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Fertilizer PPM Calculator

Enter the target ppm, water volume, and nutrient percentage to get the fertilizer dose.

How it is calculated

One ppm in water equals 1 mg per litre. The tool therefore works in litres and grams. The nutrient needed is mg = target ppm × litres. Because the fertilizer only contains part nutrient, the product weight is mg of fertilizer = mg of nutrient ÷ (nutrient % ÷ 100), shown in milligrams and as grams (÷ 1000). It also gives the dose per litre, grams ÷ litres. The nutrient percentage is the share of one element on the label (for example the N in N-P-K), it must be above zero, and the volume must be above zero. Each nutrient is a separate calculation, and the tool does not account for nutrients already in the water.

Worked example

You want 150 ppm nitrogen in 20 litres of water, using a fertilizer that is 15% nitrogen. Nutrient needed: 150 × 20 = 3,000 mg. Fertilizer needed: 3,000 ÷ 0.15 = 20,000 mg, which is 20.00 g. Per litre this is 20 ÷ 20 = 1.0000 g/L. Dissolve about 20 g in the tank and your solution should reach 150 ppm N.

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Common mistakes

  • Typing the nutrient percentage as a decimal, like 0.15 for 15%. The field is a percent, so 0.15 means 0.15% and the dose becomes a hundred times too large.
  • Using the wrong element's percentage. A 20-10-10 label has 20% N, 10% P₂O₅ and 10% K₂O; take the figure for the nutrient whose ppm you are targeting. Also note that P and K on labels are oxide forms, not pure P and K.
  • Entering gallons. The volume is in litres, so 5 US gallons is about 18.9 L.
  • Expecting the measured EC or ppm meter to match the target exactly. Tap water, other salts and meter conversion scales add readings that this calculation does not include.

About this calculator

Hydroponic and container growers feed plants by dissolving fertilizer in water to a target nutrient concentration measured in parts per million (ppm), since even a small excess can burn roots while too little starves the plant. The dose is calculated from the fertilizer's nutrient percentage on the label (usually nitrogen, as N-P-K) and the water volume in litres: fertilizer weight in grams = (target ppm × litres of water) ÷ (nutrient % × 10), a formula that converts a label percentage and a target concentration into a real gram measurement.

This calculator takes your target ppm, water volume in litres (multiply gallons by 3.785 to convert), and the nutrient percentage from your fertilizer's label and returns exactly how many grams to dissolve. Hydroponic growers, greenhouse managers, and hobbyist container gardeners use it to hit precise feeding schedules — such as 150 ppm nitrogen for leafy greens versus 200+ ppm for fruiting tomatoes — instead of eyeballing a scoop per bucket.

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