Spreader Calibration Calculator

Why nobody knows their spreader's rate

Most people answer with the number on the spec plate. Rated capacity is a volume figure — how many cubic feet the box holds — and manure's weight per cubic foot swings with bedding, moisture and species. The same box that holds four tons of wet dairy slurry might hold two and a half tons of bedded pack that is half straw. Beater speed, gate setting and ground speed then decide how far that load gets stretched. The usual result is two or three times the intended rate near the barn and a fraction of it at the far end of the farm. Penn State Extension is blunt: "Calibrating the manure spreader is the only way to know actual manure application rates."

The tarp method, step by step

Use a tarp of roughly 100 square feet — 10×10, 9×12 and 10×12 all work. Measure it rather than trusting the label. Weigh it empty along with the bucket you will scrape it into and write that tare weight down. Lay it flat, weighted at the corners so the spreader's air blast does not fold it.

Now make three passes, not one: one over the center, the other two by on either side at your normal overlap spacing. A spreader lays a heap, dense in the middle and thin at the edges, so the real field rate is a center pass plus the tails of its neighbors. Catch only the center pass and you will overstate your rate badly. Drive all three at the speed, gate and PTO settings you actually use, then fold the tarp inward, scrape it into the bucket, weigh, subtract the tare.

The arithmetic is one line: pounds collected divided by tarp square feet, times 21.8, gives tons per acre. That constant is 43,560 square feet per acre divided by 2,000 pounds per ton.

Then do it again. Penn State's procedure makes several measured runs and ends with a single instruction: "Calculate the average of each of the measured manure application rates." It does not publish how far apart two consecutive tests should be expected to fall, and neither will we — but they do move, which is the reason for averaging. Recalibrate whenever the material changes — new bedding, a wet spring, a pile that composted down over winter — and whenever you change speed or gate setting.

A tarp size that does the arithmetic for you

UGA Extension Circular 825 publishes the same constant in a more usable form. It uses tarps cut to 4 ft 8 in square — 21.8 square feet — so "the application rate in tons/acre is equal to the pounds of manure on the tarp." Catch nine pounds, you are spreading nine tons to the acre. Georgia also lays seven identical tarps in a line — "equally spaced in a line perpendicular to the travel of the spreader," at "Eight-foot (on-center)" — and weighs each separately. One tarp gives you a rate. Seven give you a rate and the shape of the pattern.

The load-count method and its weak spot

If tarping is impractical, back into the rate from records: tons applied divided by acres covered. The weak spot is tons per load, because nearly everyone fills it in with rated capacity — a volume number, not your manure's weight. Weigh it instead: a loaded and then empty pass over a truck or elevator scale, or weigh loader buckets and count buckets per load. Load-count captures headlands, which the tarp test does not, but says nothing about uniformity.

Broadcast fertilizer and lime: rate is the easy half

The third calculator handles dry broadcast spreaders, where the catch is collected over a measured distance. The arithmetic is not where people go wrong. The width is, and it scales the whole answer.

Spread width and effective width are different numbers

A spinner throws a heap: dense behind the machine, tapering to nothing at the edges. The thin edge of one pass is supposed to land on the thin edge of the next, and the two added together are supposed to equal the middle. How far granules fly is the spread width. How far apart you drive is the effective swath width, always the smaller number.

Four extension services define it the same way. NC State: "The effective swath width is the portion of the total swath width that receives at least 50% of the application rate," and "the center-to-center distance between adjacent passes should be the same as the effective swath width to obtain the desired overlap." Iowa State, UGA Circular 798 and Texas A&M E-539 restate the same half-of-the-center-tray rule. So: find the half-of-the-middle point on each side; the distance between them is your driving spacing.

LSU AgCenter names the failure: "A common error in using this method is to measure the total throw width and then use that as the effective pattern width." Two things happen at once. Your calculated rate reads too light, because you divided by too big a number. And in the field the middle of every pass gets a double helping while the seam gets almost nothing — the June stripes.

Which way you drive changes the pattern

Back and forth in adjacent lands means, in LSU's words, "the right side of the pattern overlapping with the right side of the adjacent pattern." A racetrack circuit puts right against left, which cancels a lopsided pattern — the machine lays its heavy side against its own light side, where back-and-forth doubles the error. If the machine is not skewed the extra driving buys nothing: once the back-and-forth pattern is already at an acceptable CV, LSU reports "there is statistically no advantage to using the more cumbersome circuitous mode of operation."

Coefficient of variation: the number that grades the pattern

Rate and uniformity are separate problems, and a rate test cannot see uniformity. A perfect 200 lb/ac average can be 320 down the tire tracks and 80 at the seams. The measure of the second problem is coefficient of variation — LSU defines it as "the standard deviation of individual application rate values across an overlapped spreader pattern divided by the mean application rate and expressed as a percentage."

The targets are from the same publication: "CV values of 10% are desired, but CV values of 20% are generally acceptable with most products." That is the only land-grant CV threshold we located, and it is a general granular figure. We could not find an extension publication that sets a different acceptable CV for lime, or for seed. The intuition that lime tolerates more scatter is widely repeated, but nobody we found has put a number on it, so this page will not either.

The word "overlapped" is the part people miss. CV is computed after passes are added together at your chosen spacing — a single pass always looks terrible, and is supposed to. That is why changing your spacing changes your CV without touching the machine.

Running a pan test, in the field, on your own

The formal version is ASABE Standard S341, currently S341.5 (R2022). It is paywalled and we have not read the full text; what follows is the procedure as the extension services publish it, and Ohio State's FABE-561 cites S341.4 and ISO 5690 as its basis.

1. Get pans, all identical. Ohio State specifies pans "between 1 and 3 ft² in area, 4 to 6 in. deep" with "a gridded baffle (half of the total collection pan height) placed in the bottom." Without the baffle granules bounce back out, and the pans nearest the machine lose the most. NC State uses disposable aluminum baking pans, "21 in. × 13 in. × 3 in."
2. Lay them across the direction of travel. Ohio State uses "2.5-ft intervals" for agronomic spreaders, "with pans on either side of the center pan removed to allow for unobstructed passing of the spreader tires." UGA Circular 798 uses 15 pans at 5-ft intervals.
3. Cover more than you think you need. NC State: "Place at least 10 pans in the effective swath width and five more on each side to cover the total swath width." Ohio State's table runs from 23 pans for a 30-foot swath to 71 for a 90-foot swath. If the pans stop before the granules do, you cannot find the half-rate point.
4. Drive it like you mean it. Open the gate before the pan line so the meter is running, hold normal ground speed, pass over the center. Texas A&M suggests several passes if one catch is too small to weigh accurately.
5. Weigh each pan, in grams. UGA tips each into a matching test tube in a rack, so the pattern draws itself. Find the half-of-the-middle points and measure between them.

What the pattern shape tells you

A flat top, an oval and a pyramid all overlap cleanly; UGA says these three "are the most desirable because they allow a more uniform overlapping of the swaths." Three others you do not want:

Most of those are a five-minute fix with a wrench: the test tells you which bolt to turn. On disc speed, Ohio State's tolerance is tight — readings "should not vary more than 10 RPM" between the two discs.

Where this goes wrong

The product sorts itself in the air. A spinner is a centrifuge, and mass climbs steeply with size — LSU: "doubling the diameter of a granule will increase the mass by eight times." With a blend, that separates the blend in flight: "the resulting pattern will have a higher percentage of nitrogen near the center of each pass with a higher percentage of P and K near the overlap points." Total pounds per acre can be dead right while individual nutrients run in stripes, and a rate calibration cannot see it. The fix is at the blender: Iowa State's screening test says the amount of each material caught on a given screen size "should not differ by more than 10 percent of the total for that material."

Fines wreck a pattern. Iowa State: "if urea or other materials have a significant percentage of particles less than 1 mm it is difficult to throw very far with a spinner and a narrower swath is required." Missouri's Peter Scharf puts it memorably — "you can't throw dust very far" — and notes that every pass through an auger breaks granules down further. Put a hand in the hopper; if it comes out dusty, tighten your spacing.

Moisture changes the product under you. LSU: "As the fertilizer picks up moisture, it will become less flowable and its angle of repose will increase. The delivery rate through the port(s) will thus decrease." A morning calibration on dry product can be wrong by afternoon, in the direction of under-applying.

Wind — and two authorities that disagree. UGA Circular 798 says it is difficult to keep a uniform rate "if wind speeds exceed 10 miles per hour." Ohio State FABE-563 puts the threshold at 15 mph. Both are land-grant publications and we are not going to average them into a fake number. Read it as a band: somewhere between 10 and 15 mph you stop controlling where material lands, and the finer the product, the closer to 10 that point sits.

Worn and bent metal. The vanes are the only thing deciding where a granule leaves the machine. Ohio State: "Replace worn and bent spinner-discs and/or blades." Nothing breaks; the pattern just widens or skews a few percent a season, so the calibration goes stale with no event you would notice.

Ground speed matters if the meter is fixed-rate rather than ground-driven. LSU: a spreader at 2 lb/min over 500 ft²/min applies 4 lb per 1,000 ft², and at 10% more speed "the rate will decrease to 3.6 lb/1,000 ft²." Speed moves the pattern less than you would guess — "pattern changes do not become statistically significant until the speed change exceeds approximately 25%."

The setting chart is a starting point. UF/IFAS: "Settings based on charts can be off by as much as 50%." It describes a test the manufacturer ran once, with one product at one density, through a machine that was new and clean and dry.

A fully worked broadcast example

All the numbers below are made up so you can follow the arithmetic. Nothing here is a published rate.

Nine one-square-foot pans on 5-foot centers, three passes at 5 mph, each weighed in grams. Say the three pans nearest center average 96 grams; half of that is 48. Work outward until the catch drops through 48 — suppose that happens 22 feet left of center and 20 feet right. Effective swath = 22 + 20 = 42 feet. It is not symmetrical; almost nothing is. Granules were still landing 35 feet out, which would have tempted you into calling it a 70-foot machine. Now run a measured 200 feet with a catch tray under the discharge and collect 4.2 pounds.

The mistake, priced. Had you used the 70-foot throw width: 200 × 70 = 14,000 sq ft = 0.3214 acres, and 4.2 ÷ 0.3214 = 13.1 lb/acre. You would have read 13 when the machine was putting down 22 — a 40% under-read — then opened the gate and ended up two-thirds heavy. The width error always errs toward over-applying.

What this calculator does not know

• Your pattern. All three return an average rate. None can see whether that average is even or piled in stripes. That is what a pan test and a CV are for.
• Whether your effective width is right. It takes whatever width you type. Type the throw width and it returns a wrong answer with the same confidence as a right one.
• Your headlands. The tarp test measures the middle of a field driven straight at constant speed; load-count captures the turns. That is why they disagree, and the disagreement is informative rather than an error.
• What is in the manure. Tons per acre is not pounds of nitrogen per acre. That needs a lab analysis plus availability factors that vary by nutrient, by application method and by how fast you incorporate.
• Today's weather, or anything lime-specific. Humidity, wind and hopper dwell time all move the result and none is an input. The CV target we found is a general granular figure; if a separate lime standard exists, we did not find it in a land-grant publication.

How this differs from calibrating a drill

A drill's working width is a fact of geometry — opener spacing times opener count — and does not change with product, speed, wind or wear. What a drill gets wrong is metering: worn flutes, a slipping drive wheel, one plugged tube. A spreader is the mirror image. Metering is easy to verify — Ohio State expects it "within 5% (1%–2% is attainable) of the target amount" — and the hard, invisible problem is the pattern. The drill test on a spreader tells you the rate and nothing about the stripes.

Frequently asked questions

How do I calibrate a manure spreader with a tarp? Weigh an empty tarp of about 100 square feet with the bucket you will scrape it into. Make three passes at normal speed — one over the center, one each side at your overlap spacing. Weigh the catch, subtract the tare, divide pounds by tarp square feet and multiply by 21.8. Repeat and average.

What is the difference between spread width and effective swath width? Spread width is how far granules fly. Effective swath width is how far apart you drive: the point on each side where the catch falls to half of the center pans. Using throw width instead is, in LSU AgCenter's words, "a common error," and it makes your calculated rate read light.

What CV should I be aiming for? LSU AgCenter: "CV values of 10% are desired, but CV values of 20% are generally acceptable with most products." That is the only land-grant threshold we could find, and it is a general granular figure — no publication we located sets a different acceptable CV for lime or for seed.

Can I just use the spreader setting chart? Start there, then check it. UF/IFAS says settings from charts "can be off by as much as 50%." Clemson puts it more gently: a label's recommended setting "is a great place to start, but is not always accurate."

How windy is too windy? The publications disagree, so treat it as a band: UGA puts the limit at 10 mph, Ohio State at 15 mph. Fine and dusty products lose control first.

Why does blended fertilizer spread unevenly? Because the spinner sorts it. Granule mass rises with the cube of the radius, so big particles fly farther — LSU describes nitrogen concentrating near the center of each pass and P and K near the overlap points.

Sources

• Penn State Extension — Manure Spreader Calibration (Martin, Beegle)
• UGA Extension Circular 825 — Calibration of Manure Spreaders (Ritz)
• LSU AgCenter — Granular Spreaders: Selection, Calibration, Testing, and Use (Parish)
• Ohio State Extension FABE-561 — Spinner-Disc Spreader Set Up and Calibration
• Ohio State Extension FABE-562 — Correcting Irregular Spread Patterns
• Ohio State Extension FABE-563 — Proper Spinner-Disc Spreader Operation, Terms and Definitions
• NC State Extension — Spinner Spreader Calibration for Land Application of Poultry Litter
• UGA Extension Circular 798 — Calibration of Dry Broadcast Fertilizer Applicators
• Iowa State Extension and Outreach — Equipment considerations: dry granular fertilizer (Hanna, Sawyer)
• UF/IFAS Extension SS-AGR-90 — Calibrating Forage Seeding Equipment
• Texas A&M AgriLife Extension E-539 — Spreader Calibration for Turfgrass
• Clemson Cooperative Extension HGIC 1657 — Calibrating Spreaders
• University of Missouri IPM — Streaky Nitrogen Applications (Scharf)
• ASABE Standard S341.5 (R2022) — Procedure for Measuring Distribution Uniformity and Calibrating Granular Broadcast Spreaders. Paywalled; cited for designation, title and scope only, full text not read.

Doing the same exercise on the seeding side? See drill calibration, and the food plot calculator for the rates you would be calibrating toward.