Fertilizer NPK Blend Calculator
When no single fertilizer matches your soil test recommendation, blending two products lets you hit a custom N-P-K ratio. This calculator takes the analysis (N%, P2O5%, K2O%) of two fertilizers and the proportion of each in the blend (by weight, must sum to 100%) and returns the blended NPK percentages. Adjust the proportion slider until the blended analysis matches your target. Proportion is entered as the percentage of the total blend weight contributed by fertilizer A; fertilizer B makes up the rest.
NPK blend formula
Blended N% = (propA / 100) x NA + (1 - propA / 100) x NB
Blended P% = (propA / 100) x PA + (1 - propA / 100) x PB
Blended K% = (propA / 100) x KA + (1 - propA / 100) x KB
Where propA is the percentage of Fertilizer A in the blend by weight, and NA, PA, KA are the N, P, K percentages of Fertilizer A (and similarly B). The proportion of B = 100 - propA.
Common fertilizer analyses for blending
- Urea: 46-0-0. High N, economical nitrogen source.
- Triple superphosphate (TSP): 0-46-0. Pure phosphate source.
- Muriate of potash (MOP): 0-0-60. Pure potassium source.
- Diammonium phosphate (DAP): 18-46-0. N and P source.
- Ammonium sulfate: 21-0-0. N source with sulfur.
Fertilizer NPK blend calculator: frequently asked questions
What does NPK stand for?
NPK stands for nitrogen (N), phosphorus (P, reported as P2O5), and potassium (K, reported as K2O). The three numbers on a fertilizer bag show the percentage by weight of each nutrient in that order. A 10-10-10 fertilizer contains 10% of each.
Why would I blend two fertilizers?
Blending allows you to match a specific soil test recommendation when no single off-the-shelf product has the right ratio. For example, mixing a high-nitrogen fertilizer with a high-potassium fertilizer can achieve a custom ratio appropriate for tomatoes or turf.
How is the blended NPK calculated?
Multiply the proportion of each fertilizer (as a decimal) by its N, P, and K percentages, then sum the results. If you mix 60% of fertilizer A (28-0-4) and 40% of fertilizer B (0-20-20), the blended N = 0.6 x 28 + 0.4 x 0 = 16.8%. Adjust proportions until the blend matches your target.
Should I base my fertilizer on a soil test?
Yes. USDA Cooperative Extension strongly recommends a certified soil test every 2 to 3 years before applying any fertilizer. Excess phosphorus, in particular, does not help plants and can run off into waterways. Apply only what is needed based on test results.
What is the difference between N as urea vs. ammonium nitrate?
Both supply nitrogen but in different chemical forms. Urea (46-0-0) converts to ammonium in soil and is the most concentrated dry nitrogen source. Ammonium nitrate (34-0-0) provides both ammonium and nitrate forms and is faster-acting. Your state may restrict ammonium nitrate sales; check local regulations.
Official sources
- USDA NRCS, nutrient management: nrcs.usda.gov.
- USDA Economic Research Service, fertilizer: ers.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.