What Is the Mare Gestation Calculator?
A horse's pregnancy lasts roughly 11 months. This calculator estimates a mare's foaling due date by adding her gestation length to the breeding (cover) date. The widely used average for horses is 340 days, though normal foaling can occur anywhere between about 320 and 360 days, which is why we also show a likely two-week foaling window.
How to Use It
Enter the date your mare was bred or covered, then choose a gestation length. Leave it at the default 340 days unless your veterinarian or breed records suggest a different average (ponies and some breeds can run a little shorter or longer). The calculator returns the estimated due date and a \(\pm 14\)-day window around it.
The Formula Explained
The core calculation is simply:
$$\text{Due Date} = \text{Breeding Date} + \text{Gestation Length}$$With the default value, that is breeding date + 340 days. The foaling window is the due date \(\pm 14\) days, reflecting natural variation between individual mares and pregnancies.
Worked Example
Suppose a mare is bred on May 1, 2024 with a 340-day gestation. Adding 340 days lands on April 6, 2025. The expected foaling window would run roughly from March 23, 2025 to April 20, 2025.
$$\text{May 1, 2024} + 340\ \text{days} = \text{April 6, 2025}$$
FAQ
How long are horses pregnant? On average about 340 days (11 months), with a normal range of roughly 320–360 days.
Is the due date exact? No. It is an estimate. Mares foal across a wide window, so monitor for foaling signs as the date approaches.
Does breed affect gestation? Yes, somewhat. Draft and some warmblood mares may carry slightly longer, while ponies can foal a touch earlier. Adjust the gestation length if you have breed-specific data.
