Harvest Timing Calculator
Estimate the optimal harvest date for your crop by combining planting date, growing degree day accumulation, and field drying rate. Use it when planning harvest logistics for corn, soybeans, or grain crops.
About this calculator
Harvest timing depends on two sequential phases: crop physiological maturity and grain drydown in the field. Growing Degree Days (GDD) measure heat accumulation above a base temperature (typically 50°F for corn) that drives crop development. Once a crop reaches its required GDD total, it achieves maturity, but grain moisture may still be too high for safe storage. The formula used here is: harvestDay = plantingDate + (gddRequired / avgDailyGDD) + ((25 - moistureTarget) / dryingRate). The first term converts GDD requirements into calendar days. The second term estimates additional days needed for field drying from an assumed starting moisture of 25% down to your target. Combining these gives a projected day-of-year for harvest, helping farmers align equipment, labor, and storage capacity.
How to use
Suppose you planted corn on day 120 (April 30), the crop requires 2,700 GDD, your region averages 18 GDD/day, your target moisture is 15%, and the field dries at 0.5%/day. Step 1 — GDD days: 2,700 / 18 = 150 days to maturity. Step 2 — Drying days: (25 − 15) / 0.5 = 20 additional days. Step 3 — Harvest day: 120 + 150 + 20 = day 290, which is approximately October 17. Plan your harvest equipment and grain bins around that target date.
Frequently asked questions
What are growing degree days and how do they affect harvest timing?
Growing degree days (GDD) are a measure of heat accumulation calculated as the average of a day's high and low temperature minus a crop-specific base temperature. Crops need a fixed GDD total to reach physiological maturity, regardless of the calendar date. A warmer-than-normal season means fewer calendar days to maturity, while a cool season stretches the timeline. Tracking GDD gives a far more accurate maturity estimate than simply counting days from planting.
Why does target moisture content matter when calculating harvest date?
Harvesting grain at too-high moisture risks spoilage during storage and increases propane drying costs. Most grain elevators require corn below 15% and soybeans below 13% moisture for standard discounts. The field drying phase after physiological maturity can take days to weeks depending on weather and hybrid. Building drydown time into your harvest date calculation prevents costly discounts and storage losses.
How does field drying rate vary by crop and weather conditions?
Field drying rate is highly dependent on temperature, humidity, wind, and sunlight exposure after crop maturity. Corn typically dries at 0.4–0.8 percentage points per day in favorable fall weather, but this can drop below 0.2%/day during cold, wet periods. Soybeans dry faster than corn once pods are set. Checking local historical drying rates from your extension service will make your harvest date estimate much more reliable than using a generic default.