Fertilizer Calculator (NPK)

Pounds of fertilizer to apply for a target pound of nitrogen (or P or K) per 1,000 sq ft, from the N-P-K numbers.

Product per 1,000 sq ft = target lb of nutrient ÷ (nutrient % ÷ 100). With a 24-0-6 fertilizer and 1 lb N per 1,000 sq ft: 1 ÷ 0.24 = 4.17 lb per 1,000 sq ft, so a 5,000 sq ft lawn needs 20.8 lb.

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How to use it

Enter the area and the three N-P-K numbers from the bag.
Choose the target nutrient and rate (1 lb N per 1,000 sq ft is typical for lawns).
Read the pounds of product, bags and nutrients delivered.

What you enter

Area
N-P-K analysis
Target nutrient and rate
Bag weight

What you get

Pounds of product
Rate per 1,000 sq ft
Bags
N, P₂O₅ and K₂O delivered

Formula

Product (lb/1,000 ft²) = target nutrient (lb/1,000 ft²) ÷ (% nutrient / 100) | Total = rate × area / 1,000

Worked example

Input: 5,000 sq ft lawn, 24-0-6, 1 lb N per 1,000 sq ft

Result: 20.8 lb of product

Less than half a 50 lb bag; also delivers 1.25 lb of potash.

Questions

What do the three numbers on fertilizer mean?

The percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 10-10-10 contains 5 lb of each.

How much nitrogen does a lawn need?

Most cool-season lawns need 2–4 lb of nitrogen per 1,000 sq ft per year, applied in feeds of no more than 1 lb each, mostly in fall.

How much 10-10-10 should I use per 1,000 sq ft?

For 1 lb of nitrogen, 1 ÷ 0.10 = 10 lb of 10-10-10 per 1,000 sq ft.

Do I need phosphorus?

Only if a soil test shows low phosphorus. Many states restrict phosphorus on established lawns to protect waterways.

References

University of Minnesota Extension (2023) Fertilizing lawns.

More Home & Garden

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.