The short answer: aim for about one pound of actual nitrogen per 1,000 square feet per application, and to find how much product that is, divide 100 by the first number on the bag. A 16-4-8 fertilizer needs roughly 6.25 lb per 1,000 square feet. A 6-4-0 like Milorganite needs about 16.7 lb for the same nitrogen. That is the entire calculation, and it is the only one you need.
Everything else on this page is worked examples and a calendar. But that division is worth internalising, because it is also how you compare bags honestly: a cheap bag with a low analysis is not cheap once you account for how much more of it you have to spread, and a premium bag with a high analysis often costs less per pound of nitrogen delivered. Shelf price per bag tells you very little.
A caveat we would rather state than bury: these are general rates for cool-season lawns, and they are not a substitute for knowing your own soil. A soil test from your state extension service costs very little and tells you what your soil is actually short of, which no generic rate can. We do not test soil and we are not going to pretend our arithmetic replaces one — see how we pick.
Step one: measure the lawn
Pace out each area of grass — a pace is close enough to three feet — multiply length by width, and add the areas. Do this properly, because every rate on this page is per 1,000 square feet and an error here scales straight through. People consistently underestimate their lawns, which is also why so many mowers are bought a size too small; the same measurement is useful for choosing a mower.
Step two: do the division
Divide 100 by the first number on the bag. That gives you pounds of product per 1,000 square feet for a one-pound nitrogen application:
16-4-8(The Andersons PGF Complete): 100 ÷ 16 = 6.25 lb per 1,000 sq ft.
6-4-0(Milorganite): 100 ÷ 6 = 16.7 lb per 1,000 sq ft.
32-0-5: 100 ÷ 32 = 3.1 lb per 1,000 sq ft.
Then multiply by your lawn's thousands of square feet. A 5,000 sq ft lawn needs about 31 lb of a 16-4-8 product, or about 84 lb of Milorganite, for the same nitrogen. That comparison is why analysis matters as much as price.
Step three: sanity-check against the bag's own coverage
Every bag states a coverage figure. Compare it with your calculation; if they disagree wildly, the manufacturer is assuming a different nitrogen rate than one pound per thousand — which is common and usually fine. Follow the label. The arithmetic is for comparing products, not for overriding the instructions on the bag you bought.
Worked examples by lawn size
2,500 sq ft (small lawn). About 16 lb of a 16-4-8 per application, so one 5,000 sq ft bag covers two feeds. A 12.5 lb bag of Scotts Turf Builder is roughly one application.
5,000 sq ft (typical suburban back garden). About 31 lb of a 16-4-8 per application. A 32.3 lb bag of Scotts WinterGuard is about one full autumn application for this lawn, which is why that bag size exists.
10,000 sq ft (quarter acre). About 62 lb per application. At this size hopper capacity starts to matter and you want the EdgeGuard DLX rather than the Mini, or you will spend the application walking back to the bag.
21,780 sq ft (half acre).About 136 lb per application — three or four bags. This is where a 130 lb push hopper or a tow-behind spreader stops being a luxury.
The feeding calendar for a cool-season lawn
Early spring. A light feed as growth starts, or skip it and put a crabgrass pre-emergent down instead — but not if you intend to sow grass seed this spring, because a pre-emergent stops your fescue germinating too.
Late spring. A full nitrogen feed. This is the application that buys you visible green and a lot of mowing.
High summer. Go light, or use iron rather than nitrogen. Pushing growth into heat-stressed cool-season grass invites disease, and this is the season to raise your mower deck instead.
Early autumn. The most valuable application of the year. Cool-season grasses build roots and store carbohydrate now, so this feed does more for next summer's lawn than anything else you will do. If you only feed once, feed now.
Late autumn. A winterizer feed, and the one time a weed-and-feed genuinely makes sense — perennial broadleaf weeds are translocating to their roots, so the herbicide goes where it does most damage.
Three ways people waste fertilizer
Spreading it on compacted ground.Nutrients cannot reach roots that have no air. If the lawn is hard underfoot, aerate before you feed — see best lawn aerator.
Spreading it unevenly. Half the rate in one direction, half at right angles. Otherwise you get stripes you then live with all season.
Spreading it onto paving. Fertilizer on a driveway goes into the storm drain, which is the nutrient-pollution pathway the EPA describes. Sweep it back onto the lawn, and use a spreader with edge control.
