Why a drill needs calibrating at all
A grain drill does not meter seed by weight. It meters by volume — a fluted roll or a cup wheel turning at a speed geared to the drive wheel, dropping a certain volume of seed per revolution. The rate chart riveted to the lid was produced by the manufacturer with one particular seed, at one particular test weight, through a meter that was new. Everything that has happened to your drill since then has moved the number.
Seed size and test weight move it most. A bushel of large, plump wheat and a bushel of shriveled wheat occupy the same volume and weigh differently, so the same meter setting puts down a different poundage. Coated clover is dramatically bulkier than raw clover of the same pure-live-seed content. Seed treatment changes how freely the seed flows. Beyond the seed itself, worn flutes pass more volume than new ones, a stretched or slipping drive chain under-turns the meter, a partially plugged tube passes less than its neighbors, and a drive wheel running in soft ground slips enough to matter over a long field.
None of that shows up while you are drilling. You find out at emergence, and by then the seed is in the ground and the money is spent. Twenty minutes with a tarp and a scale is the only way to know what the drill is actually doing today, with this seed lot, in this condition.
Running the catch test
There are two ways to collect, and both are in Penn State Extension's published procedure.
Drive a measured distance. Stake out a known run — 100 or 200 feet is typical — with the drill loaded and set to your intended rate. Disconnect the seed tubes and catch into bags, or run them into a tarp or catch pans. Drive the run at the speed you will actually plant at, from a rolling start so the meter is already turning when you cross the first stake. Weigh what you caught, remembering to subtract the weight of the bags or pans.
Turn the drive wheel on a jack. If you would rather not move the drill, jack the drive wheel clear, measure its circumference by rolling it one full turn and marking the ground, then turn it the number of revolutions that equals your test distance: revolutions = distance ÷ circumference. A wheel with a 6.5-foot circumference needs about 31 turns to represent 200 feet. Turn it at a steady pace close to field speed; cranking it slowly can under-feed some meter types.
Either way, the arithmetic is the same. Working width is opener spacing times the number of openers, converted to feet. Acres covered is working width times distance divided by 43,560. Rate is pounds collected divided by acres covered. If you caught from only some of the tubes rather than all of them, the calculator scales the catch up by total openers divided by openers sampled before it does the division.
Penn State's own worked example is the sanity check: 2.3 pounds caught from a 10-foot drill over 200 feet is 0.046 acre, and 2.3 ÷ 0.046 is 50 pounds per acre. That example is wired into this site's test suite, so this calculator is verified against the extension service's published answer rather than against itself.
Check the openers individually
A whole-drill catch tells you the average. It does not tell you that opener four is delivering half rate and opener five is making up for it. Penn State's consistency check is worth the extra few minutes: catch each of several tubes separately, weigh them, and compare each to the average of the group. Any opener off by 10 percent or more from the average needs attention — usually a plugged tube, a damaged flute, a seed cup out of adjustment, or trash in the box that has bridged over one opening.
This failure mode is invisible in the field until harvest, when you find alternating thin and thick rows. It is the single most common thing a catch test catches that a rate chart never would.
What a miss actually costs
It is worth doing this arithmetic once, because it is what justifies the twenty minutes.
Say you intend to drill 60 pounds of wheat per acre across 100 acres and the drill is actually running 20 percent heavy. That is 12 extra pounds per acre, 1,200 pounds over the field — roughly 20 bushels of seed wheat you did not need to buy, plus a stand thicker than you planned, which competes with itself, lodges more readily, and can pull yield down rather than up. Run 20 percent light instead and you have a thin stand that never closes the canopy, gives weeds the room they were waiting for, and cannot be fixed after emergence at any price.
Small-seeded forages are less forgiving still, because the rates are small enough that a modest absolute error is a large proportional one. On a clover seeded at 4 pounds per acre, being off by 2 pounds is a 50 percent error. A quarter turn on the wrong adjuster can double a clover rate, and clover seed is expensive enough that the mistake is felt immediately.
When to re-check
Calibrate at the start of every planting season, and again any time you change seed lot, species, variety, or seed treatment. Re-check after the first pass in a new field if the ground conditions are noticeably different, since drive-wheel slip in soft or trashy ground under-turns the meter and quietly reduces the rate. If you change ground speed substantially from what you calibrated at, check again — some meter designs are more speed-sensitive than their charts admit.
Keep the numbers. A short note of seed lot, setting, catch weight, and resulting rate, kept year over year, turns into the most useful record on the drill: it tells you when the meter is wearing, because the same setting with the same seed starts drifting.
Once the drill is putting down what you intend, the companion question is what to put down on the manure side — see spreader calibration for the tarp-test version of the same idea.