The three numbers are percentages, and that is the whole trick
An N-P-K analysis of 24-0-6 means the bag is 24 percent nitrogen, 0 percent available phosphate and 6 percent soluble potash by weight. Nothing on the bag tells you how much to put on your lawn until you decide on a rate, and rates are set in pounds of actual nitrogen per 1,000 square feet, not pounds of product.
The conversion is one division. Pounds of product per 1,000 square feet equals your target pounds of N per 1,000 divided by the N percentage expressed as a decimal. One pound of N from a 24 percent bag is 1 divided by 0.24, which is 4.17 lb of product per 1,000. Over 5,000 square feet that is 20.83 lb of product delivering 5 lb of actual nitrogen. From a 10 percent bag the same job takes 50 lb of product. Same lawn, same nitrogen, two and a half times the material — which is why comparing bags on the shelf price is meaningless and comparing them on cost per pound of actual N is not.
Where people go wrong
| The mistake | What happens |
|---|---|
| Reading 24-0-6 as pounds | Wildly under-application. The bag is a percentage, not a dose. |
| Emptying the bag over the lawn | Rate depends on how big the lawn is. A 40 lb bag on 2,000 sq ft is four times the rate it is on 8,000. |
| Trusting the coverage claim without checking the area | "Covers 5,000 sq ft" is calculated at the manufacturer's assumed rate. If your lawn is 3,200 sq ft you need to set the spreader, not just walk until it is empty. |
| Using a spreader setting from a different bag | Settings are product-specific. Prill size, density and flow all differ. |
| Comparing prices per bag | Compare cost per pound of actual nitrogen. A cheap low-analysis bag is often the expensive one. |
Rates, and why one pound is the number you keep hearing
Roughly one pound of nitrogen per 1,000 square feet is the conventional ceiling for a single application of a mostly quick-release product on lawn turf. It is not a law of nature. It is a practical limit that comes from two things: above it, soluble nitrogen salts start scorching leaf tissue, especially on dry turf in heat; and the growth flush you get outruns the root system, giving you soft top growth that mows into a mess and is more vulnerable to drought and disease.
Slow-release and coated products can go higher in one pass because the nitrogen is metered out over weeks rather than dissolved at the first watering. The percentage of the N that is slow-release is stated on the bag in the guaranteed analysis, and it is the difference between a product you can put down at 1.5 lb per 1,000 and one you cannot. Season totals are a separate question again and depend on grass type, region, clipping return and how the lawn is used — they are typically achieved across three to five applications rather than one.
Spreading it evenly is harder than calculating it
An exact calculation applied with a badly walked spreader gives you a striped lawn, and the stripes last for weeks. Three habits fix most of it. Walk at a consistent pace, because drop rate is per unit of time and your speed sets the rate per unit of area. Overlap on the wheel tracks of the previous pass with a broadcast spreader, since the pattern is heaviest under the middle and feathers out at the edges — the effective swath is narrower than the throw. And split the job: half the product in one direction, half at right angles, which turns any consistent error into a cross-hatch nobody can see.
Start and stop the flow while moving, shut the hopper before you turn at the end of a run, and load the spreader on a hard surface where you can sweep up a spill rather than on the lawn where you cannot.
Runoff, and the rules you need to look up yourself
Nitrogen and phosphorus that leave your lawn end up in surface water, and once there they do the same thing they do on grass: feed growth. Phosphorus in particular drives algal blooms in fresh water at very small concentrations, which is why it is the nutrient that regulators have targeted. A number of states and many local authorities restrict phosphorus in lawn fertiliser on established turf, several have seasonal application bans around frozen or saturated ground, and buffer strips near water are commonly required.
This calculator does not know which of those apply to you, and it deliberately does not name any of them, because they change and they vary by county. What is worth doing regardless of the rules: get a soil test before buying anything with phosphorus in it, keep granules off paved surfaces and sweep back what lands there, do not apply before heavy rain or onto frozen ground, and leave an unfertilised strip along any watercourse. Your local extension service or agriculture department will tell you what is actually required.
What this does not tell you
It does not tell you whether your lawn needs feeding, what it needs, or when. That comes from a soil test and from knowing your grass and your region — a calculator that has been given an analysis and a target has no view on whether the target was sensible. It also does not convert to a spreader setting, because settings are specific to the machine and the product and no cross-reference is reliable enough to publish. Calibrate: weigh a known quantity into the hopper, spread a measured area, and weigh what is left.
Questions people ask
How much 24-0-6 do I need for 5,000 square feet?
At a target of one pound of nitrogen per 1,000 square feet, 20.83 pounds of product. The working: 5,000 sq ft is 5 thousands, so you want 5 lb of actual nitrogen; the bag is 24 percent nitrogen, so 5 divided by 0.24 gives 20.83 lb of product. That is 4.17 lb per 1,000 square feet, which is the number you would use to calibrate the spreader. Change the analysis and the answer moves a long way — the same job from a 10-0-4 bag takes 50 lb of product.
What do the three numbers on fertilizer mean?
They are percentages by weight of nitrogen (N), available phosphate (P2O5) and soluble potash (K2O), always in that order. A 50 lb bag of 10-10-10 contains 5 lb of each. Note the second and third are reported as the oxide forms by convention rather than as elemental phosphorus and potassium, which is a historical quirk of fertiliser labelling and matters only if you are converting between analyses and soil test reports that use elemental figures. For working out how much to spread, take them at face value as percentages of the bag.
How much nitrogen per 1,000 square feet should I apply?
Around one pound in any single application of a mostly quick-release product, which is a practical ceiling rather than a recommendation — going above it risks leaf burn and produces soft growth. What the lawn should get across a season is a different question and depends on the grass species, your climate, whether you return clippings, and how the lawn is used, so it is answered locally rather than by a calculator. Products with a high slow-release fraction can be applied heavier in one pass because the nitrogen is released over weeks. Check the guaranteed analysis on the bag for the slow-release percentage.
Is it better to buy a high-analysis or low-analysis fertilizer?
Compare them on cost per pound of actual nitrogen and the question mostly answers itself. Divide the bag price by the bag weight times the N percentage: a $22 bag of 40 lb 24-0-6 holds 9.6 lb of N, so $2.29 a pound; an $18 bag of 40 lb 10-0-4 holds 4 lb, so $4.50 a pound. High analysis also means less weight to carry and fewer hopper refills. What pushes the other way is that low-analysis and organic products often carry a large slow-release or organic-matter component that the N percentage alone does not capture, and that has value the price-per-pound comparison misses entirely.
Do I need the phosphorus number?
Often not, and in some places you are not permitted to apply it to established lawns anyway. Phosphorus moves very slowly in soil and most established lawns have enough, so the middle number is genuinely useful mainly at seeding and on soils a test has shown to be deficient. Because phosphorus running off into surface water is the main driver of algal blooms, it is the nutrient most commonly restricted by state and local rules, sometimes with exceptions for new lawns and soil-test-documented deficiency. Get the soil test, then find out what your jurisdiction allows — do not assume either way.