Fertilizer NPK Calculator: How Much Fertilizer to Apply
Enter the three numbers from the fertilizer bag (such as 10-10-10), pick the nutrient you want to apply and the rate, and see how much product to spread and what N, P2O5 and K2O it delivers. Typical rates are labelled as typical; always follow your soil test and the label.
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Formula
- Product to apply = target nutrient rate / (analysis percentage / 100)
- Nutrient delivered = product x (analysis percentage / 100), for each of N, P2O5 and K2O
- Total product = product per 1,000 sq ft x area / 1,000 (or product per acre or hectare x area)
- Elemental conversion:
P2O5 = P x 2.29andK2O = K x 1.2, soP = P2O5 / 2.29andK = K2O / 1.2
Worked example
A 10-10-10 fertilizer, with a goal of 1 lb of nitrogen per 1,000 sq ft (a typical figure for one lawn application):
- Product = 1 / 0.10 = 10 lb per 1,000 sq ft
- Delivered: 1 lb N, 1 lb P2O5 (0.44 lb P) and 1 lb K2O (0.83 lb K)
- On 2,500 sq ft: 10 x 2,500 / 1,000 = 25 lb of product
Other checks: urea (46-0-0) at 100 lb N per acre needs 100 / 0.46 = 217.39 lb per acre. A 12-12-12 at 60 kg K2O per hectare needs 60 / 0.12 = 500 kg per hectare.
Assumptions
- The analysis percentages are the guaranteed values on the bag; the three together cannot exceed 100.
- Rates are for the nutrient you choose. A balanced fertilizer applied for one nutrient delivers the others in proportion, which may be more or less than the plants need.
- Typical rates vary by crop, soil and region. A soil test is the best guide to what to apply.
Frequently asked questions
How do I work out how much fertilizer to apply from the N-P-K numbers?
Divide the nutrient rate you want by the percentage on the bag, as a decimal. For 1 lb of nitrogen per 1,000 sq ft with a 10-10-10 fertilizer: 1 / 0.10 = 10 lb of product per 1,000 sq ft. The same 10 lb also delivers 1 lb of P2O5 and 1 lb of K2O.
What do the three numbers on a fertilizer bag mean?
They are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O), always in that order. A 50 lb bag of 10-10-10 holds 5 lb of N, 5 lb of P2O5 and 5 lb of K2O, with the rest being carrier and filler.
How do I convert P2O5 to phosphorus and K2O to potassium?
Labels state phosphorus as P2O5 and potassium as K2O. To get the elements, divide P2O5 by 2.29 to get P, and divide K2O by 1.2 to get K. So 1 lb of P2O5 is 1 / 2.29 = 0.44 lb of P, and 1 lb of K2O is 1 / 1.2 = 0.83 lb of K. To go the other way, multiply P by 2.29 and K by 1.2.
How do I convert pounds per 1,000 sq ft to kilograms per hectare?
1 lb per 1,000 sq ft is about 48.8 kg per hectare (1 lb is 0.45359237 kg and 1,000 sq ft is 92.903 m2). So 1 lb of N per 1,000 sq ft equals about 49 kg N per hectare, and 100 lb per acre is about 112 kg per hectare. Use the metric option to work in kg per hectare directly.
How much urea do I need for 5,000 square feet?
Urea is 46-0-0, so 1 lb of nitrogen per 1,000 sq ft needs 1 / 0.46 = 2.17 lb of product per 1,000 sq ft. On 5,000 sq ft that is about 10.87 lb of urea in total.
How much 16-4-8 fertilizer gives 1.5 lb of nitrogen per 1,000 sq ft?
Divide the target by the nitrogen share: 1.5 / 0.16 = 9.38 lb of product per 1,000 sq ft. On 3,000 sq ft that is 28.13 lb, which also delivers about 0.38 lb of P2O5 and 0.75 lb of K2O per 1,000 sq ft.
How much 34-0-0 per acre gives 150 lb of nitrogen?
Divide 150 by 0.34 to get 441.18 lb of product per acre. On 10 acres you would spread about 4,412 lb in total. A soil test and local advice decide what rate is right for your crop.