Read the foaling date and range together
Nebraska Extension gives an average mare gestation of 335–340 days and a range of 320–370 days. This calculator uses the specified 340-day reference for the central date and displays the broader cited range beside it. The answer is an estimate for organizing records and preparations, not a scheduled foaling date.
Enter the known breeding date, or select an exposure period if the mare was with the stallion across several dates. The latter keeps the uncertainty in your record visible. Calendar results use elapsed days instead of adding months, so months of different lengths cannot quietly change the arithmetic.
Why the range is wide
Nebraska reports that colts may be carried 2–7 days longer than fillies. Its discussion also describes longer gestation in thin mares and shorter pregnancies with spring or summer foaling than with January or February foaling. These tendencies help explain variation, but they are not dependable adjustments for a particular mare.
The calculator does not ask for a guessed fetal sex, body condition, or season-based correction. Combining such guesses would give a more precise-looking answer without improving the underlying evidence. Keep prior breeding and foaling records as context, and discuss the mare's circumstances with your veterinarian.
The premature cutoff is a separate warning
Foals born before day 326 are considered premature. This matters because the lower end of the reported pregnancy range is earlier than that marker. A date appearing inside the broad range does not establish that a foal born then is mature or safe. The result therefore shows the cutoff separately.
A calendar cannot assess the mare or foal. If foaling timing or any observation concerns you, call your veterinarian. Use the breeding record and the displayed dates to describe the situation, rather than relying on a colored card or a day count as reassurance.
Months, heat watches, and practical records
The “11 months after last breeding” shortcut is a rule of thumb some breeders use. It is not the method used here, because calendar months differ in length. Keep the exact recorded date available when comparing that shortcut with an elapsed-day estimate or a printed chart.
MSD gives an average estrous cycle of 21 days. The calculator shows an approximate return-to-heat watch date without inventing a cited minimum-to-maximum range for mares. MSD notes that a mare not returning to estrus 2–3 weeks after breeding may be pregnant; pregnancy still needs confirmation with your veterinarian.
For several mares, the herd planner keeps each estimate under a separate tag or name and retains the full window on its calendar. The horse gestation guide includes a worked example and source context. Read the heat-cycle guide for seasonal cycling and the difference between predicted and observed heats.