Fertilizer Rate Calculator

How do you calculate fertilizer from a soil test?

Two divisions. First get the recommendation into one unit — a figure in pounds per acre divides by 43.56 to become pounds per 1,000 square feet, because an acre is 43,560 square feet. Then divide that nutrient rate by the bag’s percentage written as a decimal. A report asking for 50 lb N per acre, spread with a 21-0-0 product over 1,000 sq ft, comes to about 5.5 pounds of product.

Step 0: your recommendation is written in one of three units

This is where most fertilizer arithmetic goes wrong, and it happens before any calculating starts. The same rate gets written three different ways depending on who wrote it down:

The per-acre conversion is not a rule of thumb. University of Maryland Extension’s soil-test guide prints it as “pounds per acre ÷ 43.56 = pounds per 1,000 square feet”, and NMSU Guide H-119 opens its worked example with the same instruction: “Divide pounds N per acre by 43,560.” The kg/ha row is the identical arithmetic routed through the pound and the square metre. Pick the basis on the toggle above and everything below it follows — there is no second calculator and no separate per-acre page to hunt for.

The nitrogen number does not come from the lab

Worth saying plainly, because “fertilizer calculator based on soil test” gets searched constantly and the answer has a catch in it. A standard soil test reports phosphorus, potassium, pH and organic matter. It does not measure plant-available nitrogen for turf in any way you can act on, which is why the University of Maryland tells readers to “always follow the University of Maryland lawn fertilization guidelines for nitrogen regardless of the recommendations given in the soil test report.”

So your N figure comes from a program or an extension guideline; the report supplies the P and K side. Rutgers Cooperative Extension’s FS839 puts the single-application convention at “only one pound of nitrogen per 1,000 square feet” when the product is a soluble inorganic nitrogen source. UNH Extension lands slightly lower at “0.9 lbs of actual nitrogen per 1,000 square feet per application” and caps the year so that “total nitrogen applied does not exceed 3.25 lbs annually.” Those are the numbers behind the note the tool prints when your target sits above the band, and they are general reference — actual caps vary by state, turf type and your own soil test, and the bag label and local rules govern.

Then one division gets you the product weight

Purdue Turfgrass Science states the formula in two steps: “Desired rate in lbs N / 1000 sq ft ÷ % nutrient = Total fertilizer needed / 1000 sq ft”, then “Total fertilizer needed / 1000 sq ft X Area to be treated in sq ft = lbs fertilizer needed to treat the area.” Its worked example runs a 16-8-8 product at 0.75 lb N per 1,000 sq ft across an 8,000 sq ft lawn and gets “4.69 lbs 16-8-8 / 1000 sq ft” and “37 lbs 16-8-8 needed to treat an 8000 sq ft lawn.”

NMSU’s example starts from the per-acre side instead: 50 lb N per acre becomes 0.00115 lb per square foot, then 1.15 lb of nitrogen over 1,000 sq ft, then “1.15 ÷ 0.21 = 5.48 pounds = 5 pounds 8 ounces” of ammonium sulfate. This tool prints 5.5 for the same inputs rather than 5.48, and the difference is worth understanding: the guide rounds its intermediate to 1.15 lb before dividing, while the calculator carries full precision (1.1478 ÷ 0.21 = 5.466) and rounds only at the end. Both are right; one just rounds sooner.

Per 1,000 sq ft and per acre are the same answer

The tool prints both because “how much fertilizer per acre” and “how much per 1,000 square feet” are the same question in different denominators, and neither one depends on the area you entered. Omni Calculator’s worked example takes a 28-0-6 product at 43 lbs N per acre and gets 154 lbs of fertilizer per acre; run the identical inputs here and you get 153.6, the same figure before rounding. Type a 400-square-foot repair patch or a five-acre site and the per-acre number does not move — only the total does. If the area is the part you are unsure about, measure it with the lawn area calculator and bring the number back.

What the soil-test P and K comparison actually does

Enter the phosphate and potash figures from your report and the tool subtracts what your bag co-delivers at the entered nitrogen rate. That is all it is: subtraction of two numbers you supplied. A negative result means the product delivers less than the report asked for; a positive one means more.

Two things to hold onto. The second and third numbers on a bag are P₂O₅ (phosphate) and K₂O (potash) on an oxide basis, not elemental P and K, so the comparison only works if your report is on the same basis — most turf reports are, but check. And a surplus against a zero phosphate recommendation is the one worth looking at twice, because phosphorus on established turf is restricted in some states and municipalities; the calculator flags the surplus and leaves the rule-checking to you. Nothing here selects a product or tells you when or how to put it down.

Fertiliser, in metric and in en-GB

Recommendations written in kilograms per hectare convert at about 0.0205 to pounds per 1,000 square feet, so roughly 48.8 kg N/ha is the same rate as 1.0 lb N per 1,000 sq ft. Choose the kg/ha basis for the input, flip the weight toggle to kg, and the answers come back as kg per 100 m² and kg per hectare with bag weights in kilograms. Spelling it fertiliser changes nothing about the arithmetic, and there is no separate UK version of this page.

Where this sits next to the other numbers

Working out the weight is one step. What the spreader actually puts down is another, and the two disagree more often than people expect — check yours with the spreader calibration calculator or, if you would rather weigh a real catch than trust the dial, the granular spreader calibration back-calculates your true rate from a measured test pass. When you are choosing between two products rather than sizing one, the nitrogen cost comparison calculator prices them per pound of actual N instead of per bag. pH sits alongside all of it — a soil test that flags pH has its own math in the lime and pH amendment calculator. For a single visit inside a season-long program, the annual fertilization schedule template shows where that application lands against the rest of the year, seeding weights come from the grass seed calculator, and the lawn agronomy hub routes between all of them.

None of those numbers says what the round is worth. Once the bag weight is settled, what a season-long fertilization and weed-control program should bill turns the visit count into a price, and anything that goes through a sprayer instead of a spreader gets its material side from the blended chemical cost per 1,000 square feet.

Nothing is stored, there is no account, and the CSV, print, PDF and copy-as-text exports are ungated so the buy list travels to the yard or the supplier with you. Rates here are general reference values, not a compliance record and not agronomic or legal advice.