Combine Harvest Loss Calculator (Kernels per Square Foot to Bushels per Acre)
Measure what the combine is leaving behind: turn seed counts — or the weight of grain collected in a pan — into bushels per acre, lb/ac and kg/ha for corn, soybeans, wheat, durum, barley, oats, sorghum, peas, canola, sunflower or any crop by thousand-kernel weight; average several counts, separate pre-harvest, header, dropped-ear and threshing losses, correct for a narrow residue spread, compare with loss targets and put a dollar figure on it per acre and per field.
How it works
seeds per ft² = count ÷ frame area (several counts are averaged) · bu/ac = seeds per ft² ÷ seeds per ft² in 1 bu/ac (corn 2, soybeans 4, wheat & sorghum 20, durum 16, barley 14, oats 10, peas 3, sunflower 3, canola 110 — or bushel weight × 453.6 ÷ (thousand-kernel weight ÷ 1,000) ÷ 43,560) · by weight: 0.0104 g/ft² = 1 lb/ac, bu/ac = lb/ac ÷ bushel weight · header loss = behind-header − pre-harvest (+ dropped ears: one ¾-lb ear per 1/100 acre ≈ 1 bu/ac) · threshing loss = (behind-combine − behind-header) ÷ (header width ÷ spread width) · % loss = machine bu/ac ÷ yield
Grain on the ground behind a combine is yield already grown and paid for, and a few counts with a square frame turn it into bushels per acre because the number of kernels in a bushel is known for each crop — two corn kernels or four soybeans on every square foot is a bushel an acre. Counting in three places separates what the crop dropped before the machine arrived, what the header failed to gather, and what the threshing and cleaning system threw over; since the machine’s losses fall in a strip as wide as the discharge rather than the header, that count is scaled back before comparing. Single frames vary a lot, so “Several counts” lets you type the count from each spot and averages them. For small seeds such as canola, where counting is impractical, weigh what a frame or drop pan collects instead — 0.0104 g on every square foot is one pound an acre. Seed size changes the conversion, so a thousand-kernel weight under More options replaces the rule of thumb and makes any crop possible. In corn, whole dropped ears often outweigh loose kernels: count them over 1/100 acre (the calculator shows how far to walk for your row spacing and head) and each ¾-lb ear is about a bushel an acre. The split points straight at the adjustment to make, and the dollar figure is usually persuasive.
Worked example
In 60 bu/ac soybeans with a 10 ft² frame: 6 seeds ahead of the combine is 0.15 bu/ac of shatter; 30 behind the header means the header is dropping (30 − 6) ÷ 10 ÷ 4 = 0.6 bu/ac; 70 behind the combine with the spreader covering the full 35 ft header adds (70 − 30) ÷ 10 ÷ 4 = 1.0 bu/ac of threshing loss. Machine loss of 1.6 bu/ac is 2.7% of yield — high, about $17.60 an acre and $3,520 on 200 acres — with the header responsible for 38% and the separator for the rest; getting down to the 1 bu/ac target is worth $1,320 on the field. If the spreader only covered 17.5 ft, the threshing count would be halved before comparing.
Sources: NDSU Extension — Measuring harvest losses in corn and soybean · Iowa State Extension — Verifying combine adjustments during crop harvest · University of Georgia Extension B 973 — Measuring field losses from grain combines · Canola Council of Canada / PAMI — Combine seed loss guide · K-State Research and Extension — Sunflower harvesting (PM33)
Frequently asked questions
Why count behind the header with the combine backed up?
The area under the header has been cut but the residue has not yet been discharged over it, so what lies there is only pre-harvest loss plus what the header shattered or missed. Subtracting the standing-crop count isolates the header’s share.
My spreader throws residue 20 ft behind a 40 ft header — does that matter?
Yes — all the machine’s losses land in that 20 ft strip, so a count there is double the true per-acre rate. Enter both widths and the calculator divides the threshing loss by header width ÷ spread width (2 in that case), or count across the full header width.
How many soybeans on the ground equal a bushel per acre?
About four beans per square foot (43 per m²) is one bushel per acre, roughly 67 kg/ha. For corn it is two kernels per square foot, for wheat and sorghum about twenty, for barley fourteen and oats ten. Large- or small-seeded varieties shift these numbers — enter a thousand-kernel weight to use your own.
What harvest loss is acceptable?
Extension services such as NDSU suggest holding corn and soybean losses to 1–2 bu/ac; many operators aim for 1 bu/ac or about 1–2% of yield, and the University of Georgia puts typical good harvesting in the 2–4% range for total losses. Above that, a stop to adjust almost always pays.
How do I measure canola losses?
Canola seed is too small to count reliably. Put a pan or frame on the ground, weigh the seed collected (a pocket scale reading to 0.01 g helps) and choose Weight — the calculator converts grams per area to lb/ac and bu/ac, correcting for the residue spread width.
Related calculators
Agronomic rules of thumb and standard grain-trade formulas from extension services. Local elevators, labs and equipment dealers publish their own schedules and specifications — use those where they differ, and confirm lime and fertilizer rates with a current soil test.