Enter your lawn size, your fertilizer bag's NPK numbers and a target nitrogen rate to get the exact pounds of product per 1,000 sq ft, total pounds and bags to buy.
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Enter your values and press Calculate.
This calculator turns a target nitrogen rate into the pounds of fertilizer product to spread. Enter your lawn area in square feet, the three NPK numbers from your fertilizer bag, and your target nitrogen rate in pounds per 1,000 square feet. It divides the target rate by the nitrogen fraction on the bag to get the product weight per 1,000 square feet, scales that to your whole area, and tells you how many bags to buy. Safety note: never apply more than about 1 lb of nitrogen per 1,000 square feet in a single application, because excess nitrogen burns grass, weakens roots and pollutes waterways. Slow release vs quick release fertilizer matters here too: slow release feeds over weeks with a lower burn risk, while quick release greens up fast but punishes over-application. These results are estimates for planning, not professional agronomic advice.
Turf and garden recommendations are written in pounds of actual nitrogen per 1,000 square feet, so the calculator starts from your target nitrogen rate. Your fertilizer bag's first NPK number is the nitrogen percentage by weight, which tells the calculator what fraction of each bag is real nitrogen. The calculator divides your target nitrogen rate by the nitrogen fraction (N percent divided by 100). That gives pounds of fertilizer product per 1,000 square feet. It then multiplies by your area in thousands of square feet to get the total product weight, and divides the total by your bag weight to show how many bags to buy.
Type in your lawn or garden area in square feet. Copy the three NPK numbers from your fertilizer bag (the nitrogen, phosphorus and potassium percentages). Enter your target nitrogen rate in pounds per 1,000 square feet; 1.0 is the common maintenance target for most lawns, with 0.5 a light feed and 1.5 the safe maximum per application. Enter your bag weight so the calculator can show how many bags to buy. Press calculate and you get the pounds of fertilizer product per 1,000 square feet, the total pounds for the whole area, the actual nitrogen you are delivering, and the number of bags to buy. For a fertilizer rate calculator result you can trust, the only field that needs care is the nitrogen percentage: double check it is the first of the three bag numbers.
Every fertilizer bag in the US prints a guaranteed analysis of three numbers, for example 25-5-10. This NPK ratio is a percentage by weight: nitrogen (N), phosphate (P2O5) and potash (K2O). Example: on a 5000 sq ft lawn at 1 lb of nitrogen per 1,000 sq ft with a 25-5-10 bag, that is 4 lb per 1,000 sq ft times 5, or 20 lb of product. A 50 lb bag of 25-5-10 holds 12.5 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash; the remaining 30 lb is filler and carrier material such as limestone or sand, which dilutes the nutrients so the product spreads evenly. The first number, nitrogen, is the one that drives the fertilizer rate calculator, because lawn recommendations are written in nitrogen. Higher numbers do not mean stronger or better; they mean more concentrated, so you spread less product to get the same nitrogen rate.
Different fertilizer types deliver very different amounts of product for the same nitrogen target. Urea, labeled 46-0-0, is the most concentrated: to deliver 1 lb of nitrogen per 1,000 square feet you spread only about 2.2 lb of product. A 32-0-0 maintenance blend needs about 3.1 lb of product for the same 1 lb of nitrogen. A 24-0-8 or 28-0-8 blend needs roughly 4.2 and 3.6 lb respectively. Starter fertilizer such as 18-24-12 is used at about 5.6 lb per 1,000 square feet to hit 1 lb of nitrogen, while a balanced 10-10-10 needs a full 10 lb of product to deliver that same 1 lb of nitrogen. Organic options such as feather meal (12-0-0) or compost (1-1-1) are far less concentrated, so the fertilizer rate calculator may show 8 lb or even 100 lb of product per 1,000 square feet for the same nitrogen target. Concentrated products save hauling and money, but they punish over-application, because a small measuring error puts down a lot of nitrogen.
Measure your area first; most people overestimate their lawn size and buy too much. For a rectangular lawn, multiply length by width in feet. Walk at a steady pace with your spreader and use the setting the bag recommends for your product. Spread half the calculated amount walking north to south and the other half walking east to west; a single pass in one direction leaves overlap stripes that show up as dark green bands. After spreading, water with about one quarter to one half inch so the granules move into the root zone, which also prevents leaf burn. Most lawns need 2 to 4 lbs of nitrogen per 1,000 sq ft per year, split into several lighter feeds rather than one heavy dose. If you switch products, run the fertilizer rate calculator again before you spread, because the new bag's nitrogen percentage changes the product weight completely.
Over fertilizing a lawn is more common than under fertilizing, and it causes real damage. Excess nitrogen forces fast, weak top growth that the roots cannot support, which leaves the grass vulnerable to disease and drought. The salts in the fertilizer pull water out of the roots and burn the leaf blades into yellow or brown patches. Nitrogen the grass cannot absorb runs off into storm drains and streams, feeding algal blooms. Never apply more than about 1 lb of nitrogen per 1,000 square feet in a single application, and split a season's total into several lighter feeds. A soil test from your local extension service tells you whether you even need phosphorus or potassium; many established lawns need nitrogen only. These results are estimates for planning purposes, not professional agronomic advice.
It depends on the nitrogen percentage on your bag. Use the calculator: for 1 lb of nitrogen per 1,000 sq ft, a 10-10-10 bag needs 10 lb of product per 1,000 sq ft, a 24-0-8 bag needs about 4.2 lb, and urea (46-0-0) needs only about 2.2 lb. Multiply by your area in thousands of square feet for the total.
The three numbers are the percentage by weight of nitrogen, phosphate and potash (N-P-K). Only the first number is used in the nitrogen rate math. A 40 lb bag of 20-0-0 holds 8 lb of actual nitrogen, because 20 percent of 40 is 8.
It is a guaranteed analysis of 25 percent nitrogen, 5 percent phosphate and 10 percent potash by weight. The remaining 60 percent is filler and carrier. Because it is 25 percent nitrogen, 1 lb of nitrogen per 1,000 sq ft needs 1 / 0.25 = 4 lb of product per 1,000 sq ft, so a 5,000 sq ft lawn needs 20 lb total.
Most cool-season grasses, such as fescue and Kentucky bluegrass, perform well at 0.5 to 1.0 lb of nitrogen per 1,000 sq ft per application. Warm-season grasses such as bermuda and zoysia can handle up to 1.5 lb. Never exceed 1.5 lb of nitrogen per 1,000 sq ft in one application, to avoid fertilizer burn.
Most lawns benefit from 2 to 4 applications per year. Cool-season lawns are usually fed in early spring and again in fall, with fall feeding the most valuable. Warm-season lawns are fed during the growing season from late spring through summer. The best time to fertilize a lawn is fall for cool-season grasses and late spring through summer for warm-season grasses. A soil test and your grass type set the exact schedule.
Yes. Too much nitrogen burns leaf blades into yellow or brown patches, forces weak rapid growth that invites disease, and washes into waterways. Quick release nitrogen above 1 lb per 1,000 sq ft in a single application is the most common cause. Stick to the bag label, split feeds across the season, and always water after applying.
A soil test is strongly recommended before you fertilize. It reveals your soil pH and existing nutrient levels, so you choose the right formula and skip nutrients you already have enough of. Many established lawns need nitrogen only, which saves money and keeps excess phosphorus out of waterways. Tests are available through local extension services and garden centers at a modest cost.
Yes. About one quarter to one half inch of water moves the granules off the blades and into the root zone, which prevents leaf burn and reduces runoff. Watering is especially important with quick release products. If heavy rain is forecast within a day of your planned application, wait until it passes.