Understanding your result
The headline is the total weight of fertilizer to spread over the whole lawn. The line under it gives the spreading rate, which is the number your spreader setting has to deliver.
Product to spread is the figure to work from on the day. It tells you how many pounds of the actual product go on each 1,000 square feet. Weigh that amount into the spreader for the first 1,000 square feet and you will know straight away whether the setting is right.
One bag covers shows the area a full bag treats at your chosen rate. It will often differ from the coverage printed on the bag, because the manufacturer assumed a particular rate. Neither number is wrong. They answer different questions.
If the product contains phosphate or potash, the table shows how much of each goes down alongside the nitrogen. This matters where phosphate is restricted, and it is useful if a soil test has told you what your lawn is short of.
How we calculate this
The phosphate and potash applied are worked out the same way in reverse: the product rate multiplied by each percentage.
In metric the calculator works in grams per square meter. One pound per 1,000 square feet is very close to 4.9 grams per square meter, so the common “one pound” rate is about 5 grams of nitrogen per square meter.
The assumptions behind the numbers
| Assumption | Value used | Why |
|---|---|---|
| Analysis | Percent by weight, in the order N-P-K | This is the legal labeling convention for fertilizer |
| Default nitrogen rate | 1 lb per 1,000 sq ft (5 g per m²) | The usual upper limit for one application of quick-release nitrogen |
| Warning threshold | Above 1 lb per 1,000 sq ft | Heavier single applications risk scorching the grass |
| Bags | Rounded up to whole bags | The leftover weight is shown so you can plan the next feeding |
| Spreader overlap | Not included | Overlap depends on your spreader. Calibrate with a weighed test run |
Assumptions last reviewed October 3, 2026.
The calculator does not tell you which fertilizer to buy or how often to feed. Those depend on your grass, your soil and your climate, and a soil test is the best guide. What it does is make sure that whatever you choose goes down at the rate you intended.
Two worked examples
A standard lawn feed
A lawn of 7,500 square feet is being fed with a 32-0-4 product sold in 12.5 pound bags. The target is three quarters of a pound of nitrogen per 1,000 square feet.
- Product per 1,000 sq ft: 0.75 ÷ 0.32 = 2.34 lb
- Total product: 7.5 × 2.34 = 17.6 lb
- Bags of 12.5 lb: 17.6 ÷ 12.5 = 1.4, so 2 bags
- One bag covers: 12.5 × 0.32 ÷ 0.75 × 1,000 = 5,333 sq ft
- Left over: 25 − 17.6 = 7.4 lb
The leftover is not waste. It is a little under half the next feeding, so the second purchase is one bag instead of two.
A low-analysis organic product
A 5,000 square foot lawn is being fed with a 6-4-0 organic fertilizer in 32 pound bags, also at three quarters of a pound of nitrogen per 1,000 square feet.
- Product per 1,000 sq ft: 0.75 ÷ 0.06 = 12.5 lb
- Total product: 5 × 12.5 = 62.5 lb
- Bags of 32 lb: 62.5 ÷ 32 = 1.95, so 2 bags
- Phosphate applied: 12.5 × 0.04 = 0.5 lb per 1,000 sq ft
Compare the two examples. The same nitrogen rate takes 2.3 pounds per 1,000 square feet of the first product and 12.5 pounds of the second. Anyone who spreads the organic product at the setting they used for a synthetic one will put down less than a fifth of what they meant to.
Common mistakes
- Spreading by the bag instead of by the rate. “One bag on the front, one on the back” only works if your lawn happens to match the bag.
- Confusing pounds of product with pounds of nitrogen. A recommendation of “1 pound of nitrogen” never means 1 pound of fertilizer.
- Using an old spreader setting for a new product. Settings are specific to the product. A different analysis or granule size needs a different setting.
- Guessing the lawn area. Take the lot size and subtract the house, drive, beds and patio. Most lawns are a good deal smaller than their owners think.
- Feeding at the heavy rate to make up for a missed application. Grass cannot use a double dose. The excess washes away or burns the turf.
- Ignoring the second number. If a soil test shows plenty of phosphorus, or local rules restrict it, choose a product with a zero in the middle.
- Filling the spreader on the lawn. A spill of concentrated fertilizer leaves a dead patch. Fill it on the driveway and sweep up what you drop.
Printable shopping list
Print shopping list produces a page with the product analysis, the number of bags, the total weight and the spreading rate. Tape it inside the shed door. The spreading rate is the number you will want again in two months, when it is time for the next feeding and the bag with the instructions on it has long since gone out with the trash.
Questions people ask
- What do the three numbers on a fertilizer bag mean?
- They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O), always in that order. A 50 pound bag of 24-0-6 contains 12 pounds of nitrogen, no phosphate and 3 pounds of potash. The rest is carrier material and other ingredients.
- How much nitrogen should I apply to my lawn?
- A common guideline is between half a pound and one pound of nitrogen per 1,000 square feet in a single feeding, and no more than one pound if the product is quick-release. How many feedings a year depends on your grass and climate. Your state or regional extension service publishes a schedule for local lawns.
- How do I work out how much fertilizer to use?
- Divide the nitrogen you want by the first number on the bag, as a decimal. For 1 pound of nitrogen from a 24-0-6 product, 1 ÷ 0.24 = 4.2 pounds of product per 1,000 square feet. Multiply by your lawn area in thousands of square feet.
- Why does a low-number fertilizer need so many more pounds?
- Because less of each pound is nitrogen. A 6-4-0 product is 6% nitrogen, so it takes four times as much of it as a 24-0-6 product to deliver the same nitrogen. That is not a fault. Low-analysis and organic products release slowly and are harder to over-apply.
- How do I set my spreader?
- Use the setting printed on the bag for your spreader model as a starting point. To check it, weigh out the amount for 1,000 square feet, mark out that area, and spread it. If you run out early, close the setting down. If you have product left, open it up.
- Can I use the calculator for garden fertilizer?
- The arithmetic is the same for any granular fertilizer, so yes. Enter the bed area and the nitrogen rate recommended for your crop. The lawn-specific warnings will not apply.
Built and reviewed by Bertha
Last updated · How we work out our numbers