest Lab's hydroponic EC chart lists lettuce and other leafy greens around 0.8 to 1.2 mS/cm, basil a bit higher at 1.0 to 1.8, and fruiting crops like tomatoes considerably higher, 2.0 to 3.5 mS/cm and sometimes up to 5.0 during late fruiting.

This calculator's crop comparison checks an entered EC reading against these same ranges directly, since knowing the raw number matters less than knowing whether it's actually appropriate for what's growing.

Conductivity Factor (CF): The Bluelab/Truncheon Alternative

Some meters, notably Truncheon and Bluelab models common in Australian and New Zealand hydroponics, display a conductivity factor (CF) reading instead of PPM. Botanicare's Q&A on Bluelab meter readings explains CF as simply EC in mS/cm multiplied by 10, so a 1.4 mS/cm reading is a CF of 14, a separate but directly related figure from any of the PPM scales.

Knowing CF matters because a nutrient chart written in CF can't be read directly against a PPM-based chart without converting one to the other first, mixing the two without converting produces a dosing error even though both figures ultimately trace back to the same EC measurement.

Accuracy and Limitations

EC to PPM conversion is only ever an estimate, not a direct measurement, since it assumes a specific relationship between conductivity and dissolved solids that varies somewhat by which salts are actually present in solution. Waternity Lab's technical notes on EC to PPM conversion point out that this is exactly why lab-grade TDS testing, which measures dissolved solids more directly, can disagree slightly with a meter's PPM estimate even at an identical EC reading. If your water's alkalinity is also relevant to nutrient uptake, our Alkalinity Calculator and, for broader concentration unit conversions, our Concentration Calculator cover the related water chemistry questions this tool doesn't.