Lawn Fertilizer Calculator
Nitrogen, product, and bags from lawn area and feed rate.
What it does
Fertilizer bags lead with three numbers for nitrogen, phosphorus, and potassium, and lawns hunger mostly for the first. The math divides the nitrogen target by the bag percent to get product weight, then by bag size for the count.
The tool above takes lawn area, feed rate, bag nitrogen, and bag size in imperial or metric. It returns nitrogen needed, product weight, and bags.
One pound of nitrogen per thousand square feet per feeding suits most lawns, with yearly totals near two to four pounds split across seasons. Warm and cool grasses peak in different months, so timing beats quantity.
Overfeeding backfires twice: it burns leaf tips in stripes and washes into waterways feeding algae instead of grass. Calibrated spreaders and measured areas prevent both.
How to use
- Switch the unit system first: imperial for ft2 and lb, metric for m2 and kg.
- Measure the lawn, subtracting house and beds.
- Pick the standard rate unless a soil test says otherwise.
- Read the bag N number off the label and buy the count.
Formula
- rate
- Nitrogen per area: 1 lb per 1000 ft2 or 20 g per m2 standard.
- N%
- First bag number as a rate. 24 means 0.24 nitrogen.
- ceil
- Round bags up. Partial bags store sealed and dry.
Phosphorus bans cover many regions, which is why zero middle number bags dominate shelves. Soil tests confirm whether P or K need adding.
Worked example
Example: 5000 ft2 lawn, 1 lb rate, 24% bag, 50 lb bags
Lawn 5000 ft2, standard rate, 24-0-0 product in 50 lb bags.
- Nitrogen: 5000 / 1000 x 1 = 5.00 lb N.
- Product: 5 / 0.24 = 20.83 lb.
- Bags: 20.83 / 50 = 0.42, so 1 bag.
- Leftover product feeds the next round.
Reference table
| Lawn | Nitrogen | Product | Bags |
|---|---|---|---|
| 2,500 ft2 | 2.50 lb | 10.42 lb | 1 |
| 5,000 ft2 | 5.00 lb | 20.83 lb | 1 |
| 10,000 ft2 | 10.00 lb | 41.67 lb | 1 |
| 20,000 ft2 | 20.00 lb | 83.33 lb | 2 |
Limits
- Nitrogen only sized. P and K follow soil tests.
- Flat rate assumed. Shade and wear vary needs.
- Runoff rules restrict timing and rates locally.
- New seed and sod follow starter programs instead.
Common mistakes
- Doubling rates for faster green, burning the lawn.
- Spreading by eye instead of calibrating the spreader.
- Feeding dormant grass that cannot use it.
- Piling granules that wash off drives.
- Skipping soil tests for years while guessing.
Try it yourself
Quick checks. Answers sit tucked under each question.
3000 ft2 at 1 lb rate, 32% bag, 50 lb bags?
N = 3 lb. Product 3 / 0.32 = 9.38 lb. So 1 bag.
Metric: 200 m2 at 20 g, 24% bag, 20 kg bags?
N = 4 kg. Product 4 / 0.24 = 16.67 kg. So 1 bag.
Questions
What do bag numbers mean?
Percent nitrogen, phosphorus, potassium by weight. A 24-0-0 bag is 24% nitrogen.
How much nitrogen per year?
Two to four pounds per thousand square feet yearly, split across two to four feedings.
When should I fertilize?
Peak growth seasons: fall for cool grasses, summer for warm ones. Skip dormant months.
Can fertilizer burn lawns?
Yes from excess or unwatered granules. Measured rates plus light watering prevent it.
What about phosphorus bans?
Many areas restrict P without soil tests. Zero middle number bags comply.
Should I test soil first?
Yes every few years. Tests tune pH, P, and K beyond nitrogen math.