Landscaping
Lawn Fertilizer Calculator
Enter your lawn size and the first number on the bag, and this tells you the pounds of fertilizer to spread and to buy.
| Lawn area | — |
|---|---|
| In thousands of sq ft | — |
| Product per 1,000 sq ft | — |
| Product for this round | — |
| For the whole year | — |
| Bags, rounded up | — |
UNITS LB
SOURCE /SOURCES/#fertilizer
What to buy
Paid links. If you buy through one, we earn a commission. Your price is the same. Nothing is listed here unless the calculator above says you need it.
Why this is more than you need
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Lawn fertilizer
The pound figure above is product, not nitrogen. Check the first number on the bag against the one you entered, because a different analysis changes the weight you need.
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Broadcast spreader
The number above is pounds to spread over a known area. A spreader is what turns that into an even application instead of stripes.
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Slow-release nitrogen fertilizer
Same math, gentler delivery. Extension guidance for fall feeding asks for a quarter to a half of the nitrogen in slow-release form.
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Soil test kit
This calculator sizes the nitrogen. Only a soil test tells you whether the phosphorus and potassium in that bag are needed at all.
What the turf programs state
| Penn State Extension | 1.0 lb N per 1,000 sq ft | divide by the bag's nitrogen fraction: 1.0 / 0.26 = 3.8 lb of a 26-5-10 |
|---|---|---|
| MSU Extension, E0001TURF | 1 lb N per 1,000 sq ft | stated as a per-application maximum, not a target |
| MSU Extension, E0001TURF | 1 to 5 lb N per year | 1 to 2 low maintenance, 3 medium, 4 to 5 high |
Not a manufacturer spec. A fertilizer maker publishes a setting for its own spreader and its own bag; these are the underlying rates those settings are built on, from three university turf programs.
Gear for the job
The calculator above does not count any of this. It is what the job takes to actually do. Paid links, and your price is the same.
- Spreader calibration scale Weigh what goes in the hopper and what comes back out. It is the only way to know a setting actually delivered the pounds you calculated.
- Push spreader edge guard Keeps granules off the drive and out of the storm drain, which is the whole point of the water-quality guidance behind these rates.
- Long measuring wheel Lawn area is the input that decides everything here, and pacing it out is where most of the error comes from.
- Garden hose sprinkler Granular fertilizer needs watering in. Left dry on the blades it can burn the grass it was meant to feed.
- Leaf blower For sweeping stray granules off hard surfaces back onto the lawn rather than hosing them into the gutter.
Part of a bigger job
This calculator sizes one material. These jobs need it along with others, and the list says what else to buy and in what order.
- Reseed a patchy lawn grass seed, topsoil or compost
Every turf recommendation is written in pounds of nitrogen, and every bag is sold in pounds of product. They are not the same number. One pound of nitrogen means 3.8 pounds of a 26% fertilizer and 3.1 pounds of a 32% one, so the bag's own scoop setting tells you nothing until you do this division.
How to estimate lawn fertilizer
Every lawn feeding recommendation you will ever read is written in pounds of nitrogen. Every bag on the shelf is sold in pounds of product. Those are different numbers, and converting between them is the entire calculation.
product per 1,000 sq ft = nitrogen wanted ÷ (bag nitrogen % ÷ 100)
total product = product per 1,000 × (lawn area ÷ 1,000)Penn State Extension states the method as "divide the amount of nitrogen desired (1.0 lb nitrogen per 1000 sq ft) by the percentage of nitrogen in the bag".
Measuring the lawn
Grass only. This is where most of the error comes from, and it goes one way: people use the lot size. A quarter-acre lot is about 10,900 square feet, but once the house, the driveway, the patio and the beds are out, the lawn might be 5,000. Fertilize 10,900 square feet worth onto 5,000 and you have applied double the rate.
Measure the grass in rectangles and add them up. Front, back, both sides. A curved edge can be averaged; being off by a few feet changes the answer by ounces, not pounds.
Reading the bag
The three numbers on the front are nitrogen, phosphorus and potassium, as percentages by weight. A 26-5-10 bag is 26 percent nitrogen. For this calculation the second and third numbers do not matter at all.
It is worth seeing what that percentage means in weight. A 40 pound bag of 26-5-10 holds 10.4 pounds of actual nitrogen. The remaining 29.6 pounds is phosphorus, potassium and carrier. That is not padding you are being cheated with, since granules have to be spreadable, but it does explain why the pounds on the bag and the pounds in the recommendation are so far apart.
The rate, and the ceiling on it
One pound of nitrogen per 1,000 square feet is the standard application. MSU Extension states it as a limit rather than a goal: do not apply more than 1 pound of nitrogen to 1,000 square feet of lawn per application.
That ceiling is about two things at once. Too much nitrogen at once burns the grass, and whatever the lawn cannot take up runs off into storm drains and groundwater. The bulletin the figure comes from is called "Fertilizing Home Lawns to Protect Water Quality", which tells you which of the two the researchers were more worried about.
Lower rates are perfectly normal. Half a pound is common on a light spring feed or on fine fescue, which wants less nitrogen than bluegrass or ryegrass. Going below the rate costs you nothing but a slower green-up. Going above it is the only direction with a real downside.
Working it out
Penn State publishes the arithmetic directly: 1.0 pound of nitrogen per 1,000 square feet, divided by 0.26, is 3.8 pounds of a 26-5-10 fertilizer. That is the rate for one thousand square feet.
Multiply by your lawn. A 5,000 square foot lawn is five of those, so 19.2 pounds of product for one application. If your bag is 20 pounds, that is one bag with a little left over.
Change the analysis and the weight changes with it. The same pound of nitrogen takes 3.1 pounds of a 32 percent product, or 8.3 pounds of a 12 percent one. This is why a spreader setting copied off a different bag is close to meaningless.
One round is not the year
The number above covers one application. The season is the figure that decides what you actually carry home, and it is the one a fertilizer maker's own calculator will not give you.
MSU puts a low-maintenance lawn at 1 to 2 pounds of nitrogen a year, a medium one at 3, and a high-maintenance lawn at 4 to 5. At the standard pound per application, medium maintenance is three rounds. For that 5,000 square foot lawn on a 26 percent product, three rounds is about 58 pounds of fertilizer across the season, or three 20 pound bags.
Timing is a regional question and this page will not pretend otherwise. Cool-season and warm-season grasses want feeding at opposite ends of the year. Your state extension service publishes a calendar for your grass; use theirs, not a national one.
When the bag states its own coverage
Most consumer bags print an area rather than a rate, and you can work backwards to see what rate they assumed. Multiply the bag weight by the nitrogen percentage to get pounds of actual nitrogen, then divide by the stated coverage in thousands of square feet.
A 12.5 pound bag of 32-0-4 that claims 5,000 square feet delivers 4 pounds of nitrogen over five thousand, which is 0.8 pounds per 1,000. That is a sensible rate. It is just not the one pound you might have assumed, and if you were counting on a full rate you are 20 percent short.
A quick cost check
The 5,000 square foot lawn again, medium maintenance, 26 percent nitrogen. One round is 19.2 pounds. The year is about 58 pounds, so three 20 pound bags. At $25 a bag that is $75 of fertilizer for the season.
Switching to a 32 percent product drops the season to 47 pounds and might fit in two bags plus a bit. Whether that saves money depends entirely on the price per bag, which is why the price field above is yours to fill in rather than ours to guess.
Common estimating mistakes
- Using lot size instead of lawn size. The single biggest error, and it always overapplies.
- Confusing pounds of nitrogen with pounds of fertilizer. One pound of nitrogen is 3.8 pounds of a 26 percent bag.
- Copying a spreader setting from a different product. Settings are specific to one granule size and one analysis.
- Doubling the rate to skip a round. The pound per application is a ceiling, and the reason is in the title of the bulletin it comes from.
- Trusting the bag's coverage claim as a rate. It is an area at an unstated rate. Work it backwards and see.
- Buying phosphorus you do not need. Many states restrict it on established lawns, and a soil test is the only way to know.
When to hand it off
Two honest boundaries here, and neither is about the arithmetic.
The first is the soil test. This calculator sizes nitrogen, and nitrogen is the one nutrient a soil test cannot usefully measure because it moves through soil so quickly. Phosphorus and potassium are the opposite: a test tells you exactly whether the second and third numbers on that bag are doing anything for you. Several states restrict phosphorus on established lawns unless a test shows a deficiency, so this is a legal question in places and not only an agronomic one.
The second is diagnosis. Fertilizer is not a treatment for grubs, compaction, shade, drought or the wrong grass for your climate. If a lawn is thin and feeding has not fixed it, more nitrogen will not either, and a local extension office or a lawn service will identify the actual problem faster than another bag will.
Questions
How much fertilizer do I need for my lawn?
Divide the nitrogen you want by the nitrogen fraction in the bag, then multiply by your lawn area in thousands of square feet. Penn State Extension gives the worked example: 1.0 lb of nitrogen per 1,000 sq ft divided by 0.26 is 3.8 lb of a 26-5-10 fertilizer. On a 5,000 sq ft lawn that is about 19 lb of product for one application.
What do the three numbers on a fertilizer bag mean?
Nitrogen, phosphorus and potassium, as percentages by weight. A 26-5-10 bag is 26 percent nitrogen. Only the first number matters for this calculation, because lawn feeding rates are written in pounds of nitrogen. A 40 lb bag of 26-5-10 contains 10.4 lb of actual nitrogen, and the other 29.6 lb is phosphorus, potassium and filler.
How many times a year should I fertilize a lawn?
It follows from the annual total rather than from a calendar. MSU Extension puts a low-maintenance lawn at 1 to 2 lb of nitrogen a year, a medium one at 3, and a high-maintenance lawn at 4 to 5. At the standard 1 lb per application, that is one or two rounds a year at the low end and four or five at the high end. Your state extension service publishes the timing, which varies a lot by grass type and region.
Can I apply more than 1 pound of nitrogen at once?
MSU Extension states it plainly: do not apply more than 1 pound of nitrogen to 1,000 square feet of lawn per application. That is a ceiling, not a target. Doubling up to save a trip risks burning the lawn and sends the excess into groundwater and storm drains, which is what the guidance exists to prevent. If the lawn needs more for the year, add another round, not a heavier one.
My bag says it covers 5,000 square feet. Why calculate anything?
Because that claim already assumes a rate, and it is often not 1 lb of nitrogen. Work it backwards: multiply the bag weight by the nitrogen percentage to get pounds of actual nitrogen, then divide by the stated coverage in thousands. A 12.5 lb bag of 32-0-4 covering 5,000 sq ft delivers 4 lb of nitrogen over 5 thousand sq ft, which is 0.8 lb per 1,000. That is a fine rate, but it is not the one you may have assumed.
Every constant on this page is logged with its source and the date it was checked on the sources page.