Lawn Fertilizer Nitrogen Calculator
How much of the bag to spread for a pound of actual nitrogen — and why a "balanced" 10-10-10 is really 1 : 0.44 : 0.83.
What the bag actually contains
The second and third numbers are phosphate and potash, not phosphorus and potassium. Only 43.6% and 83.0% of them is the element.
The same nitrogen from other bags
Identical result on the lawn. The difference is how much material you carry, spread and pay for.
A "balanced" 10-10-10 is 1 : 0.44 : 0.83
The three numbers on a fertiliser bag are per cent by weight of nitrogen, phosphate and potash — not nitrogen, phosphorus and potassium. Phosphate is P2O5 and potash is K2O, and the element is only part of each: 43.64% of the phosphate figure and 83.02% of the potash one. Those are ratios of atomic masses, not agronomy — 2P over P2O5, 2K over K2O. So a bag sold as balanced actually delivers 10 parts nitrogen, 4.4 phosphorus and 8.3 potassium, and the phosphorus is out by more than a factor of two.
The bag number is a divisor, not a rate. To put down one pound of actual nitrogen you spread 100/N pounds of product: 2.2 lb of urea at 46%, 4.2 lb of a 24% turf food, twenty pounds of a 5% organic bag. Nine times the material, identical nitrogen. Which is also why one spreader setting is not transferable — at 4 lb per 1,000 sq ft a 30% bag delivers 1.2 lb of nitrogen and a 10% bag 0.4. That factor of three is how lawns get burnt.
Compare price per pound of nitrogen, not per bag. A cheap-looking 10% bag at $18 for 50 lb is $3.60 a pound of nitrogen; 46% urea at a higher sticker price of $25 is $1.09. The cheap bag is mostly filler and you pay to haul, store and spread four times as much of it. Slow-release and organic products genuinely cost more per pound of nitrogen — but they buy something, and a cheap synthetic blend does not.
- One pound of quick-release nitrogen is the cap for a single application. Soluble nitrogen is a salt: above that it draws water out of the leaf faster than the roots replace it. Slow-release fractions don't count against the limit, which is the entire reason they exist.
- Autumn is the application that matters for cool-season grass — it goes into roots and carbohydrate reserves rather than leaf. Warm-season grass is the opposite: feed it once it's fully green and growing, never while it's still breaking dormancy.
- Centipedegrass is the trap. Feeding it properly means barely feeding it at all; excess nitrogen causes centipede decline, which is real and often fatal. If in doubt, apply nothing.
- Don't apply phosphorus without a soil test. Established lawns rarely need it, it doesn't move in soil the way nitrogen does, and the surplus washes into waterways. Several places restrict its sale for established turf for exactly that reason.
- Calibrate the spreader. Settings aren't a standard and they vary with walking speed. Weigh out what the lawn needs, put exactly that in the hopper, and cover the lawn in two passes at right angles at half the rate each.
How to use
- Enter the three numbers printed on the bag and the lawn area.
- Set the nitrogen you want for this application, normally one pound.
- Check the burn warning, which watches the quick-release fraction only.
- Compare the price per pound of nitrogen before buying, not the price per bag.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
Per cent by weight of nitrogen, phosphate and potash — not nitrogen, phosphorus and potassium. Phosphate is P2O5 and potash is K2O, and only 43.64 per cent and 83.02 per cent of those figures is the element itself. Those are ratios of atomic masses rather than agronomy, and they are why the label is not telling you quite what it appears to.
Is a 10-10-10 fertilizer balanced?
No, despite the name. Elementally it delivers 10 parts nitrogen, 4.4 phosphorus and 8.3 potassium — a ratio of 1 : 0.44 : 0.83, with the phosphorus out by more than a factor of two. The three numbers look equal because two of them are quoted as oxides. A genuinely balanced elemental blend would have to read roughly 10-23-12.
How much fertilizer do I need per 1000 square feet?
One hundred divided by the nitrogen percentage, times the pounds of nitrogen you want. For one pound of actual nitrogen that is about 2.2 lb of 46 per cent urea, 4.2 lb of a 24 per cent turf food, or twenty pounds of a 5 per cent organic bag. Nine times the material for identical nitrogen, which is the single most useful thing to understand about a bag.
Why did my lawn burn after fertilizing?
Almost always too much quick-release nitrogen at once. Soluble nitrogen is a salt, and above about a pound per thousand square feet it draws water out of the leaf faster than the roots replace it. The commonest route to it is reusing a spreader setting from a different bag: at four pounds per thousand square feet a 30 per cent product delivers 1.2 lb of nitrogen where a 10 per cent one delivers 0.4.
How much nitrogen can I apply at once?
One pound per thousand square feet of quick-release nitrogen. The slow-release fraction does not count against that limit, which is the entire reason it exists — a bag that is half slow-release can carry two pounds total while still putting down only one pound of soluble nitrogen. Read the label for the slow-release percentage; it is usually the most valuable number on the bag after the first one.
How do I compare fertilizer prices properly?
Price per pound of actual nitrogen, never price per bag. A 50 lb bag of 10 per cent product at $18 works out at $3.60 a pound of nitrogen; 46 per cent urea at a higher sticker price of $25 is $1.09. The cheap bag is mostly filler and you are paying to haul, store and spread four times as much of it to get the same result.
How much nitrogen does a lawn need in a year?
Between about one and six pounds per thousand square feet depending entirely on the grass. Fine fescue wants one to two, tall fescue two to three, Kentucky bluegrass two to four, and bermudagrass four to six. Centipedegrass is the outlier at half a pound to one — feeding it properly means barely feeding it at all.
When should I fertilize my lawn?
For cool-season grasses the autumn application matters most, going down around the last mowing when top growth has slowed but roots are still active — it goes into roots and carbohydrate reserves rather than leaf. Warm-season grasses are the reverse: feed once the lawn is fully green and growing, never while it is still breaking dormancy, and stop about six weeks before the first expected frost.
Why does centipedegrass die when I fertilize it?
Excess nitrogen causes centipede decline, a real condition and often a fatal one. Centipede evolved on poor, acidic soil and it is the only common lawn grass that is actively harmed by generous feeding. Half a pound to a pound of nitrogen a year is the whole programme, and applying nothing is a much safer error than applying a normal amount.
Should I use a fertilizer with phosphorus?
Not without a soil test. Established lawns very rarely need phosphorus, most soils already hold plenty, and unlike nitrogen it does not move down through the soil — so the surplus stays on the surface or washes into waterways, where it feeds algae. Many places restrict its sale for established turf for that reason. It earns its place at seeding, where seedling roots genuinely benefit.
How do I calibrate a fertilizer spreader?
Weigh out exactly what the lawn needs, put that much in the hopper, and cover the whole lawn in two passes at right angles at half the rate each. If you run out early you were walking too slowly; if product is left you were walking too fast. Spreader settings are not a standard between machines or products, and two light passes also hide the overlap stripes a single heavy pass makes obvious.
What is slow-release nitrogen worth paying for?
It buys two things: headroom above the one-pound burn limit, and a feeding that lasts weeks rather than days, which means fewer applications and far less of the growth surge that turns into extra mowing. It genuinely costs more per pound of nitrogen than urea does, and that premium is real value — unlike a cheap synthetic blend, which costs more per pound of nitrogen and buys nothing at all.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.