Water Soluble Fertilizer Calculator
Greenhouse and hydroponic feeding is prescribed in ppm — parts per million of an actual nutrient — but fertilizer comes in bags labelled with percentages. The bridge between the prescription and the bag is one division, and this calculator walks it in both directions a grower needs: how many grams of a given product, per tank, to deliver a target ppm of nitrogen (or phosphorus, or potassium — whichever nutrient the prescription names); and, for injector systems, how strong the concentrated stock must be at a stated injection ratio.
The Division at the Heart of Fertigation
One fact organises everything: ppm is mg/L of the nutrient, and the bag is only part nutrient. A 20-10-20 fertilizer is 20% nitrogen by weight, so delivering 150 ppm of nitrogen needs 150 / 0.20 = 750 mg of product per litre — the nutrient divided by its fraction in the bag. From there the arithmetic is volume: 750 mg/L × 100 L = 75,000 mg = 75 g for the tank, the calculator's default run. The same division prices every nutrient on the label simultaneously — that 75 g simultaneously delivers phosphorus at 750 × 0.10 = 75 ppm (as P₂O₅ equivalent, on the label's oxide convention) and potassium at 150 ppm (as K₂O) — because a blended product doses its whole analysis at once, in fixed proportions. That fixedness is the real subject of fertilizer selection: matching the ratio of a product to the crop's appetite matters more than hitting any single ppm, and this calculator's per-nutrient field exists so each element of a candidate bag can be priced against its target before the bag is chosen.
Worked Examples: Tank Mixing and the Injector
Direct tank. Defaults as above: 150 ppm N from a 20% product into 100 L = 75 g. Scale intuition comes free: at 1,000 L the same prescription is 750 g, and a grower who has weighed the 75 g once can sanity-check any larger batch against it. Injector stock. A 1:100 injector dilutes stock a hundredfold at the hose, so the stock must be a hundredfold stronger: product strength = 750 × 100 = 75,000 mg/L = 75 g per litre of stock. For a 10-litre stock tank: set volume 10 and injector 100 in the calculator → 750 g — a serious dissolution exercise, done warm, in stages, and the reason stock tanks are mixed the day before and checked against solubility limits (most common soluble fertilizers dissolve in the hundreds of grams per litre, but the margin narrows in cold water and vanishes for some calcium-bearing mixes — the classic A/B tank separation exists because concentrated calcium and concentrated phosphate/sulfate precipitate each other in the stock even when they coexist happily at final dilution).
EC: The Meter That Audits the Arithmetic
Calculated ppm is a promise; the EC (electrical conductivity) meter is the audit. Dissolved fertilizer salts conduct in rough proportion to their concentration, so a mixed tank's EC can be checked against the recipe's expectation — as a field rule, many common greenhouse blends contribute on the order of 1 mS/cm per ~700–800 ppm of total salts, though the factor varies by formulation, which is why growers calibrate their product once and thereafter steer by the meter. Two disciplines keep the audit honest. Subtract the water: source water carries its own EC (near zero for rain and RO water; appreciable for well and municipal supplies), and the fertilizer's contribution is the rise above the source reading. Temperature compensates: conductivity climbs about 2% per °C, and meters correct for it — trust a compensated reading at tank temperature. When calculated ppm and measured EC disagree, believe the discrepancy and hunt it: an uncalibrated injector ratio is the usual suspect, and the calculator's numbers, re-run with the measured ratio, become the corrected recipe.
Reading a Label Like a Formulator
Three label conventions decide whether the calculator gets honest inputs. The guaranteed analysis lists N as elemental nitrogen, but P as P₂O₅ and K as K₂O equivalents — oxide conventions from the age of ash analysis. If your target is elemental phosphorus, convert: P = P₂O₅ × 0.436; K = K₂O × 0.830. Inside the calculator, the percentage field must be the percentage of the quantity your ppm names — label percent for N, label percent if your ppm is stated as P₂O₅ (common in older recipes), converted percent if elemental. Nitrogen's forms (nitrate, ammonium, urea) share one total-N line but different pH and growth effects; the ppm math is deaf to the difference, the crop is not. And chelated micronutrients ride along at percentages the calculator handles identically — a 0.1% iron product dosed by the same division — where the precision required is exactly why stock solutions, not tank additions, are the sane route for the trace elements.
A Second Worked Product: Calcium Nitrate
Variety is the test of a formula, so price a straight (single-salt) fertilizer through the same division. Calcium nitrate — a common greenhouse staple — analyses at about 15.5% N and 19% Ca. For the same 150 ppm N target: 150 / 0.155 = 968 mg of product per litre, noticeably more mass than the 20-10-20 needed, because the nutrient is more dilute in the bag. The same weighing unavoidably co-delivers calcium at 968 × 0.19 ≈ 184 ppm Ca — generous, deliberate in soft-water districts, and precisely the kind of coupled delivery the label-reading section warned of: every straight salt is a two-nutrient decision. The calculator handles each in turn; the recipe is the set of its answers that are mutually compatible in one tank.
The Water Is an Ingredient Too
No recipe survives contact with unexamined source water. Hard, alkaline water carries bicarbonate that neutralises acidifying fertilizers and nudges substrate pH upward across a crop's life — the ppm of nitrogen stays exactly as calculated while its effect drifts — and water already rich in calcium or magnesium delivers a silent base dose of those elements before the bag is opened. Professional recipes therefore begin with a water analysis and set the fertilizer's acid balance against it; the household version of the same wisdom is to test the source once, keep the report with the fertilizer, and remember that this calculator prices what you add — the water's own contribution is the recipe's zeroth line.
Related Tools
More from the Biology Calculators hub: the Vegetable Seed Calculator for the planting that precedes the feeding, the Bulb Spacing Calculator for densities of a different crop, and the MLVSS Calculator for mg/L discipline at the other end of the water cycle.
Frequently Asked Questions
How do I convert ppm to ounces or tablespoons?
Stay in grams — a pocket scale is the grower's honest instrument, because soluble fertilizers vary in bulk density enough to make spoon measures lie by tens of percent. If a conversion is unavoidable: the calculator's grams per litre × 3.785 gives grams per US gallon, and grams ÷ 28.35 gives ounces. But weigh the product once, note your scoop's actual fill for that product, and the volume shortcut becomes a calibrated one.
Is ppm the same as EC?
No — ppm (as used here) is a mass concentration of a named nutrient; EC is a bulk electrical property of all dissolved salts together. They correlate within a fixed recipe and water source, which is why EC steers day-to-day operations, but no universal conversion exists between them: two solutions of identical EC can carry different nitrogen ppm. Calculate by ppm, audit by EC, and keep both numbers in the log.
What ppm of nitrogen do crops want?
As orientation, not prescription: many greenhouse vegetables and ornamentals are fed in the 100–200 ppm N range at each watering (constant liquid feed), leafy greens toward the lower half, heavy-fruiting crops toward the upper, with seedlings and sensitive species well below. The crop, stage, light level and leaching fraction all move the target — use your crop guide's figure in this calculator, whose job is the weighing, not the agronomy.
Why is my stock precipitating?
Almost always incompatibility at concentration: calcium with phosphates or sulfates forms insoluble salts in strong stock (hence A-tank/B-tank separation, calcium alone in one), or the total concentration exceeds a component's solubility in cold water. Dilute further, warm the water, separate the antagonists, and add products one at a time to a partly filled tank — the order of addition is process design, not superstition.
Does this work for soil drench and garden use?
Yes — the ppm arithmetic is medium-blind. A watering-can mix for containers is a small tank: enter its litres and the same division prices it. Garden guidance given as "a tablespoon per gallon" can be translated once through this page (weigh the tablespoon of your product, back out its ppm, and thereafter dose by weight) — the garden and the greenhouse share one arithmetic, as they should.
What about fertilizers with several percentages — which do I enter?
The percentage of the nutrient your ppm target names. Dose the whole prescription by its most limiting or most carefully targeted nutrient (usually nitrogen), enter that percentage, then read what the same weighing delivers of the others (product mg/L × their percentages) and check those against their targets. If one product cannot satisfy all the targets in proportion — the usual case — a second product enters as a second run of this calculator, and the two weighings are added to the same tank or separate stocks.