The only formula you need
lbs of product = (target lb N ÷ (bag N% ÷ 100)) × (lawn sq ft ÷ 1,000)
Example: 0.75 lb N target with a 30-0-4 bag on 5,000 sq ft → 0.75 ÷ 0.30 = 2.5 lbs of product per 1,000 sq ft → 12.5 lbs total. A 40 lb bag of 30-0-4 carries 12 lbs of actual N — enough for three-plus standard feedings of that lawn, which is roughly a cool-season lawn’s whole year.
When to feed matters more than how much
Cool-season lawns take most of their nitrogen in fall (main feeding 4–8 weeks before first frost, winterizer 2–4 weeks before), with only a light spring application. Warm-season lawns feed monthly-ish from full green-up to 6–8 weeks before first frost, then stop hard. Your local dates are in the lawn calendar lookup and on your city’s calendar page. And water it in: ¼″ of irrigation after spreading moves granules off the blades and into the soil — the watering calculator knows how long that takes.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
N-P-K: the percentage by weight of nitrogen, phosphate, and potash. A 30-0-4 bag is 30% nitrogen — so a 40 lb bag contains 12 lbs of actual nitrogen. Lawn feeding is planned in pounds of actual nitrogen per 1,000 sq ft, which is exactly the conversion this calculator does.
How much nitrogen per application?
The standard rate is 0.75–1.0 lb of actual N per 1,000 sq ft per application, and never more than 1 lb of fast-release N at once — more than that risks burn and surge growth. Low-maintenance grasses like centipede want half that, which is why the target is adjustable.
Why do you recommend two half-rate passes?
Spreader striping. Applying half the product in one direction and half perpendicular to it evens out coverage errors, which is where most burnt stripes and pale streaks come from. Set the spreader lower than the bag says and make two passes.
Do I need phosphorus and potassium?
Only a soil test can say — established lawns very often need no phosphorus at all, and several states restrict phosphorus lawn fertilizer for water-quality reasons. Test every few years (your county extension office does it cheaply) and buy the bag that matches the result.