How much fertilizer do I need?
The bag sells you product. The lawn eats nitrogen. This converts one into the other, which is the step most fertilizer advice skips.
The formula
lb of product per 1,000 sq ft = lb of nitrogen wanted ÷ (bag % N ÷ 100)
total = that × (area ÷ 1,000)
Example. Penn State's own: 1.0 lb of nitrogen ÷ 0.26 for a 26-5-10 bag is 3.8 lb of product per 1,000 sq ft. Over a 2,000 sq ft lawn that is 7.7 lb of the bag — not 2 lb, which is what you get if you read the target as if it were the product.
How much across a year
Per application is not per season. Penn State puts Kentucky bluegrass and perennial ryegrass at 2–4 lb of nitrogen per 1,000 sq ft per year and fine fescues at 1–2 lb, spread over several feedings. High-maintenance lawns take 1.0 lb of nitrogen per application, low-maintenance ones half that.
Timing is its own question. Late summer is the feeding that matters most; midsummer heat is skipped. The fertilizing guide has the windows, the annual ranges by species, and why a single April dump is the program that fails.
Your spreader setting is not this number
The number above is how much product to put down. What dial gets you there depends on the spreader, your walking speed and the granule, which is why extension services publish a calibration procedure: weigh what actually comes out over a measured strip and adjust. Bag settings are a starting guess.
Questions people actually ask
How much fertilizer do I need per 1,000 square feet?
Divide the pounds of nitrogen you want by the nitrogen percentage on the bag, in decimal form. Penn State Extension's example: 1.0 lb of nitrogen ÷ 0.26 for a 26-5-10 product = 3.8 lb of that product per 1,000 sq ft.
What do the three numbers on the bag mean?
Percent nitrogen, phosphate and potash by weight. A 26-5-10 bag is 26% nitrogen, so about a quarter of what you carry home is the nutrient the lawn is actually short of.
How much nitrogen does my lawn need in a year?
Penn State puts Kentucky bluegrass and perennial ryegrass at 2–4 lb of nitrogen per 1,000 sq ft per year and fine fescues at 1–2 lb, split across several applications rather than dumped at once.
Can I just apply it all in one go?
No. Rates are per application — typically 1.0 lb of nitrogen per 1,000 sq ft for a high-maintenance lawn, or half that for low maintenance. Applying a season's worth at once burns turf and sends the excess into runoff.
Sources
- Penn State Extension — Calculations Used to Determine the Amount of Fertilizer Needed to Treat Turf: the division by percent nitrogen, and the 26-5-10 worked example.
- Penn State Extension — Turfgrass Fertilization: annual nitrogen ranges by species and per-application rates.
- Penn State Extension — Calibrating Your Fertilizer Spreader: why the bag setting is only a starting point.