Fertilizer Calculator

What this fertilizer calculator does — and its lane

This is the nutrient-rate tool: it answers how much of this N-P-K product do I apply, and how much nitrogen, phosphate and potash does that actually deliver. It owns fertilizer rate, NPK grade and ratio, lime and amendment rate, and solution ppm. It is not a soil-volume tool — for compost or topsoil in cubic yards, use the soil calculator, and for plain area and volume math the cubic-yards calculator. Whether you write it fertilizer or fertiliser, work per 1,000 sq ft, per 1,000 square feet, per acre or per hectare, the four modes above cover it.

How much fertilizer do I need?

The honest friction this fixes — the “%N trap.” The number you want to apply is usually pounds of nitrogen, but the bag is sold as pounds of product, and only a fraction of that product is nitrogen. People multiply when they should divide, or treat “1 lb N” as 1 lb of fertilizer. The fix is one division:

product per 1,000 sq ft = (lb N you want per 1,000 sq ft) ÷ (N% ÷ 100)
total product = product per 1,000 × (area in ft² ÷ 1,000)
lb N applied = product × N%/100  ·  lb P₂O₅ = product × P%/100  ·  lb K₂O = product × K%/100

So 1 lb N per 1,000 sq ft from a 10-10-10 is 1 ÷ 0.10 = 10 lb of product per 1,000 sq ft; over a 5,000 sq ft lawn that is 50 lb. From a 50 lb bag of 26-5-10 at the same 1 lb N, you need 1 ÷ 0.26 = 3.85 lb per 1,000 sq ft, and one bag covers about 50 ÷ 3.85 × 1,000 ≈ 13,000 sq ft. The Rate mode does all of this and shows the phosphate and potash you are adding alongside.

How much fertilizer per acre (and per 1,000 sq ft)

Recommendations arrive in different units — pounds per 1,000 square feet, pounds per acre, or kilograms per hectare — but the arithmetic is the same. To convert a per-1,000-sq-ft figure to a per-acre figure, multiply by 43.56, because an acre is 43,560 sq ft. So the 10 lb of 10-10-10 per 1,000 sq ft above is 10 × 43.56 = 435.6 lb per acre. Metric users can read kg per hectare directly. The same chain answers “how much urea per acre,” “how many 50 kg bags per hectare of maize,” or “how much 19-19-19 per acre” — set the grade, set the target, and switch the rate basis.

NPK ratio and what the numbers mean

The three numbers are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 10-10-10 is 10% of each by weight; a 50 lb bag therefore holds 5 lb each of N, phosphate and potash, and the rest is carrier. The NPK ratio is that grade simplified — 10-10-10 reduces to 1-1-1, and 24-8-16 to 3-1-2 — which is what lets you compare two products that feed in the same proportion at different strengths. The grade-and-ratio mode shows the simplified ratio, the pounds of each nutrient per 100 lb and per bag, and a side-by-side compare of two grades.

P₂O₅ vs P, and K₂O vs K

One quiet source of error: labels in the US report phosphorus as P₂O₅ (phosphate) and potassium as K₂O (potash), not as the elements. Plants take up elemental P and K, and many soil tests and research papers report those, which are smaller numbers. To convert, multiply P₂O₅ by 0.4364 to get elemental P, and K₂O by 0.8301 to get elemental K (divide to go back). So an 11-52-0 phosphate fertilizer is about 11-23-0 in elemental terms. The elemental toggle in the ratio and ppm modes does this for you.

Common fertilizer grades and formulas

These are typical commodity grades with their chemical formulas — useful for the “chemical formula for fertilizer” and “formula of DAP / MAP / MOP / SOP” questions — but the actual percentages are always whatever your bag’s label states, so read that. DAP, MAP, MOP, SOP, SSP, TSP, urea, AN and AMS below are generic fertilizer types, not products.

Typical grades for reference; the bag label is the legal statement of analysis — use those numbers in the calculator.

Mixing two products: the resulting grade

Want a rough blend? In the grade-and-ratio mode you can read two grades side by side; if you physically mix equal weights of two products, the blended grade is just the weight-average of each number. This is a quick check, not a full inverse blend solver — for a precise prescription blend, a supplier or agronomist runs the formulation.

Lime and soil amendment rates

The lime mode turns a rate you already have into a total and a bag count. The rate has to come from a soil test, a buffer-pH reading or your local extension — the amount of lime to move soil pH depends on the soil, and no calculator can guess it from nothing. You can enter your own rate in lb per 1,000 sq ft, tons per acre or kg per hectare, or start from a clearly labelled, fully editable soil-texture guide and adjust it. The tool then multiplies rate by area; it never reproduces a copyrighted soil-test or buffer-pH rate table.

Fertilizer to ppm for liquid and hydroponics

For liquid feeding and hydroponics the question flips to concentration: how many ppm of N, P and K does a given mass of fertilizer in a given volume of water give — or, working backwards, how many grams hit a target ppm. Because 1 ppm = 1 mg/L:

ppm (mg/L) = (mass in g × 1000 × nutrient%/100) ÷ volume in L
mass in g = (target ppm × volume in L) ÷ (10 × nutrient%)

So 10 g of a 20-20-20 in 5 L gives (10 × 1000 × 0.20) ÷ 5 = 400 ppm N. Flip the elemental toggle if your recipe is written in elemental P and K rather than P₂O₅ and K₂O.