Corn Yield Calculator - Estimate Bushels Per Acre

What is a corn yield calculator?

A corn yield calculator estimates how many bushels of corn a field will produce before harvest. It uses the kernel count method, a technique first described by the University of Illinois and still taught by extension agents today. A farmer counts kernels on a sample of ears and counts ears in a sample of row. The calculator converts those counts into bushels per acre and total bushels for the field.

Corn yield formula

The kernel count method uses one formula:

$\text{Yield (bu/acre)} = \frac{\text{Kernels per Ear} \times \text{Ears per Acre}}{\text{Kernels per Bushel}}$

To find the total harvest for a field, multiply the per-acre yield by the field size:

$\text{Total Yield (bushels)} = \text{Yield per Acre} \times \text{Field Size (acres)}$

• Kernels per ear is the average number of kernels counted on sample ears.
• Ears per acre is the number of harvestable corn ears growing in one acre.
• Kernels per bushel is the number of kernels assumed to make up one bushel. The calculator uses 75,000 by default, and the value can be changed.

How many kernels are in a bushel?

A bushel of corn is defined as 56 pounds of grain at 15.5% moisture, the standard used in commercial grain trading. The method was first published with a fixed "fudge factor" that represents 90,000 kernels per 56-pound bushel at 15% grain moisture. Kernel weight varies with the hybrid and the season. Large, well-filled kernels mean fewer kernels per bushel. Small, stressed kernels mean more. In recent Purdue trials, kernel weight ranged from 67,000 to 94,000 kernels per bushel, with an average of about 76,000. Purdue now suggests running the estimate with factors of 65, 75 and 85 (thousand kernels per bushel), and using values of 90 to 100 for seasons with stressful grain fill.

The calculator's default of 75,000 is the middle of Purdue's suggested set. A user can enter 65,000 or 85,000 to see a range, or 90,000 to 100,000 after a drought or heat during grain fill. A lower factor gives a higher yield estimate.

How to measure the two inputs

Kernels per ear

A typical ear holds 400 to 600 kernels, arranged in 16 to 20 rows with 20 to 40 kernels per row. To count one ear, count the number of rows around the ear, count the kernels in one row from base to tip, then multiply the two numbers. Weather and nutrition after pollination affect this count most: good rain and nitrogen produce fuller ears, while heat or drought during grain fill produces fewer kernels, especially near the tip.

Purdue's version of the method records rows and kernels per row for every fifth ear in the sample row, and repeats the sampling in several parts of the field, not just at the edge. Edge rows get more sunlight and less competition, so they are not representative of the whole field.

Ears per acre

This number depends on plant population and row spacing. Plant populations of 28,000 to 36,000 plants per acre are common in commercial fields, though not every plant produces a usable ear. For 30-inch row spacing, 17.4 feet of row equals 1/1,000 of an acre. Counting the harvestable ears, not bare stalks, along that stretch and multiplying by 1,000 gives ears per acre. Repeating this count in several spots across the field, not only near the road, gives a more reliable average.

Best timing

The method can be used as early as the milk stage (R3), once kernel number is set. Later counts, from the dent stage (R5) onward, are more reliable because kernel weight has mostly stopped changing. Counting early risks overestimating yield, because grain fill may still fail under late-season stress.

Worked examples

Example 1: standard field. A field covers 80 acres. Sampling gives an average of 500 kernels per ear and a stand of 30,000 ears per acre.

• Yield per acre = (500 × 30,000) ÷ 75,000 = 200 bushels per acre
• Total yield = 200 × 80 = 16,000 bushels

Running the same counts with Purdue's other factors gives a range. At 65,000 the yield is 230.77 bushels per acre. At 85,000 it is 176.47 bushels per acre.

Example 2: dense planting. A field covers 40 acres. Higher plant density gives more ears but slightly smaller ears: 450 kernels per ear and 36,000 ears per acre.

• Yield per acre = (450 × 36,000) ÷ 75,000 = 216 bushels per acre
• Total yield = 216 × 40 = 8,640 bushels

Example 3: drought-affected field. A field covers 60 acres. Drought during grain fill cut kernel counts to 350 per ear, with a stand of 28,000 ears per acre. Stress also makes kernels smaller, so the factor is raised to 90,000, as Purdue suggests for stressful grain fill.

• Yield per acre = (350 × 28,000) ÷ 90,000 = 108.89 bushels per acre
• Total yield = 108.89 × 60 = 6,533.33 bushels

At the default 75,000 the same counts give 130.67 bushels per acre, which would likely overestimate this field.

Each total is calculated from the unrounded per-acre figure, so multiplying the rounded per-acre value shown above by the field size can give a slightly different result than the total shown.

How accurate is this method?

Purdue describes the method as probably accurate only to within plus or minus 20 bushels per acre. One source of error is the kernels-per-bushel factor. Too low a factor in a year of poor grain fill makes the estimate run high. Too high a factor in a year of excellent grain fill makes it run low. The rest of the gap comes from harvest losses (ears left on the ground, kernels lost through the combine), weather between the estimate and harvest, and natural field variability. It is an estimate, not a substitute for weighed grain.

Other ways to estimate corn yield

• Weight-based sampling: weighing a set of sample ears and applying moisture and test-weight conversions. Faster than counting kernels, generally less precise.
• Combine yield monitors: sensors on the combine that measure grain flow during harvest and build a yield map. Accurate only when properly calibrated.
• Satellite or drone imagery: plant-health data (NDVI) combined with historical yield records to predict yield before harvest. Mostly used on large operations with data to train the models.
• Crop simulation models, such as APSIM or DSSAT, which combine weather, soil, and hybrid data. Used mainly in research settings.

Frequently asked questions

How do you calculate bushels of corn per acre? Multiply the average kernels per ear by the number of ears per acre, then divide by the kernels per bushel. For example, 500 kernels per ear and 32,000 ears per acre gives (500 × 32,000) ÷ 75,000 = 213.33 bushels per acre.

What is the standard number of kernels in a bushel of corn? There is no fixed number. The method traditionally used 90,000 kernels per 56-pound bushel. Purdue trials have measured 67,000 to 94,000 kernels per bushel, with an average of about 76,000, so the true figure changes from year to year and hybrid to hybrid. This calculator defaults to 75,000 and lets the user change it.

When is the best time to estimate corn yield before harvest? From the milk stage (R3) onward. Counts from the dent stage (R5) to physiological maturity (R6) are the most reliable, because kernel number and kernel fill have mostly stabilized by then.

What is considered a good corn yield per acre? It depends on the region, the hybrid, and whether the field is irrigated. The USDA National Agricultural Statistics Service publishes national average corn yields by year, and state averages are available from the same agency. Comparing a field with its state average for the same season is the usual benchmark.

How accurate is the kernel count method? Purdue puts the method's accuracy at roughly plus or minus 20 bushels per acre. Sampling several spots across the field improves the estimate, but it remains an estimate.

Can this method be used for sweet corn? No. This formula applies to field corn (grain corn), which is measured in bushels at maturity. Sweet corn is harvested earlier, while kernels are still immature, and is sold by the ear or by weight, not by the bushel.

History

Before systematic sampling methods, farmers estimated yield mostly by visual inspection and comparison with past harvests. The kernel count formula was first described by the University of Illinois, and extension services at other land-grant universities have taught and refined it since. Combine-mounted yield monitors, satellite imagery, and machine-learning yield models have added new estimation tools since the 1990s, but the kernel count method remains in wide use for its simplicity and low cost.

References

1. Nielsen, R.L. Purdue University Department of Agronomy. Estimating Corn Grain Yield Prior to Harvest. Purdue: Estimating Corn Grain Yield Prior to Harvest — the yield component method, first described by the University of Illinois; the traditional factor of 90,000 kernels per 56-lb bushel at 15% moisture; recent trial kernel weights of 67,000 to 94,000 (average about 76,000); suggested factors of 65, 75 and 85 thousand, or 90 to 100 after stressful grain fill; accuracy of about plus or minus 20 bu/acre; 17.4 ft of 30-inch row as 1/1000th acre.
2. Ohio State University Extension. Corn Production Facts and Conversions (AGF-502). Ohioline AGF-502 — 56.00 lb of shelled corn at 15.5% moisture equals one bushel of No. 2 corn; plant population table 28,000 to 36,000 plants per acre.
3. USDA National Agricultural Statistics Service. Corn: Yield by Year, US. NASS corn yield chart — national average corn yield published each year.