How Much Fertilizer Per 1,000 Sq Ft: Bag Analysis to Pounds on the Ground
Divide 100 by the first number on the bag and you get the pounds of product that deliver 1 lb of nitrogen per 1,000 sq ft. A 21-7-14 bag needs 4.8 lbs per 1,000; a 46-0-0 bag needs 2.2. Multiply by your lawn area in thousands of square feet and you have the whole answer.
One pound of nitrogen per 1,000 square feet is the standard single application for almost every lawn in the country. To get there from a bag, divide 100 by the first number in the analysis. That is your pounds of product per 1,000 sq ft. Multiply by your lawn area divided by 1,000, and you are done.
So a 21-7-14 bag: 100 divided by 21 is 4.76 lbs per 1,000. A 6,000 sq ft lawn is 6 units of a thousand. 4.76 times 6 is 28.6 lbs of product. That is the entire calculation, and it is the calculation that almost nobody does on the driveway, which is why lawns come out striped.
The trouble is never that the recommendation is unclear. Every extension sheet in America says the same thing in the same unit. The trouble is that the recommendation is in pounds of nitrogen and the bag is in percentages, and those two things live on opposite sides of a division sign that nobody wants to do while holding a spreader handle.
The formula, once, with the numbers filled in
Write this on the inside of the shed door:
Pounds of product = (100 divided by %N on the bag) multiplied by (lawn square feet divided by 1,000) multiplied by (target lbs N per 1,000).
The last term is 1.0 for a standard feed. Drop it to 0.5 for a light summer pass or a burn-shy product, push it to 1.25 only if the label says the nitrogen is mostly slow-release.
Worked, with a 5,000 sq ft lawn and a 30 lb bag of 24-0-4: 100 / 24 = 4.17. 5,000 / 1,000 = 5. 4.17 x 5 x 1.0 = 20.8 lbs. Your 30 lb bag covers this application with 9 lbs left in the corner. It does not cover two applications, whatever the front of the bag says about coverage.
- The first number is the percentage of nitrogen by weight. Nothing else on the front of the bag changes the arithmetic.
- A 50 lb bag of 46-0-0 contains 23 lbs of actual nitrogen. A 50 lb bag of 10-10-10 contains 5. Same weight, four and a half times the nitrogen.
- Coverage claims printed on bags are usually calculated at somewhere between 0.5 and 0.9 lb N per 1,000, not 1.0. That is not dishonest, but it means the stated square footage and the agronomic rate are not the same number.
Pre-solved: pounds of product by lawn size and analysis
If you would rather not do arithmetic in a garage, find your lawn size down the side and your bag analysis across the top. Every cell is the pounds of product for one application at 1.0 lb N per 1,000 sq ft.
Lawn area (sq ft) 10-10-10 16-4-8 21-7-14 24-0-4 30-10-10 46-0-0 urea 1,000 10.0 lb 6.3 lb 4.8 lb 4.2 lb 3.3 lb 2.2 lb 2,500 25.0 lb 15.6 lb 11.9 lb 10.4 lb 8.3 lb 5.4 lb 5,000 50.0 lb 31.3 lb 23.8 lb 20.8 lb 16.7 lb 10.9 lb 7,500 75.0 lb 46.9 lb 35.7 lb 31.3 lb 25.0 lb 16.3 lb 10,000 100.0 lb 62.5 lb 47.6 lb 41.7 lb 33.3 lb 21.7 lb 15,000 150.0 lb 93.8 lb 71.4 lb 62.5 lb 50.0 lb 32.6 lb 43,560 (1 acre) 435.6 lb 272.3 lb 207.4 lb 181.5 lb 145.2 lb 94.7 lb One application at 1.0 lb N per 1,000 sq ft. Read your lawn size, read your bag, weigh out that many pounds before you open the hopper. Halve every figure for a 0.5 lb N pass. Notice the spread across a single row. A 10,000 sq ft lawn takes 100 lbs of 10-10-10 or 21.7 lbs of urea to do the same job. Two bags versus half a bag. If you have ever wondered why the guy down the road buys one small sack and you buy a pallet, that is the whole reason, and it has nothing to do with lawn quality.
Measure the turf, not the deed
Lot size is the wrong input. A quarter-acre lot is 10,890 sq ft, but the house, driveway, deck, beds and that gravel strip beside the garage are not grass. Most quarter-acre properties carry somewhere between 4,000 and 7,000 sq ft of actual turf, and people routinely buy fertilizer for the larger number.
Pace it. A comfortable adult stride is close enough to 3 feet that a 30-pace side is about 90 feet. Break the lawn into rectangles, pace each one, multiply, add them up, then subtract nothing else because you already left out the hard surfaces.
Two shapes worth knowing by heart. A circle is radius times radius times 3.14. A triangle is base times height divided by two. Between rectangles, circles and triangles you can approximate any suburban lawn to within about five percent, which is far tighter than your spreader will ever be.
What a single bag actually covers
Flip the arithmetic around and the shopping question answers itself. Bag weight times %N gives you pounds of nitrogen in the bag. Multiply by 1,000 and divide by your target rate, and you have square feet of real coverage.
Bag Actual lbs N in bag Coverage at 1.0 lb N/1,000 Coverage at 0.75 lb N/1,000 Coverage at 0.5 lb N/1,000 15 lb of 29-0-4 4.35 4,350 sq ft 5,800 sq ft 8,700 sq ft 16 lb of 32-0-4 5.12 5,120 sq ft 6,830 sq ft 10,240 sq ft 30 lb of 24-0-4 7.20 7,200 sq ft 9,600 sq ft 14,400 sq ft 37.5 lb of 21-7-14 7.88 7,880 sq ft 10,500 sq ft 15,750 sq ft 40 lb of 16-4-8 6.40 6,400 sq ft 8,530 sq ft 12,800 sq ft 36 lb of 6-4-0 biosolids 2.16 2,160 sq ft 2,880 sq ft 4,320 sq ft 50 lb of 46-0-0 urea 23.00 23,000 sq ft 30,670 sq ft 46,000 sq ft Check this against the coverage claim on the front of the bag before you buy. If the bag claims more square feet than the 1.0 lb column, it is quoting a lighter rate. That biosolids row is the one that surprises people. A 36 lb sack of 6-4-0 looks substantial and covers about the size of two decent front yards at full rate. It is a fine product. It is just a low-analysis product, and low analysis means you carry more bags.
The over-application band, and why 1.0 is the ceiling
Burn is an osmotic event. Soluble salts in the fertilizer pull water out of leaf tissue faster than the roots replace it, and the blade goes straw-coloured in strips exactly where the spreader overlapped. It is not a slow decline. Most people see it inside 72 hours.
The safe ceilings are worth memorising because they do not change much between products.
Quick-release soluble nitrogen: 0.5 to 0.75 lb N per 1,000 sq ft in one pass. Coated or slow-release nitrogen: up to 1.0, and up to 1.25 if the label says at least half the N is slow-release. Anything above 1.5 lb N per 1,000 in a single application is a gamble regardless of source.
Half-rate double passes at right angles are strictly better than a single full-rate pass, and they cost you eight extra minutes. Do that on any lawn you care about.
- Under-applying by 20 percent produces a slightly paler lawn and no other consequence.
- Over-applying by 20 percent on a hot, dry week produces damage you will be looking at until the next mowing cycle grows it out.
- The asymmetry is the whole reason to round down when you are unsure.
Season totals: how many bags to buy in one trip
Annual nitrogen budgets run from about 1 lb per 1,000 for centipede to 5 or 6 for a hard-run bermuda lawn. Take your species total, multiply by lawn area in thousands, and you have pounds of nitrogen for the year. Divide by the %N of your preferred bag and you have pounds of product. Divide by bag weight and you have bags.
For a 6,000 sq ft tall fescue lawn at 3 lbs N per 1,000 per year: 18 lbs of nitrogen. Buying 24-0-4 in 30 lb bags, that is 75 lbs of product, so three bags and a bit. Buy four. The leftover keeps fine in a sealed tub with the desiccant pack from a shoe box.
Do not store fertilizer in the paper bag on a concrete floor. It wicks moisture, sets into a brick, and then it will not meter through any spreader you own.
Where the arithmetic goes wrong in practice
- Using lot size instead of turf area. Consistently produces a 30 to 50 percent overdose.
- Reading the second number as nitrogen. On 5-10-31 the nitrogen is 5 percent, not 10, and you need 20 lbs of product per 1,000 to deliver a pound of N.
- Dividing by the whole analysis. Adding 21 plus 7 plus 14 gets you 42 and it means nothing. Only the first number drives the rate.
- Assuming the bag's coverage claim equals a full feed. It usually does not.
- Applying the whole bag because it is open. This is the single most common cause of a striped September lawn.
- Forgetting that a partly full bag no longer weighs what the label says. Weigh it. Bathroom scale, hold the bag, subtract yourself.
How this number feeds the Feed Index
The Feed Index is a 0 to 100 score built from four axes: how well the nitrogen carrier's release curve matches the window you are feeding in (35 points), burn headroom as read from salt index and maximum safe single rate (25 points), delivered cost per actual pound of nitrogen (20 points), and spread behaviour, meaning granule uniformity and how predictably the product meters (20 points).
Every one of those axes is downstream of the number you just calculated. You cannot score cost per pound of nitrogen without knowing pounds of product per pound of N. You cannot score burn headroom without knowing what rate you are actually putting down. The division comes first. Everything else on this site is built on top of it.