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Fertilizer / Nutrient Dilution Calculator

Dilute liquid fertiliser concentrate to a target PPM or EC for soil or hydroponics. NPK aware and tank-volume aware.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on August 26, 2026 · Corrections log

N (%)

P (%)

K (%)

Concentrate dose (mL) e.g. 5 mL per gal/L of mixed solution.

Tank volume

Volume unit

Concentrate density (g/mL)

Your TDS meter's ppm scale Changes only the meter ppm reading. EC does not depend on it.

Nitrogen concentration in tank

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Estimated meter EC for a complete feed ≈ — mS/cm · a 500-scale TDS meter reads ≈ — ppm

N + P + K actually dissolved ≈ — ppm (mg/L)

Nutrient	ppm (mg/L)	mg per tank

Quick reference

Concentrate grams = dose mL × density. Element ppm = (grams × element % × 10000) / tank mL. NPK percentages are given as elemental N, P₂O₅, K₂O — multiply P by 0.4364 and K by 0.8301 for elemental P/K.

EC (mS/cm) is what a meter physically measures. A TDS meter then shows ppm = EC × its scale factor: 500 (NaCl, most US meters), 640 (common in Europe) or 700 (the 442 scale). So one solution at EC 1.4 reads 700, 896 or 980 ppm depending on the meter, while its EC and the nutrients in it stay the same. The scale setting here changes only that meter ppm line.

The N + P + K figure is the real mass of those three elements per litre. A meter also sees calcium, magnesium, sulphur and the counter-ions everything arrives with, so EC is estimated from a complete-feed reference instead: full-strength Hoagland solution carries 476 ppm N + P + K and meters about 2.0–2.5 mS/cm, so EC ≈ N + P + K × 2.2 ÷ 476 (about ÷ 216). That assumes a balanced feed. Urea-heavy or N-P-K-only products read lower, and your tap water adds its own EC on top. Measure the mixed tank and trust the meter.

Plant EC targets as read on a meter: seedlings 0.5–0.8, veg 1.2–1.8, flower 1.8–2.5, fruiting tomatoes 2.5–3.0. The presets are N + P + K targets that land in those ranges: Seedling 150 ppm ≈ EC 0.7, Veg 275–375 ≈ 1.3–1.7, Flower 475 ≈ 2.2, Tomato/fruit 600 ≈ 2.8.

How it works

The chain is short and fully linear. Concentrate grams = dose mL × density, defaulting to 1.2 g/mL. Element ppm = grams × element fraction × 10⁶ ÷ tank millilitres, with a US gallon taken as 3.78541 litres. Because label NPK is stated as N, P₂O₅ and K₂O rather than pure elements, phosphorus is multiplied by 0.4364 and potassium by 0.8301 first. The dose is read as millilitres per gallon or per litre, not per tank.

Concentrate dose (mL) and Tank volume set the ratio, while the N, P and K percentage fields should be copied straight off the bottle — 6-2-4 is loaded by default. EC is estimated from the dissolved N + P + K on one fixed basis: full-strength Hoagland solution carries 476 ppm N + P + K and meters about 2.2 mS/cm, so EC ≈ N + P + K × 2.2 ÷ 476. The meter-scale setting (500 NaCl, 640 EU or 700 442-scale) changes nothing chemical and does not move the EC. It only sets the ppm a TDS meter would display, EC × scale, so EC 1.2 shows 600 ppm on a 500 meter and 840 on a 700 meter.

The EC is an estimate for a balanced, complete feed, the kind that also carries calcium, magnesium and sulphur. A urea-heavy or N-P-K-only product will meter lower than shown, and your tap water adds its own EC on top, so measure the mixed tank and trust the meter. Density also defaults to 1.2 g/mL for every product; check your own label, because thin liquid feeds sit nearer 1.0 and that error propagates into every ppm shown.

Frequently asked questions

Why does my EC meter read differently from the estimated EC?

The estimate scales your N + P + K by a complete-feed reference (Hoagland solution, 476 ppm N + P + K at about 2.2 mS/cm). Your tap water adds its own EC on top, so a meter reads higher by roughly the plain-water reading. A product without calcium and magnesium, or one whose nitrogen is mostly urea, which barely conducts, reads lower. Measure the mixed tank and adjust from there.

How do I turn P₂O₅ and K₂O on the label into actual P and K?

Multiply the P figure by 0.4364 and the K figure by 0.8301, which the tool does automatically. A 6-2-4 label therefore delivers 0.87% elemental phosphorus and 3.32% elemental potassium, which is why the phosphorus row always looks smaller than the bottle suggests.

Should I pick the 500, 640 or 700 conversion factor?

Match whatever your TDS meter uses. US meters typically use the 500 NaCl scale, many European meters 640, and some hydroponic meters the 700 (442) scale. EC is the measured quantity and does not change with the scale. The setting only changes the ppm figure a meter would display, which is EC × scale.

What do the ppm preset buttons actually set?

They back-solve the concentrate dose so the tank reaches that figure as elemental N + P + K combined. The figures are chosen so the estimated EC lands in the usual meter ranges: Seedling 150 ppm shows EC 0.69, Veg 275 and 375 show 1.27 and 1.73, Flower 475 shows 2.20 and Tomato/fruit 600 shows 2.77.

How much 6-2-4 concentrate goes into a one gallon tank?

At the default 5 mL per gallon and 1.2 g/mL density that is 6 grams of concentrate, delivering roughly 95 ppm nitrogen. Halving the dose halves every ppm figure, since the whole chain is linear in dose, density and NPK percentage.

NPK numbers are percentages by weight

The three figures on a fertiliser label are percentages by weight of nitrogen, phosphorus (as P₂O₅) and potassium (as K₂O). That means a 20-20-20 product is twice as concentrated as a 10-10-10, and mixing both at the same rate applies twice the nutrient.

The phosphorus and potassium figures being expressed as oxides rather than elements is a historical convention that survives on labels, and it is why elemental comparisons need conversion.

Over-application is the common failure

More fertiliser does not produce more growth past the point of sufficiency. Excess salts accumulate in soil or growing medium and cause root burn, leaf-tip scorch and osmotic stress — symptoms that look like drought and are frequently misdiagnosed as needing more water or more feed.

Diluting further and feeding more often is generally safer than concentrated infrequent feeding, particularly in containers where there is little buffer.

Runoff from over-application is the other cost, and it is an environmental one: nutrients leaving the site contribute to algal blooms in waterways. Applying what the plant can use is both cheaper and better practice.

Where these numbers come from

The standard or formula behind this tool, so you can check it rather than take it on trust.

van Delden, Nazarideljou & Marcelis (2020), Nutrient solutions for Arabidopsis thaliana, Plant Methods 16:72
Lists full-strength Hoagland and Arnon solution (476 ppm N + P + K) at EC 2.0 dS/m and half strength at 1.1; freshly made batches are often measured nearer 2.5. The tool scales EC by 2.2 ÷ 476 per ppm of N + P + K, and treats the 500/640/700 ppm scales only as meter display conventions (ppm = EC × scale).