Why nobody knows their spreader's rate
Ask ten people what their manure spreader puts down per acre and most will answer with the number on the spec plate, or with a shrug. Both answers are the same answer. A spreader's rated capacity is a volume figure — how many cubic feet the box holds — and manure's weight per cubic foot swings enormously with bedding type, moisture, and species. The same box that holds four tons of wet dairy slurry might hold two and a half tons of bedded-pack material that is half straw. The beaters, the apron chain speed, the gate setting and your ground speed then decide how that load gets spread out over how much ground.
The practical consequence is that manure is routinely applied at two or three times the intended rate near the barn and a fraction of it out at the far end of the farm, which is exactly backwards from what most nutrient plans call for. In states with nutrient management requirements, "I don't know my rate" is also not an acceptable answer on paper. A tarp, a bathroom scale and twenty minutes fixes it.
The tarp method, step by step
This is Penn State Extension's published procedure, and the three-pass part is the piece people skip.
Use a tarp of roughly 100 square feet — 10×10, 9×12 and 10×12 all work, and the exact area matters more than the shape, so measure it rather than trusting the label. Weigh the tarp empty, along with whatever bucket or tote you will scrape it into, and write that tare weight down. Lay the tarp flat in the field, weighted at the corners so a gust or the spreader's air blast does not fold it.
Now make three passes, not one. The first goes straight over the center of the tarp. The second and third go by on either side at your normal overlap spacing — the same spacing you use when you are actually covering a field. This matters because a spreader does not lay down an even rectangle; it lays a heaped pattern that is dense in the middle and thin at the edges, and the real field rate is the sum of a center pass plus the tails of its neighbors. Catch only the center pass and you will overstate your rate badly.
Drive all three passes at the speed you actually spread at, with the gate and PTO settings you actually use, and with a typical load in the box rather than the last few hundred pounds scraped off the floor. Then fold the tarp inward, scrape everything into the bucket, weigh it, and subtract the tare.
The arithmetic from there is one line: pounds collected divided by tarp square feet, times 21.8, gives tons per acre. That constant is not a fudge factor — it is 43,560 square feet per acre divided by 2,000 pounds per ton, so it converts pounds per square foot straight into tons per acre.
Then do the whole thing again. Penn State recommends repeating the test one or two more times and averaging, and the reason becomes obvious the first time you do it: spreaders are not precision instruments, and two consecutive tests can differ by a fifth. One test tells you the order of magnitude. Three tests averaged tell you your rate.
When to recalibrate
Recalibrate whenever the material changes, because the material is most of the variable. New bedding — straw to sawdust, or sawdust to sand — changes both density and how the beaters throw it. A wet spring changes moisture content. Manure from a different group of animals, or from a pile that has composted down over the winter, is a different product than what came out fresh. Changing ground speed or gate setting changes the rate directly. And a stretched apron chain or worn beater will drift the number slowly over a season without any single day where you notice.
A reasonable habit is to calibrate at the start of each spreading season and again any time you would answer "well, it's different manure" to the question of whether anything changed.
The load-count method and its weak spot
If tarping is impractical, you can back into the rate from records: total tons applied divided by acres covered. The arithmetic is trivial and the calculator above will do it. The weak spot is the "tons per load" figure, and it is a serious one, because nearly everyone fills it in with the spreader's rated capacity — which, as above, is a volume number and not your manure's weight.
Get a real weight instead. A loaded and then empty pass over a truck scale, a feed-mill scale, or a grain elevator scale gives you the number directly. Failing that, weigh several loader buckets of the same material on a platform scale and count buckets per spreader load. Either approach is far better than the plate. The load-count method is best treated as a check on the tarp method rather than a replacement for it: it captures the whole-field average including the headlands, which the tarp test does not, but it cannot tell you anything about uniformity.
Broadcast fertilizer and lime
The third calculator handles dry broadcast spreaders, where the catch is collected over a measured distance rather than on a tarp, and the swath width you enter should be the effective width — the spacing you actually drive on — not the width at which a few granules still land. Spinner spreaders throw a pattern that tapers at the edges and is designed to be overlapped; entering the maximum throw distance instead of your driving spacing will make the rate look far lighter than it is.
If you want to check uniformity as well as rate, set a row of shallow catch pans across the full swath, spaced evenly, make a pass, and compare the depth in each. A pattern that is heavy on one side usually means a worn or misaligned spinner, an off-center drop point, or product that has segregated in the hopper.
From tons per acre to nutrient credits
Knowing tons per acre is step one, not the finish line. Turning that into nutrient credits — pounds of nitrogen, phosphorus and potassium available to this year's crop — requires a laboratory manure analysis for your actual material plus availability factors that differ by nutrient, by application method, and by how quickly the manure was incorporated. Surface-applied manure left on top can lose a large share of its ammonium nitrogen to volatilization within a day or two; the same manure injected or incorporated immediately keeps most of it. Book values for "dairy manure" are averages across a range wide enough to be misleading for any particular pile.
That calculation deserves a tool built carefully rather than badly, and it is not on this site yet. Until it is, your state's nutrient management guidance and a manure testing lab are the right stop. What this page gives you is the number that every one of those calculations starts from.
Doing the same exercise on the seeding side? See drill calibration for the catch-test version.