How it works
Fertilizer is labeled by its analysis: the percent nitrogen (N), phosphate (P₂O₅), and potash (K₂O), in that order. A 46-0-0 bag is 46% nitrogen. To deliver a given number of pounds of a nutrient, divide by the product's fraction of that nutrient.
Some products carry more than one nutrient. Diammonium phosphate (DAP, 18-46-0) is the phosphate source here and also supplies 18% nitrogen. This tool figures the phosphate product first, credits its nitrogen, and only then works out how much nitrogen source you still need. The blend analysis shows the N-P-K of the combined product, which is the number a bulk blender would put on the ticket.
Worked example
Rates of 100 lb N, 40 lb P₂O₅, and 60 lb K₂O per acre on 10 acres, using DAP, urea, and muriate of potash:
- Phosphate product = 40 lb P₂O₅ ÷ 0.46 = 87 lb/ac (also supplies 15.7 lb N)
- Nitrogen product = (100 − 15.7) lb N ÷ 0.46 = 183 lb/ac
- Potash product = 60 lb K₂O ÷ 0.6 = 100 lb/ac
- Total = 87 + 183 + 100 = 370 lb/ac; 10 ac = 3,703 lb
| Input | Value |
|---|---|
| Nitrogen wanted (N) | 100 lb/ac |
| Phosphate wanted (P₂O₅) | 40 lb/ac |
| Potash wanted (K₂O) | 60 lb/ac |
| Area to fertilize | 10 ac |
| Phosphate source | Diammonium phosphate, DAP (18-46-0) |
| Nitrogen source | Urea (46-0-0) |
| Potash source | Muriate of potash, potassium chloride (0-0-60) |
| Other phosphate product: % N | 11 % |
| Other phosphate product: % P₂O₅ | 52 % |
| Other nitrogen product: % N | 34 % |
| Other potash product: % K₂O | 50 % |
| Result | Value |
|---|---|
| Phosphate product per acre | 87 lb/ac |
| Nitrogen product per acre | 183 lb/ac |
| Potash product per acre | 100 lb/ac |
| Total product per acre | 370 lb/ac |
| Total product for the whole area | 3,703 lb |
| Blend analysis: % N | 27 % |
| Blend analysis: % P₂O₅ | 10.8 % |
| Blend analysis: % K₂O | 16.2 % |
| Phosphorus (P) supplied per acre | 17.5 lb/ac |
| Potassium (K) supplied per acre | 49.8 lb/ac |
Assumptions and limits
- Analyses are the guaranteed analyses printed on the bag or in extension tables. The actual product can differ; use the tag on your load.
- Nutrients are expressed as N, P₂O₅, and K₂O, the way soil tests and labels state them. The elemental P and K figures are given for reference.
- Only one product supplies each nutrient. Products with several nutrients other than DAP's nitrogen are entered as "Other" with their analysis, and other nutrients they carry (such as sulfur) are not tracked.
- This calculator does not recommend rates and does not check local restrictions on nitrogen, phosphorus, or application timing.
Common questions
How much urea do I need for 50 pounds of nitrogen per acre?
Divide 50 by 0.46: about 109 pounds of urea per acre, which is the standard extension example.
How much DAP gives 80 pounds of P₂O₅?
Divide 80 by 0.46: about 174 pounds of DAP per acre, which also supplies about 31 pounds of nitrogen.
What do the three numbers on a fertilizer bag mean?
The percent nitrogen (N), phosphate (P₂O₅), and potash (K₂O) by weight. A 0-0-60 bag is 60% K₂O.
Why credit the nitrogen in DAP?
It is real nitrogen that will be in the soil. If you ignore it and add a full rate of urea, you over-apply nitrogen, which costs money and can leach.
Sources
- University extension fertilizer calculation guides: pounds of product = pounds of nutrient wanted ÷ fraction of nutrient in the product. Worked examples: 50 lb N ÷ 0.46 = 109 lb urea; 80 lb P₂O₅ ÷ 0.46 = 174 lb DAP.
- Guaranteed analyses of common materials (urea 46-0-0, ammonium sulfate 21-0-0-24S, diammonium phosphate 18-46-0, triple superphosphate 0-46-0, muriate of potash 0-0-60) from Virginia Cooperative Extension publication 424-035.
- P₂O₅ to P and K₂O to K factors from atomic masses.
- Read them: Virginia Cooperative Extension 424-035, Fertilizer types and calculating rates
Updated 2026-09-30