Calculators · Lawn Fertilizer Calculator

Landscaping

Lawn Fertilizer Calculator

Enter your lawn area and the numbers on the bag to get the pounds of fertilizer for the nitrogen rate you want.

Area 1

Shape

Length

Width

Counts as

Nitrogen, first number (N %)

On a 20-5-10 bag this is 20.

Phosphate, second number (P %)

Optional. On a 20-5-10 bag this is 5.

Potash, third number (K %)

Optional. On a 20-5-10 bag this is 10.

Nitrogen rate guide

Fills the rate box from an extension service. Choose “Custom rate” to use your own.

Nitrogen rate (lb N per 1,000 sq ft)

Type the pounds of actual nitrogen you want per 1,000 sq ft.

Bag size (lb)

Optional. Gives the number of bags and what one bag covers.

How to calculate how much fertilizer your lawn needs

Lawn fertilizer is measured by the pounds of actual nitrogen it puts on each 1,000 square feet. The bag tells you what share of its weight is nitrogen, so you divide the nitrogen you want by that share to get pounds of product. Iowa State University Extension describes it this way, and NC State Extension gives the same shortcut, dividing 100 by the first number on the bag to get the pounds of product per pound of nitrogen.

Step	Formula
1. Lawn area	area (sq ft) = lawn areas − house, driveway and beds
2. Nitrogen on the bag	first number of the bag, as a decimal (20 → 0.20)
3. Pounds per 1,000 sq ft	lb of N wanted ÷ nitrogen decimal
4. Whole lawn	lb per 1,000 × area ÷ 1,000
5. Bags	pounds ÷ bag weight, rounded up

What the three numbers on the bag mean

NC State Extension says the three numbers are the percentages by weight of nitrogen (N), phosphorus (as P₂O₅) and potassium (as K₂O), always in that order. On a 20-5-10 bag, 20% of the weight is nitrogen, 5% is P₂O₅ and 10% is K₂O. The calculator takes the nitrogen number to size the application and shows how many pounds of the other two nutrients that application brings.

How much nitrogen per 1,000 square feet?

Extension services give ranges, and the right figure depends on the grass and the month. The calculator can fill the rate box from these guides. It uses the midpoint of the range, which is our own arithmetic, and you can type any other number. The figures were checked in October 2026.

Guide	lb of N per 1,000 sq ft	Source
Iowa lawns, depending on application time and lawn quality	0.5 to 1	Iowa State
Tall fescue, Kentucky bluegrass, ryegrass: September, October	0.75 to 1 each	University of Missouri, basic maintenance
Same grasses: November	1
Same grasses: March or April	0.5 to 1
Zoysiagrass, bermudagrass, buffalograss: May to August	0.5 to 0.75 each month	University of Missouri, basic maintenance

Example: a 20-5-10 bag on 10,000 square feet

This is the example Iowa State University Extension uses, a 20-5-10 fertilizer at 0.75 pounds of nitrogen per 1,000 square feet, now on a 100 ft by 100 ft lawn with a 50 pound bag.

Step	Operation	Result
Fertilizer per 1,000 sq ft	0.75 ÷ 0.20	3.75 lb
Whole lawn	10,000 ÷ 1,000 × 3.75	37.5 lb
Actual nitrogen	10 × 0.75	7.5 lb
Phosphate (P₂O₅)	37.5 × 5%	1.875 lb
Potash (K₂O)	37.5 × 10%	3.75 lb
Bags of 50 lb	37.5 ÷ 50 = 0.75, rounded up	1 bag
One bag covers	50 ÷ 3.75 × 1,000	13,333 sq ft

Pounds of fertilizer per 1,000 square feet

The pounds you spread depend on the nitrogen percentage. A weaker bag takes more product for the same nitrogen. The table is lb of nitrogen ÷ nitrogen decimal.

Nitrogen on the bag	0.5 lb N	0.75 lb N	1 lb N
10%	5	7.5	10
16%	3.13	4.69	6.25
20%	2.5	3.75	5
25%	2	3	4
30%	1.67	2.5	3.33
46%	1.09	1.63	2.17

How many square feet does a 50 pound bag cover?

A bag covers its pounds of nitrogen divided by the rate you want, times 1,000. The label of your product has the coverage the maker designed it for, and that figure is based on the maker’s own rate.

Nitrogen on a 50 lb bag	At 0.5 lb N	At 0.75 lb N	At 1 lb N
10%	10,000 sq ft	6,667 sq ft	5,000 sq ft
16%	16,000 sq ft	10,667 sq ft	8,000 sq ft
20%	20,000 sq ft	13,333 sq ft	10,000 sq ft
25%	25,000 sq ft	16,667 sq ft	12,500 sq ft
30%	30,000 sq ft	20,000 sq ft	15,000 sq ft

Phosphorus and local rules

University of Missouri Extension says established lawns do not require additional phosphorus, so it should be avoided unless a soil test calls for it. Cities and counties can also set their own limits. Orange County in Florida, for example, bans fertilizer with nitrogen or phosphorus from June 1 through September 30 and allows no more than 1 pound of total nitrogen applied per 1,000 square feet, and no lawn may receive more than 3 pounds per 1,000 square feet. Those numbers apply only there and are shown as an example. See the Orange County page and ask your own city or county. Seeding a new lawn first? The grass seed calculator gives the pounds of seed.

Tips for applying fertilizer

Read the label. The coverage and the spreader setting are printed on the bag for that product.
Take the house, driveway and beds out of the area with “Not lawn (subtract)”. Fertilizer does not go there.
Apply half the rate in two directions for an even coat, as University of Missouri Extension suggests.
Sweep fertilizer off sidewalks and driveways, as Iowa State University Extension advises, to keep runoff low.
Get a soil test before adding phosphorus or potassium. Prices change by product and region, so this calculator shows none.

Frequently asked questions

How much fertilizer do I need for a 10,000 square foot lawn?

It depends on the nitrogen number on the bag and on the rate you want. Divide the pounds of nitrogen per 1,000 square feet by the nitrogen percentage as a decimal, then multiply by 10. At 1 pound of nitrogen with a 20-5-10 bag that is 1 ÷ 0.20 = 5 pounds per 1,000, so 50 pounds for 10,000 square feet. At 0.75 pounds of nitrogen it is 37.5 pounds.

How many square feet does a 50 lb bag of fertilizer cover?

Multiply the bag weight by the nitrogen percentage as a decimal, divide by the pounds of nitrogen you want per 1,000 square feet, and multiply by 1,000. A 50 pound bag of a 20% nitrogen fertilizer holds 10 pounds of nitrogen, so at 1 pound per 1,000 it covers 10,000 square feet. At 0.5 pounds it covers 20,000. The bag label gives its own coverage, and University of Missouri Extension notes that most lawn fertilizer products are designed to cover 5,000 or 7,500 square feet.

How much fertilizer do I need for my lawn?

Measure the lawn in square feet, choose the pounds of actual nitrogen per 1,000 square feet, and divide that by the nitrogen percentage on the bag. Iowa State University Extension gives a range of 0.50 to 1 pound of actual nitrogen per 1,000 square feet, depending on the time of year and the quality of lawn you want. Then multiply the pounds of fertilizer per 1,000 square feet by the lawn area ÷ 1,000.

What do the three numbers on a fertilizer bag mean?

NC State Extension says they are the percentages by weight of nitrogen (N), phosphorus as P₂O₅ and potassium as K₂O, always listed in that order. On a 20-5-10 bag, 20% of the weight is nitrogen, 5% is P₂O₅ and 10% is K₂O. The calculator uses the first number for the amount of fertilizer and shows the pounds of the other two that you apply.

Does my lawn need phosphorus?

University of Missouri Extension says established lawns do not require additional phosphorus and that phosphorus and potassium should always be based on a soil test. Some local rules restrict phosphorus. Orange County, Florida, for example, prohibits fertilizer with phosphorus from October 1 to May 31 unless a soil test shows a deficiency.

Is there a limit on how much nitrogen I can apply?

Rules are local, so check yours. As one example, Orange County, Florida, says no more than 1 pound of total nitrogen may be applied per 1,000 square feet, says no lawn may receive more than 3 pounds of total nitrogen per 1,000 square feet, and bans fertilizer with nitrogen or phosphorus from June 1 through September 30. Iowa State University Extension gives 0.50 to 1 pound of actual nitrogen per 1,000 square feet, depending on application time. The calculator shows a warning when your rate is above 1.

How do I spread fertilizer evenly?

University of Missouri Extension says it is best to apply half the rate in two directions to get an even distribution, and to sweep or blow fertilizer off sidewalks and driveways back into the lawn. Iowa State University Extension also says to sweep fertilizer off hard surfaces to keep runoff to a minimum.