About the Hydroponic Nutrient Calculator
This hydroponic nutrient calculator does the two sums every hydroponic grower needs. First, it converts between electrical conductivity (EC, in mS/cm) and the parts-per-million (ppm or TDS) number shown on handheld meters. Meters do not all use the same conversion: many US meters use the 500 scale (NaCl), others use 700 (KCl, common in Europe and Australia) or 640, so the same solution can read 800, 1,024 or 1,120 ppm depending on the meter.
Second, it scales the nutrient dose on your bottle label to your reservoir. Enter the reservoir size in gallons or liters, the label rate in mL per gallon or per liter, how many parts your nutrient has (one-part, two-part A + B or three-part) and the strength you want — for example 50% for seedlings and cuttings — to get the millilitres of each part to add.
It suits deep water culture, NFT, ebb-and-flow and drip systems, as well as Kratky jars. Always add each part to the water separately and stir between parts, measure EC after mixing, and adjust toward the range your crop and nutrient manufacturer recommend.
With the default inputs, the ppm on 500 scale is 800 ppm. Change any value above to recalculate instantly.
How to use the hydroponic nutrient calculator
- 1Choose whether you have an EC or a ppm reading and enter it.
- 2If you entered ppm, pick the scale your meter uses (check its manual).
- 3Enter your reservoir volume and the unit.
- 4Enter the label dose per gallon or liter, the number of parts and the strength you want.
- 5Add each part separately, stir, then re-check EC against your target.
Formula and method
TDS meters actually measure electrical conductivity and multiply it by a fixed conversion factor to display ppm. The 500 scale (also called NaCl or TDS scale) multiplies EC in mS/cm by 500, the 700 scale (KCl, often used in Europe and Australia) by 700, and some meters use 640. None of these is the true dissolved-solids content of a nutrient solution; they are conventions, so the same water gives different ppm numbers on different meters. Conductivity factor (CF) is EC × 10 and microsiemens are EC × 1,000.
The nutrient dose is the label rate per gallon or liter multiplied by the reservoir volume in the same unit and by the strength fraction. For multi-part nutrients the same dose is usually given for each part, so the total added is the per-part dose times the number of parts. Check your own label, as some brands use different ratios between parts.
- EC
- Electrical conductivity in millisiemens per centimetre
- Scale
- Meter conversion factor: 500, 640 or 700
- Strength
- Fraction of the full label dose (50% = 0.5)
Worked examples
EC 1.6 and a 20-gallon two-part reservoir
EC 1.6 mS/cm is 800 ppm on a 500-scale meter, 1,024 on a 640 meter and 1,120 on a 700 meter. At 5 mL per gallon, 20 gallons needs 100 mL of part A and 100 mL of part B, about 6.8 fl oz in total.
840 ppm on a 700-scale meter, 50 L three-part at half strength
840 ÷ 700 = EC 1.2, which a 500-scale meter would show as 600 ppm. A 2 mL/L dose at 50% strength in 50 liters is 50 mL of each of the three parts, 150 mL total.
400 ppm (500 scale) for seedlings in a 5-gallon bucket
400 ppm on the 500 scale is EC 0.8, or 560 ppm on a 700-scale meter. A one-part nutrient at 4 mL per gallon and 75% strength needs 5 × 4 × 0.75 = 15 mL.
Frequently asked questions
How do I convert EC to ppm?+
Multiply EC in mS/cm by your meter’s scale factor: × 500 for the NaCl/TDS scale, × 640, or × 700 for the KCl scale. EC 1.5 is therefore 750, 960 or 1,050 ppm depending on the meter.
Which ppm scale does my meter use?+
Many meters sold in the US default to the 500 scale, many European and Australian meters use 700, and a few models use 640; some let you switch. Check the manual or calibration solution label; EC itself is the same on every meter.
What EC should my hydroponic solution be?+
Seedlings and clones usually do well around 0.5–1.0 mS/cm, leafy greens and herbs around 1.0–1.8, and fruiting crops such as tomatoes, peppers and cucumbers around 2.0–3.5. Follow your nutrient brand and crop guidance.
Why add nutrient parts separately?+
Concentrated parts A and B contain calcium and sulfates or phosphates that react when mixed undiluted, forming insoluble precipitates the plants cannot use. Add each part to the full reservoir water and stir before adding the next.
Does tap water affect my EC reading?+
Yes. Tap water already has dissolved minerals, often 0.1–0.5 mS/cm or more. Measure your source water first and subtract it if your nutrient targets refer to added nutrients only.