Fertilizer is sold by the weight of the bag, but lawns are fed by the weight of the nitrogen inside it. Those are different numbers, and the gap between them is where most over- and under-feeding comes from.
Reading the three numbers
The three numbers on every bag are the percentages of nitrogen, phosphorus and potassium by weight. A 10-10-10 bag is 10 percent nitrogen — fifty pounds of bag holds five pounds of nitrogen.
A 24-0-6 bag is 24 percent nitrogen, so the same fifty pounds holds twelve. To put down the same amount of nitrogen you need less than half as much product.
One pound per thousand is the standard
One pound of actual nitrogen per 1,000 square feet per application is the normal rate for most lawns, applied two to four times a year depending on grass type and climate.
To turn that into product: divide the nitrogen you want by the percentage on the bag. One pound of N from a 10 percent bag takes ten pounds of product. From a 24 percent bag, just over four.
Quick-release and slow-release behave differently
Quick-release nitrogen greens up in days and is gone in weeks. It is also what burns lawns when applied heavy or on a hot day.
Slow-release — look for sulfur-coated or polymer-coated urea on the label — feeds over six to eight weeks and is far more forgiving. Most quality lawn products are a blend, and the bag will say what fraction is slow-release.
Water it in, and sweep the hard surfaces
Granular fertilizer needs about a quarter inch of water to move it off the blades and into the soil. Left sitting on wet grass it can scorch.
Sweep anything that lands on the driveway or sidewalk back onto the lawn. It is not just tidiness — that is the fraction that washes straight into the storm drain, and several states regulate it.
Soil test before the second application
The nitrogen figure is a general guide. Phosphorus and potassium requirements vary enormously with the soil and many established lawns need none at all.
A soil test through the local extension service is inexpensive and tells you which of the three numbers you actually need, plus the pH — which governs whether the lawn can take up what you apply at all.
Applying phosphorus that the soil does not need is also restricted or banned in a number of jurisdictions, because what the grass does not use runs off into waterways.
Spreader calibration
The setting printed on a bag is a starting point for one particular spreader at one particular walking speed. Two people with the same spreader will apply different amounts.
The way to check is to weigh out a known quantity, cover a measured area at your normal pace, and weigh what is left. Adjust and repeat.
Half-rate in two perpendicular passes is more forgiving than full rate in one. It evens out the overlap and it is what stops a lawn ending up striped.