Corn Yield Estimator: Kernel Count, Sample Weight & TKW

What This Tool Does

Estimate grain corn yield in bu/ac and t/ha at 15.5% moisture and 56 lb/bu using kernel counts, harvest sample weights, or measured thousand-kernel weight.

Yield Component (Kernel Count)

Use from about 2 weeks after pollination (R3/milk stage) until black layer, when kernel number is set but kernel weight is still filling. Take at least 5 to 10 stops of 1/1,000th acre (17 ft 5 in on 30-inch rows). Expect an error band of ±20 bu/ac (Wisconsin) to ±30 bu/ac (Purdue). Not suitable for corn silage, pre-R3 fields, or severely lodged or diseased stands.

Harvest Sample Weight

Use after physiological maturity (black layer) or during harvest when you can shell, weigh, and moisture-test grain from a measured area. University of Wisconsin Extension notes this is the most accurate method short of weighing the entire field. Using 1/100th acre (435.6 sq ft) across 5 to 10 locations reduces small-plot variance.

Thousand-Kernel Weight (TKW)

Use after black layer in variety trials, research plots, or post-maturity checks when you have a 0.01 g scale to weigh 3 to 5 clean subsamples of 1,000 kernels dried or adjusted to 15.5% moisture. Do not use this tab with a guessed TKW—in this method, yield is directly proportional to TKW, so any percentage error in TKW shifts yield by that exact same percentage (for example, a 100 g error off a 300 g baseline shifts yield by about 33%), which is no better than guessing a kernel factor.

How It Is Calculated

Every calculation in this tool is anchored to the U.S. grain trade standard for No. 2 corn: a 56-pound bushel at 15.5% moisture (25,401.17 grams per bushel), converted to metric tonnes per hectare using 1 bu/ac = 0.0627676644 t/ha (1 t/ha = 15.9318 bu/ac). In the Yield Component method, multiplying ears in 1/1,000th of an acre by rows per ear and kernels per row estimates total kernels per 1/1,000th acre. Dividing by the kernel factor (default 90, representing 90,000 kernels per 56-lb bushel from University of Illinois and Purdue Extension) gives bushels per acre. Because kernel weight varies widely with hybrid genetics and grain fill, 90 is a rule-of-thumb fudge factor rather than a fixed physical constant—you can adjust the divisor to 85 for modern hybrids (NC State, Ohio State, Bayer), 80 for heavy grain fill, 100 to 105 for drought-stressed small kernels, or 89.605 (University of Wisconsin's 0.01116 multiplier). In the Thousand-Kernel Weight (TKW) method, that assumed divisor is replaced directly by your lab-measured 1,000-kernel weight in grams (adjusted to 15.5% moisture via tkw_15.5 = tkw_wet × (100 - tkw_moisture_pct) / 84.5 if weighed wet) via an algebraic identity derived from the 56 lb/bu standard (dividing kernels per acre × TKW in grams by 25,401,172.72).

When corn has reached black layer or harvest, the Harvest Sample Weight method eliminates kernel-count guesswork by weighing shelled grain from a measured area and correcting for physical moisture shrink to 15.5% using W_15.5 = W_wet × (100 - M_wet) / 84.5 (equivalent to Wisconsin Extension's 21.13 dry-matter shortcut for a 1/1,000th-acre plot). Unlike online calculators that hide their divisors or apply a 15.0% moisture constant (such as the 109.815 factor used by QLE without unit labels), this calculator exposes every step and uses the true 15.5% moisture denominator (84.5). Keep in mind that hand-harvested or combine plot weights only measure grain captured in the sample—they do not account for combine header or threshing losses, commercial elevator drying shrink factors, or test weights that deviate from 56 lb/bu.

Yield Component (Kernel Count)

1. Calculate average kernels per ear: kernel_rows_per_ear × kernels_per_row = kernels_per_ear.
2. Calculate kernels per 1/1,000th acre: ears_per_1000th_acre × kernels_per_ear.
3. Divide by the selected kernel factor (kernels per bushel / 1,000) to get bu/ac at 56 lb/bu: (ears_per_1000th_acre × kernels_per_ear) / kernel_factor.
4. Convert to metric yield: t/ha = bu/ac × 0.0627676644 (and lb/ac = bu/ac × 56).

Harvest Sample Weight

1. Determine harvested sample area in square feet: area_ft2 = row_length_ft × (row_width_in / 12) × rows_harvested.
2. Adjust wet sample weight to 15.5% standard moisture: W_15.5 = wet_weight_lb × (100 - moisture_pct) / 84.5.
3. Scale sample weight to a full acre (43,560 sq ft) and divide by 56 lb/bu: bu/ac = W_15.5 × (43,560 / area_ft2) / 56.
4. Convert to metric yield: t/ha = bu/ac × 0.0627676644.

Thousand-Kernel Weight (TKW)

1. Calculate total kernels per acre: kernels_per_acre = ears_per_acre × kernels_per_ear.
2. If TKW was weighed above or below 15.5% moisture, adjust to 15.5% moisture: tkw_15.5 = tkw_wet × (100 - tkw_moisture_pct) / 84.5.
3. Convert kernel count and TKW (grams per 1,000 kernels) to 56-lb bushels per acre: bu/ac = kernels_per_acre × tkw_15.5 / 25,401,172.72 (where 25,401,172.72 = 1,000 × 453.59237 g/lb × 56 lb/bu).
4. Convert to metric yield: t/ha = bu/ac × 0.0627676644.

Why shouldn't I always leave the kernel factor at 90?

The divisor 90 assumes 90,000 kernels weigh 56 pounds (about 282.2 mg per kernel at 15.5% moisture). University of Illinois and Purdue use 90 as an average baseline, while NC State, Ohio State, and Bayer note that modern hybrids under good grain fill often average 80,000 to 85,000 kernels per bushel (divisor 80 or 85). Under drought or late-season stress, kernels are smaller and it may take 100,000 to 105,000 kernels to make a 56-lb bushel (divisor 100 or 105). Using 90 in a dry year will overestimate yield, while using 90 in a heavy-fill year can underestimate yield by 5.6% or more.

How is the Thousand-Kernel Weight (TKW) method related to the kernel count method?

No university extension publication provides a separate standalone TKW formula. Mathematically, the TKW tab is an algebraic identity that replaces the fixed kernel-per-bushel fudge factor with your actual measured weight per 1,000 kernels at 15.5% moisture (adjusted via tkw_15.5 = tkw_wet × (100 - tkw_moisture_pct) / 84.5 if weighed wet), divided by 25,401.17 grams per 56-lb bushel. For example, at 90,000 kernels/bu, 1,000 kernels weigh 282.235 g; at 80,000 kernels/bu, 317.515 g; and at 100,000 kernels/bu, 254.012 g (note: one industry article lists 225 mg for 100,000 kernels, which equals 49.6 lb/bu and conflicts with the 56 lb/bu standard, so we use the mathematically exact 254.012 g).

Why does the Harvest Sample Weight result differ slightly from some other online plot calculators?

This tool adjusts wet weight to the U.S. standard 15.5% moisture using (100 - moisture) / 84.5. Some web calculators (such as the QLE plot calculator) use a combined constant of 109.815 without labeling input units; back-calculating 109.815 shows it is based on a 15.0% moisture standard (divisor 85.0 instead of 84.5), which understates yield at 15.5% moisture by about 0.45% (e.g., 355.53 vs. 357.14 bu/ac on a 20-lb 1/1,000th-acre sample).

What if I only have whole ear weights instead of shelled grain weight?

After black layer, University of Wisconsin Extension and Bayer provide a quick ear-weight approximation that assumes an average shelling percentage without shelling the sample: bu/ac = [(ears per 1/1,000th acre × average ear weight in lb) / (1.411 × grain moisture % + 46.2)] × 1,000. For example, 24 ears averaging 0.5 lb at 30% moisture estimates 135.55 bu/ac (about 135 bu/ac). Because shelling percentage varies, weighing shelled grain in the Harvest Sample Weight tab is more accurate.

Does the moisture adjustment match what my local grain elevator will deduct?

No. The formula W_15.5 = W_wet × (100 - M_wet) / 84.5 calculates pure physical water weight loss down to 15.5% moisture (for example, 7.69% water weight shrink from 22% moisture, or 23.08% from 35% moisture). Actual elevator total shrink and dockage vary by facility, and this tool does not calculate commercial deductions. Per SDSU Extension, physical moisture shrink to 15.5% is about 1.18% per point of moisture, while commercial elevator total shrink factors commonly run about 1.2% to 1.5% per point, with handling loss calculated separately depending on facility policy.

Limitations & Exceptions

Adjusted to 15.5% standard moisture (No. 2 yellow corn upper limit) using physical water weight conversion: W_15.5 = W_wet × (100 - M_wet) / 84.5. Actual elevator total shrink and dockage vary by facility, and this tool does not calculate commercial deductions. Per SDSU Extension, physical moisture shrink to 15.5% is about 1.18% per point of moisture, while commercial elevator total shrink factors commonly run about 1.2% to 1.5% per point, with handling loss calculated separately depending on facility policy.

Calculated on a standard 56 lb/bu weight basis at 15.5% moisture. Actual field test weight can range from 52 to 60 lb/bu and will shift volume-based bushels by 1% to 5% if not corrected separately. Always sample 5 to 10 representative spots across the field.

Sources & References

Last verified: 2026-11-02

Next scheduled review: 2027-11-02

University Extension & Industry References

For general reference only. Results are estimates and do not constitute legal, tax, or financial advice for your specific situation. Always verify with official rules and notices.

For pre-harvest and field estimation only. Results are mathematical estimates for grain corn (not silage) based on user-supplied samples and standard constants (15.5% moisture, 56 lb/bu) and carry typical sampling error bands of ±20 to ±30 bu/ac for kernel counts; in the TKW method, yield is directly proportional to TKW, so any percentage error in TKW shifts yield by that exact same percentage (e.g., a 100 g error on a 300 g baseline shifts yield by about 33%). These figures do not account for combine harvest losses, test weight deviations, or commercial elevator shrink, and must not be used as a guarantee of actual harvest yield, agronomic advice, or crop insurance/claim documentation.

Numbers you enter are calculated strictly in your browser and are never uploaded or stored.

Related Tools

• England Rent Increase Date and Form 4A Checker