How do I calibrate a spreader? Walk a measured strip and weigh it
Calibrating a spreader is one measurement: how many pounds of product it puts on a measured patch of ground at a given setting. Everything else — the setting printed on the bag, the "covers 5,000 sq ft" — is somebody else's guess about your spreader. The bag's coverage claim does give you the target, though: a 12.5 lb bag labeled for 5,000 sq ft is asking for 2.5 lb per 1,000 sq ft, and the fertilizer calculator turns a nitrogen target into the same number.
Labels give rates per 1,000 sq ft, so make the strip 1,000 sq ft: its length is 1,000 ÷ your swath. At a 10 ft swath that's 100 ft; at 12 ft it's 83.3. Weigh what came out and there is no arithmetic left — 2.5 lb caught is 2.5 lb per 1,000. A shorter strip works too; multiply the catch up.
- Know the swath. For a drop spreader it's the hopper, outer hole to outer hole. For a broadcast spreader it's the spacing between passes — measure it with the pan test below, not the box's "up to 12 ft".
- Mark the strip with flags or chalk, on a driveway, a tarp or a patch of lawn you don't mind feeding.
- Catch or weigh. A catch bag around the impeller, a gutter under a drop hopper, a sweep of the driveway — or weigh the product going in and what's left coming out.
- Walk it like the lawn. Start moving before the line, open the gate at it, close it at the end, at the pace you'll actually use.
- Enter the setting and the weight. After two runs at different settings, the calculator aims the third.
| Spreader | 1,000 sq ft strip | 500 sq ft (catch × 2) | 100 sq ft (catch × 10) |
|---|---|---|---|
| Drop, 18 in hopper | 666.7 ft | 333.3 ft | 66.7 ft |
| Drop, 22 in hopper | 545.5 ft | 272.7 ft | 54.5 ft |
| Rotary, 6 ft swath | 166.7 ft | 83.3 ft | 16.7 ft |
| Rotary, 8 ft swath | 125.0 ft | 62.5 ft | 12.5 ft |
| Rotary, 10 ft swath | 100.0 ft | 50.0 ft | 10.0 ft |
| Rotary, 12 ft swath | 83.3 ft | 41.7 ft | 8.3 ft |
Strip length = test area ÷ swath. NC State calibrates drop spreaders over 100 sq ft, because 1,000 sq ft through a 22 in hopper is a 545 ft walk.
Why the setting on the bag is only a starting point
A setting number is a position on one manufacturer's gate. The people who print them say so: Scotts' own guidance is that the numbers "are relative to each spreader model; they are not universal." The Sports Turf Managers Association goes further — two identical-looking spreaders of the same model can deliver differently, a bigger or heavier granule throws farther and flows differently, and the operator's walking speed changes the result. The bag's number was measured on the maker's spreader, with that product, at the maker's pace.
That is also why this page publishes no settings. A "settings calculator" that prints a dial number for any brand and any product isn't looking anything up — no such table can exist. What does exist is your spreader, your bag and a scale, and ten minutes of measuring replaces the lookup. Use the bag's setting as run one, never as the answer.
The dial isn't linear — so aim with two runs
"Spreader settings are not linear," NC State's calibration guide warns: half the setting does not give half the rate. That rules out the obvious fix — a run came in 30% heavy, so turn the dial down 30% — because the dial doesn't map to pounds that way. One run tells you the direction. Two runs at different settings show how this spreader, with this product, behaves between them, and the next setting can be read off the line through them.
Say the target is 2.5 lb per 1,000. Run one at setting 10 puts down 3.4 — 36% heavy. Scaling the dial by the miss says setting 7.4. Run two, at 7.5, comes in at 2.0 — 20% light — so the proportional guess had already gone past the answer. The line through both runs says 8.4; read on that same line, 7.4 would have landed 23% light. (Example arithmetic, not a setting for any spreader.)
The calculator does this with whatever runs you give it. It stops when a run lands within ±5% of the target (the STMA standard; NC State accepts ±10%), interpolates between a heavy run and a light one, extends the line — and says so — when every run so far is heavy or every run light, and refuses to aim at all if a higher setting put down less than a lower one. That isn't the dial; it's pace, a clump in the hopper or a misread scale. STMA's worksheet has room for seven runs. Two either side of the target, and a third to confirm, usually do it.
How wide does my spreader throw? The catch-pan test
A broadcast spreader throws far wider than it is, but not evenly: most lands behind you and it tapers toward the edges. The effective swath — how far apart your passes go — is where that taper falls to half. NC State and STMA measure it the same way:
- Lay a row of identical pans across your path — shallow boxes about 1 ft square, 1 or 2 ft apart, an odd number with the middle one on the centerline, the row 1½–2× as wide as the swath you expect.
- Walk over them three times in the same direction, at your normal pace, at the setting you'll use.
- Weigh each pan — a kitchen scale in grams — and enter the weights left to right as you face the way you walked.
The effective swath is the distance between the points either side where the catch falls to half the center pan. On our even example row (0.3, 1.0, 2.0, 3.2, 4.3, 5.1, 5.5, 5.1, 4.3, 3.2, 2.0, 1.0, 0.3 g) that's 6.75 ft. But the half-rate rule assumes a symmetric pattern, and the lawn gets the overlap, not one pass. So the calculator also overlaps your row at that spacing and reports the coefficient of variation (CV) — the spread of the dose across the lawn, and the number New Zealand's Spreadmark scheme certifies spreaders on: 15% or less for nitrogen fertilizer, 25% for seed, lime and other products.
| Passes apart | Even row | Lopsided row, back-and-forth | Lopsided row, one-way |
|---|---|---|---|
| 5 ft | 2% | 10% | 4% |
| 6 ft | 3% | 14% | 7% |
| 7 ft | 2% | 16% | 6% |
| 8 ft | 10% | 16% | 5% |
| 9 ft | 21% | 20% | 14% |
| 10 ft | 33% | 28% | 26% |
CV of the overlapped dose for two example rows of 13 pans 1 ft apart — even: 0.3, 1.0, 2.0, 3.2, 4.3, 5.1, 5.5, 5.1, 4.3, 3.2, 2.0, 1.0, 0.3; lopsided: 0.3, 0.9, 1.8, 3.0, 4.2, 5.1, 5.5, 5.4, 5.0, 4.2, 3.2, 2.0, 0.9 (grams). Bold = over the 15% nitrogen limit. Example readings, not measurements of any spreader.
The lopsided row is only 15 points heavier on the right: its half-rate points sit 3.2 ft left and 4.4 ft right, a 7.6 ft swath. Walked one way, every pass facing the same direction, that spacing is even — CV 4%. Walked back-and-forth, the way everyone walks a lawn, each pass turns around, so the heavy side of one pass lands on the heavy side of the next: CV 16%, over the nitrogen limit, in stripes exactly two passes apart. Back-and-forth stays under 15% only with passes 6.2 ft apart.
The fixes, cheapest first: use the spreader's side deflector or pattern adjustment if it has one (NC State: "adjust the spreader to correct the pattern"); close the gate and walk back to the start so every pass faces the same way; walk narrower passes; or put down half the rate twice, crosswise. The even row shows the other side of it — symmetric, it stays at CV 2% at its half-rate spacing and under 15% out to 8.4 ft, so on a symmetric pattern the half-rate rule has room to spare; on a lopsided one it has none.
The pans measure the rate too. NC State's formula is grams per pan ÷ pan area ÷ passes × 2.2 = lb per 1,000 sq ft, and applied across the overlapped row, our even example at 6.75 ft works out to 4.03 lb per 1,000. Give the calculator the pan size and the number of passes and it prints yours.
Does walking speed change the rate?
Yes — and it's the variable you can't see. The gate meters product per second while the ground goes by per foot, so walk slower than you calibrated and every square foot gets more. A broadcast spreader's wheel-driven spinner also throws wider when you speed up, which thins it further. NC State treats 3 mph as the standard pace — about 22.7 seconds per 100 ft. Time your test strip, then time a pass on the lawn: amble at 2.5 mph and a spreader calibrated for 2.5 lb puts down 3.0.
| Pace | Seconds per 100 ft | Lands at | Against 2.5 lb |
|---|---|---|---|
| 2 mph | 34.1 s | 3.75 lb / 1,000 | 50% heavy |
| 2.5 mph | 27.3 s | 3.00 lb / 1,000 | 20% heavy |
| 3 mph | 22.7 s | 2.50 lb / 1,000 | on target |
| 3.5 mph | 19.5 s | 2.14 lb / 1,000 | 14% light |
| 4 mph | 17.0 s | 1.88 lb / 1,000 | 25% light |
A spreader calibrated at 3 mph for 2.5 lb per 1,000. The pace effect alone; the wider throw at speed would thin the fast rows further.
How precise does my scale need to be?
More precise than a bathroom scale. Calibrating to ±5% means seeing a 5% difference in the catch, and a bathroom scale moves in 0.2 lb steps. At 2.5 lb per 1,000, a whole 1,000 sq ft strip catches 2.5 lb, and one bathroom-scale step is 8% of it — bigger than the band you're trying to hit. The 100 sq ft test one major brand's instructions use catches 0.25 lb, and one step of a bathroom scale is 80% of that. A kitchen scale reading in grams costs less than a bag of fertilizer and makes the problem disappear.
| Strip | Catch at 2.5 lb / 1,000 | One kitchen-scale step (1 g) | One bathroom-scale step (0.2 lb) |
|---|---|---|---|
| 100 sq ft | 0.25 lb (113 g) | 0.9% of it | 80% of it |
| 250 sq ft | 0.63 lb (283 g) | 0.4% of it | 32% of it |
| 500 sq ft | 1.25 lb (567 g) | 0.2% of it | 16% of it |
| 1,000 sq ft | 2.50 lb (1,134 g) | 0.1% of it | 8% of it |
A step bigger than 5% of the catch (bold) can't see the ±5% band. A bathroom scale needs a catch of 4 lb or more; weighing before and after takes two readings, so it's worse.
Half the rate, twice, crosswise
NC State and STMA both recommend it for broadcast spreaders: calibrate to half the label rate and cover the lawn twice, the second pass at right angles to the first. Skips and overlaps from one direction land on the other's, the stripes cross instead of lining up, and a lopsided pattern matters far less. It costs a second walk and nothing else. Tick "two half-rate passes" above and the calculator calibrates to half — STMA's 6.25 lb example becomes 3.13 lb per pass. It is the same move the overseeding guide makes with seed.
Drop spreaders: easy to calibrate, unforgiving to use
A drop spreader puts product only between its wheels, so the swath is the hopper width and the whole output can be caught in a length of gutter fastened under it — the easiest spreader there is to calibrate. The same thing makes it unforgiving: there's no taper to hide a miss, so passes have to meet exactly, and a gap of an inch is a stripe. NC State calibrates them over 100 sq ft and multiplies the catch by 10. They earn their place on small lawns, along beds where a broadcast throw would land in the mulch, and for anything you don't want on the neighbor's drive.
What this calculator won't do
It won't tell you a setting for your spreader or a rate for your product. The setting depends on your spreader, your product, your pace and your scale — four things this page can't see. The rate is the bag's, and for weed- and insect-control granules (see the pre-emergent timing tool for when) the label is the legal instruction, maximum included. What this page does is arithmetic on what you measured. For liquids, the sprayer calculator does the same job with a bucket instead of a scale.
Sources: NC State Extension AG-628, The Calibration of Turfgrass Boom Sprayers and Spreaders (Buhler et al., rev. 2024) — effective swath by the half-of-center rule, the pan-rate formula, 3 mph, ±10%, and "spreader settings are not linear"; Sports Turf Managers Association, Rotary Spreader Calibration — 1,000 sq ft run length, ±5%, split application at half rate; Spreadmark Code of Practice (Fertiliser Quality Council, New Zealand) — transverse CV limits of 15% for nitrogen fertilizer and 25% for other products; Scotts Miracle-Gro, How to Calibrate a Spreader and Find the Right Settings. Conversions: 453.6 g per lb, 88 ft per minute per mph.