NPK fertilizer calculator
Enter the three numbers on the fertilizer bag, choose the nutrient you are dosing for and the rate you want, then the area. The calculator gives the amount of product per area and in total, the bags to buy and the other two nutrients you apply along with it.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the N, P₂O₅ and K₂O percentages from the label.
- Choose which nutrient sets the dose (usually nitrogen) and enter the rate you want per area.
- Enter the area and the bag size, then read how much product to spread and how many bags to buy.
How it works
product per area = nutrient rate ÷ (nutrient % ÷ 100)
Multiplying by the area gives the total. Each of the other nutrients is applied at product per area × its own %. Bags to buy is the total divided by the bag size, rounded up.
Example
A lawn of 5,000 ft², a 20-5-10 fertilizer and a target of 1 lb of nitrogen per 1,000 ft²:
- product: 1 ÷ 0.20 = 5 lb per 1,000 ft²;
- total: 5 × 5,000 ÷ 1,000 = 25 lb, one 50 lb bag;
- with it you also apply 5 × 0.05 = 0.25 lb of P₂O₅ and 5 × 0.10 = 0.5 lb of K₂O per 1,000 ft².
In metric: 5 g of N per m² from the same product is 25 g/m², or 2.5 kg for 100 m².
Tips and common mistakes
- Split large nitrogen doses into several smaller ones rather than spreading them all at once.
- Calibrate the spreader: the same setting puts down different amounts of different products.
- Measure the area you actually treat, without the house, patio and beds.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
They are percentages by weight: nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 20-5-10 holds 10 lb of N, 2.5 lb of P₂O₅ and 5 lb of K₂O. The rest is other material that carries the nutrients.
How do I get 1 lb of nitrogen per 1,000 ft²?
Divide 1 by the nitrogen share: 1 ÷ 0.20 = 5 lb of a 20% product per 1,000 ft², or 1 ÷ 0.46 = 2.2 lb of urea (46-0-0). The calculator does this for any rate and analysis.
Is P₂O₅ the same as phosphorus?
No. Labels give phosphate and potash as oxides. Elemental phosphorus is about 44% of P₂O₅ and elemental potassium about 83% of K₂O. Soil test reports may use either form, so check which one your recommendation uses.
What rate should I use?
The one from your soil test, the product label or your local extension service. The default of 1 lb N per 1,000 ft² (about 5 g/m²) is only an example.