How it works
Run the spreader over a measured distance, collect the material it drops, and weigh it. The area covered is the swath width times the distance. Divide the weight by that area, then scale to an acre (43,560 square feet).
If the rate is off, change the gate or feed setting by roughly the percentage shown and test again. Calibrate at the speed you will work, with the same material, because flow depends on both.
Worked example
Collecting 12 lb over a 200 ft pass with a 12 ft swath, aiming for 200 lb/ac:
- Area covered = 12 ft × 200 ft = 2,400 ft² (0.0551 ac)
- Rate = 12 lb × 43,560 ÷ 2,400 ft² = 218 lb/ac
- Per 1,000 ft² = 12 × 1,000 ÷ 2,400 = 5 lb
- Change to reach 200 lb/ac = 200 ÷ 218 − 1 = -8.2%
| Input | Value |
|---|---|
| Material collected in the test | 12 lb |
| Swath width | 12 ft |
| Distance driven in the test | 200 ft |
| Target rate | 200 lb/ac |
| Result | Value |
|---|---|
| Actual application rate | 218 lb/ac |
| Rate per 1,000 sq ft | 5 lb/1,000 ft² |
| Change needed to hit the target | -8.2 % |
| Area covered in the test | 0.0551 ac |
Assumptions and limits
- The swath width is the effective, uniform width. Test pattern uniformity separately, since edges of a broadcast spreader usually thin out.
- Flow depends on material, moisture, speed, and the setting. The gate setting needed for a target comes from testing, not from this tool.
- Applies to dry granular material; liquid application uses the sprayer calibration calculator.
Common questions
How do I calibrate a spreader?
Run it over a measured distance, weigh what it drops, and divide by the area covered. Adjust the setting and repeat until you hit the target rate.
What is the swath width?
The effective width of even coverage, often less than the throw distance. Measure the pattern with catch pans.
How do I convert to pounds per 1,000 square feet?
Multiply pounds per acre by 1,000 and divide by 43,560; the calculator shows it directly.
How often should I recalibrate?
Whenever the material, speed, or gate setting changes.
Sources
- Area = swath × distance; 1 acre = 43,560 ft² (NIST Handbook 44).
- University extension spreader calibration guides use the same catch-test arithmetic.
Updated 2026-09-30