🧪 Lawn Fertilizer Amount Calculator

Enter your lawn area, target nitrogen application rate (lbs N per 1,000 sq ft), and your fertilizer bag's nitrogen percentage and weight to see how many bags you need.

Lawn area (sq ft)

Target N rate (lbs per 1,000 sq ft)

Bag nitrogen % (first number, e.g. 20 for 20-0-0)

Bag weight (lbs)

Pounds of nitrogen, then pounds of product

Fertiliser rates are set in pounds of actual nitrogen per 1,000 sq ft, so the bag analysis is what turns that into a weight you can spread:

lbs N needed = (area / 1000) × target N rate
lbs product  = lbs N needed / (bag N% / 100)
bags to buy  = round up(lbs product / bag weight)

The defaults work through as 5,000 / 1,000 = 5, times a 1 lb rate, so 5 lb of nitrogen. A 20-0-0 product is one fifth nitrogen, so 5 / 0.20 = 25 lb of product - 0.63 of a 40 lb bag, rounded up to one bag to buy.

The bag analysis changes everything

Same lawn, same 1 lb rate, different products:

Bag	Product for 5,000 sq ft
46-0-0 urea	10.9 lb
32-0-0	15.6 lb
24-0-8	20.8 lb
10-10-10	50.0 lb

Spread half the total in one direction and half at right angles to it. A spreader run in one pass leaves overlap stripes, and the dark green bands are the parts that got double.

More is not better. Quick release nitrogen above 1 lb per 1,000 sq ft in a single application scorches turf - the salts pull water out of the roots - and what the grass cannot take up runs off into storm drains and streams. Phosphorus, the middle number, is restricted or banned in many places except when establishing new turf or where a soil test shows a deficiency, and application blackout dates apply in others. Your local rules govern, not this calculator.

Frequently asked questions

How much 10-10-10 do I need for 5,000 sq ft?

50 lb, to deliver 1 lb of nitrogen per 1,000 sq ft. That is more than one 40 lb bag, so the tool tells you to buy two.

What do the three numbers on the bag mean?

Percentages by weight of nitrogen, phosphate and potash. A 40 lb bag of 20-0-0 holds 8 lb of nitrogen. Only the first number is used here.

How much nitrogen does a lawn need in a year?

Commonly 2 to 4 lb per 1,000 sq ft across the season, split into several feeds rather than given at once, with the higher end for warm season grasses. Follow the extension guidance for your region.