Garden / Soil
Fertilizer Calculator: How Much Product for Your N-P-K Target
Work out how much fertilizer to apply from the N-P-K analysis on the bag: choose the nutrient you are aiming for and the rate per hectare or per 1,000 ft² of lawn, and get the product per area, the amount and bags for your field or lawn, and the other nutrients it supplies.
Fertilizer Calculator: How Much Product for Your N-P-K Target: Product = nutrient rate ÷ the nutrient's percentage. 50 kg of nitrogen per hectare from urea, which is 46% N, needs 108.7 kg of urea per hectare. On a lawn, 1 lb of nitrogen per 1,000 ft² from a 20-5-10 product needs 5 lb of product, which also supplies 0.25 lb of P₂O₅ and 0.5 lb of K₂O. Runs 100% locally in your browser with zero server file uploads.
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Product needed = nutrient rate ÷ the nutrient's share of the product. Labels give nitrogen as N but phosphorus and potassium as the oxides P₂O₅ and K₂O, the way recommendations usually do; the tool also shows the elements, since some countries' advice uses them. Base the target nitrogen (N) on a soil test and local advice, and split large nitrogen doses.
Soil test first
Phosphate and potash needs depend on what is already in the soil; a soil test avoids spending money on nutrients the soil does not need.
Mulch
To work out how much mulch or compost covers a bed, use the mulch calculator.
How to use it
- Choose the target nutrient and units.
- Enter the rate and the N-P-K numbers from the bag.
- Read the product to spread and the number of bags.
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Related tools
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
The percentages by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O): a 20-5-10 bag of 50 lb has 10 lb N, 2.5 lb P₂O₅, and 5 lb K₂O.
Why are phosphorus and potassium given as oxides?
It is a long-standing labelling convention; to get the elements, multiply P₂O₅ by 0.4364 and K₂O by 0.8301.
How much nitrogen does a lawn need?
Often about 1 lb per 1,000 ft² per feeding, a few times a year, depending on the grass and climate; follow local advice and do not feed dormant grass.
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