Fertilizer Calculator (NPK)

About Fertilizer Calculator (NPK)

The Fertilizer Calculator (NPK) converts a nutrient recommendation into the exact amount of bagged fertilizer to spread on a garden bed. Enter the bed area, the target nutrient rate, and the N-P-K numbers printed on the bag to calculate product weight in pounds, ounces, kilograms, and grams. The result also shows how much nitrogen, phosphate, and potash are applied at the same time.

NPK Fertilizer Formula

If a soil test recommends 1 lb of nitrogen per 1,000 sq ft and the bag is 10-10-10, nitrogen is 10% of the product by weight. The product needed is 1 lb ÷ 0.10 = 10 lb of fertilizer per 1,000 sq ft.

How to Use the Fertilizer Calculator

1. Enter the bed area: Use the actual planting area, not the whole property, unless the whole property will be fertilized.
2. Choose the target nutrient: Match the nutrient named in your recommendation: nitrogen, phosphate, or potash.
3. Enter the target rate: Use the rate format you have, such as lb per 1,000 sq ft, oz per 100 sq ft, g/m², or kg/hectare.
4. Enter the N-P-K label: Type the three percentages from the fertilizer bag, such as 10-10-10 or 4-6-3.
5. Review the co-applied nutrients: Check the N, P2O5, and K2O breakdown before applying fertilizer.

What the N-P-K Numbers Mean

Practical Notes for Garden Beds

Frequently Asked Questions

What does N-P-K mean on fertilizer?

N-P-K is the percentage by weight of nitrogen, available phosphate as P2O5, and soluble potash as K2O in the fertilizer product.

Which nutrient should I target?

Use the nutrient named in your soil test or crop recommendation. Nitrogen is commonly targeted for leafy growth, while phosphorus and potassium should usually be based on soil test need.

Why does fertilizer amount change with a low N-P-K number?

The lower the selected nutrient percentage, the more product is needed to supply the same amount of nutrient. A 5-5-5 fertilizer requires twice as much product as a 10-10-10 fertilizer for the same target nutrient.

Does P2O5 mean elemental phosphorus?

No. Fertilizer labels commonly report phosphorus as available phosphate, P2O5, and potassium as soluble potash, K2O. This calculator follows the label convention.

Should I apply fertilizer without a soil test?

A soil test is the best basis for fertilizer rates. This calculator turns a known recommendation into a product amount; it does not diagnose whether your soil needs the nutrient.