Horse Gestation Calculator (mare foaling date, about 340 days)

Understanding the horse gestation calculator — concept and principle

This calculator gives a mare's expected foaling date and normal foaling window from the breeding date, shows how far along she is today, and marks the date before which a foal would be premature. Horse owners and breeders use it to plan foaling supervision, vaccinations and vet checks.

Counting 340 days: with the default inputs the result is Mar 30, 2027 (expected foaling).

How it is calculated

Expected foaling = breeding date + 340 days.
Normal window = day 320 to day 370; average range begins at day 335.
Premature before day 326.
Days pregnant = today − breeding date; days to due date = 340 − days pregnant.
Week = floor(days ÷ 7) + 1; progress = days ÷ 340.

The principle

Mares carry their foals for 335–340 days on average, about 11 months, but the normal range is wide, from about 320 to 370 days. Mares bred early in the year often carry a little longer than those bred in summer, and individual mares tend to repeat their own gestation length from year to year. A foal born before about day 326 is premature and usually needs a vet straight away; delivery before about day 310 is classed as an abortion. The calculator therefore shows the whole window, not just the average.

Worked example

Default inputs: bred on Apr 24, 2026, viewed on Nov 10, 2026.

Expected foaling = Apr 24, 2026 + 340 days = Tue, Mar 30, 2027.
Window = Wed, Mar 10, 2027 (day 320) to Thu, Apr 29, 2027 (day 370); average range begins Thu, Mar 25, 2027 (day 335).
Premature before day 326 = Tue, Mar 16, 2027.
Days pregnant = 200; days to due date = 140; week 29; progress = 200 ÷ 340 = 59%.
Stage = before the normal birth window.

How to read the result

Plan foaling supervision for the whole window, not just the expected date. Signs that foaling is near include udder development, waxing of the teats and relaxation of the muscles around the tail head. Keep a record of each mare's actual foaling date; her own past lengths are the best guide for the next pregnancy.

Common mistakes

Using the first breeding date when the mare was covered again later.
Expecting foaling exactly on day 340.
Treating a pregnancy past day 340 as overdue; much longer is still normal.
Missing the premature date when planning vet cover.

Keep in mind: This is an estimate, not a diagnosis. Call your vet if the mare shows signs of colic, discharge, early udder development or if foaling does not progress.

Frequently asked questions

How long is a horse pregnant?

335–340 days on average, with a normal range of about 320–370 days.

When is a foal premature?

When born before about day 326 of gestation.

Can I just plan for 11 months?

Many owners do as a quick estimate, but watch for signs from about day 320.

Sources: University of Nebraska–Lincoln Extension (K. Anderson), "Management of the Pregnant Mare", horses.extension.org (2020)

Checked 9 Nov 2026. University of Nebraska–Lincoln Extension (K. Anderson), “Management of the Pregnant Mare”, Extension Horses (horses.extension.org), 2020: average 335–340 days, range about 320–370 days, premature before 326 days, abortion before 310 days, 11-month planning estimate.

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