How to Calculate Fertiliser per Acre
Work out fertiliser rates per acre or hectare for urea 46-0-0, DAP 18-46-0, MOP 0-0-60 and compound NPK blends, with worked examples and unit conversions.
Published by Calculator.uk.com · Updated 21 October 2026 · Our methodology
Quick answer
Fertiliser per acre = nutrient needed per acre ÷ (nutrient % on the bag ÷ 100). To supply 120 lb of nitrogen with urea (46-0-0) you need 120 ÷ 0.46 = 261 lb of urea per acre. For phosphate from DAP (18-46-0), divide the P2O5 target by 0.46 and subtract the 18% nitrogen it supplies from your urea. Multiply by your acreage for the total.
Fertiliser per acre calculator
Enter your field size, your nutrient targets and the three numbers on the fertiliser bag. It works in acres, hectares or square metres, with rates in lb/acre, kg/ha or g/m². The result shows the product rate, the total amount, and which nutrient sets the rate.
Enter values to see results
Step 1: Read the three numbers on the bag
Every fertiliser shows its grade as three numbers, N-P-K: the percentage by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A 50 kg bag of 18-46-0 contains 9 kg of N and 23 kg of P2O5. The rest is the other parts of the compound and filler.
The second and third numbers are oxides, not pure phosphorus and potassium. Soil-test recommendations are almost always given as P2O5 and K2O too, so the units match. If a recommendation gives elemental P or K, convert it first: P2O5 = P × 2.291 and K2O = K × 1.205.
| Fertiliser | Grade (N-P₂O₅-K₂O) | Notes |
|---|---|---|
| Urea | 46-0-0 | Highest-N solid fertiliser. Volatilisation losses if left on the surface in warm, dry conditions. |
| Ammonium nitrate (AN) | 34.5-0-0 | Common UK straight nitrogen, with lower volatilisation losses than urea. |
| Diammonium phosphate (DAP) | 18-46-0 | Main phosphate source, also supplies some nitrogen. |
| Monoammonium phosphate (MAP) | 11-52-0 | More phosphate and less nitrogen than DAP. |
| Triple superphosphate (TSP) | 0-46-0 | Phosphate with no nitrogen. |
| Muriate of potash (MOP) | 0-0-60 | Potassium chloride, the cheapest potash source. |
| Sulphate of potash (SOP) | 0-0-50 | Chloride-free potash for sensitive crops. Also supplies sulphur. |
| Balanced compound | 20-10-10, 15-15-15 | Several nutrients in one pass, though rarely an exact match for a soil test. |
Grades vary slightly between manufacturers, so always use the numbers printed on your bag.
Step 2: Get your nutrient targets from a soil test
How much N, P2O5 and K2O a crop needs depends on the crop, the expected yield, the previous crop, soil type and existing soil nutrient levels. That means it should come from a soil analysis and a recognised recommendation system, not a generic figure. The examples below use a recommendation of 120 lb N, 50 lb P2O5 and 60 lb K2O per acre to show the method.
Step 3: Divide the target by the nutrient percentage
Example A: nitrogen from urea (46-0-0)
120 lb N ÷ 0.46 = 260.9 lb urea per acre. With ammonium nitrate (34.5-0-0) instead, you would need 120 ÷ 0.345 = 347.8 lb per acre.
Example B: building a blend from straights (DAP + urea + MOP)
Start with the nutrient that only one product supplies, then fill the gaps:
- Phosphate from DAP: 50 lb P2O5 ÷ 0.46 = 108.7 lb DAP per acre
- Nitrogen already supplied by DAP: 108.7 × 0.18 = 19.6 lb N
- Nitrogen still needed: 120 − 19.6 = 100.4 lb N, so 100.4 ÷ 0.46 = 218.3 lb urea per acre
- Potash from MOP: 60 lb K2O ÷ 0.60 = 100.0 lb MOP per acre
| Product | Per acre | 40-acre field | 50 kg bags |
|---|---|---|---|
| DAP 18-46-0 | 108.7 lb | 4,348 lb | 40 |
| Urea 46-0-0 | 218.3 lb | 8,734 lb | 80 |
| MOP 0-0-60 | 100.0 lb | 4,000 lb | 37 |
| Total | 427.0 lb | 17,081 lb | 157 |
A 50 kg bag weighs 110.2 lb. Bag counts are rounded up.
Example C: one compound fertiliser (20-10-10)
Work out the rate for each nutrient separately: N needs 120 ÷ 0.20 = 600 lb, P2O5 needs 50 ÷ 0.10 = 500 lb and K2O needs 60 ÷ 0.10 = 600 lb. The highest figure, 600 lb per acre, meets every target. At that rate you apply 120 lb N, 60 lb P2O5 and 60 lb K2O, which is 10 lb more phosphate than needed. This is the trade-off with compounds: they are convenient but rarely exact. The calculator above is pre-filled with this example for a 40-acre field.
Unit conversions
| From | To | Multiply by |
|---|---|---|
| kg/ha | lb/acre | 0.892 |
| lb/acre | kg/ha | 1.121 |
| UK units/acre | kg/ha | 1.255 |
| acres | hectares | 0.4047 |
| hectares | acres | 2.471 |
| P2O5 | P (elemental) | 0.436 |
| K2O | K (elemental) | 0.830 |
The method is the same in metric. 150 kg N/ha from urea is 150 ÷ 0.46 = 326 kg urea per hectare, which is about 291 lb per acre.
Before you spread
- Subtract other nutrient sources. Manure, slurry, digestate and crop residues supply nutrients too, so reduce the bagged fertiliser to match.
- Split nitrogen. Large nitrogen doses are usually split across the season to cut leaching and improve uptake.
- Calibrate the spreader for each product. Urea, DAP and MOP have different densities and spread widths.
- Check local rules. Nitrate Vulnerable Zone limits, closed periods and buffer strips may cap what you can apply.
Frequently asked questions
How do I calculate how much fertiliser I need per acre for my crops?
Divide the amount of nutrient your crop needs per acre by the percentage of that nutrient in the fertiliser (as a decimal). For example, if your soil test recommends 120 lb of nitrogen per acre and you use urea (46-0-0), you need 120 ÷ 0.46 = 261 lb of urea per acre. Multiply by the number of acres for the total. Do this for each nutrient (N, P2O5, K2O). The free fertiliser calculator on this page does the sums in acres, hectares or square metres.
How much urea do I need per acre?
Pounds of urea per acre = pounds of nitrogen needed ÷ 0.46, because urea is 46% nitrogen. So 50 lb N needs 109 lb urea, 100 lb N needs 217 lb urea and 150 lb N needs 326 lb urea per acre. In metric, kg of urea per hectare = kg N per hectare ÷ 0.46, so 150 kg N/ha needs 326 kg urea/ha.
How do I combine DAP and urea to meet nitrogen and phosphate targets?
Start with the phosphate, because DAP is your only phosphate source. Divide the P2O5 target by 0.46 to get the DAP rate. Then work out the nitrogen that DAP supplies (DAP rate × 0.18), subtract it from your nitrogen target, and cover the rest with urea (remaining N ÷ 0.46). For 120 lb N and 50 lb P2O5 per acre that is 109 lb DAP plus 218 lb urea.
How do I convert kg per hectare to lb per acre?
Multiply kg/ha by 0.892 to get lb/acre, and multiply lb/acre by 1.121 to get kg/ha. For example, 150 kg N/ha is 134 lb N/acre. In the UK, older recommendations use "units" per acre. One unit is 1% of a hundredweight (1.12 lb), so units per acre × 1.255 gives kg/ha.
What is the scientific source for these fertiliser calculations?
The calculations use the guaranteed-analysis convention used on fertiliser labels worldwide, in which the three numbers give the percentage by weight of total nitrogen (N), phosphate (P2O5) and potash (K2O). Nutrient targets should come from a soil test and a recognised recommendation system, such as the AHDB Nutrient Management Guide (RB209) in the UK or land-grant university extension recommendations in the US, applied in line with the 4R nutrient stewardship framework (right source, rate, time and place). The P2O5-to-P (0.436) and K2O-to-K (0.830) factors come from the molecular weights of the oxides.