Horse Gestation Calculator

Enter the date your mare was last bred and get her estimated foaling date based on the average 340-day equine gestation period, plus the normal 320–370 day due window.

Quick Facts

Method
Foaling date = breeding date + 340 days
Normal foaling window runs 320 to 370 days after breeding.

Your Results

Calculated
Estimated foaling date
-
Breeding date + 340 days
Earliest normal date
-
320 days after breeding
Latest normal date
-
370 days after breeding
Days remaining
-
From today to the due date

Ready

Enter the mare's breeding date and press Calculate.

About the Horse Gestation Calculator

A mare (a female horse) carries her foal for an average of about 340 days — roughly 11 months. This calculator takes the date the mare was last bred and adds the gestation length to estimate her foaling (due) date. Because equine pregnancy length varies naturally, it also shows the earliest and latest dates of the normal foaling window so you can plan foal watch instead of relying on a single day.

The formula

The calculation is simple date arithmetic:

Estimated foaling date = breeding date + 340 days

The tool also computes the boundaries of the normal range by adding 320 days (earliest) and 370 days (latest) to the breeding date. If you know your mare's own history, you can adjust the average gestation length input — some mares consistently carry a few days shorter or longer than the 340-day average.

Why 340 days?

Across breeds, the widely cited average equine gestation is 335–342 days, with 340 days used as the standard planning figure. The full healthy range spans roughly 320 to 370 days. A foal born before about 320 days is considered premature and may need veterinary support; a mare going past 360–370 days is unusual and worth a veterinary check, though some mares carry to 365+ days and deliver a healthy foal.

Reference points for gestation length

  • Average: ~340 days (about 11 months and 5 days)
  • Most common window: 335–345 days
  • Full normal range: 320–370 days
  • Season effect: mares bred in late winter/early spring (foaling in fall) tend to carry slightly longer than mares bred in late spring (foaling in early summer), by roughly 10 days on average
  • Foal sex: colts (males) are often carried a few days longer than fillies on average

Frequently Asked Questions

How long is a horse pregnant?
A mare's gestation averages about 340 days, or roughly 11 months. The normal healthy range is wide — from 320 to 370 days — with most mares foaling between 335 and 345 days after breeding.
Which date should I enter — first cover or last cover?
Enter the last breeding date (last live cover or the insemination date). If the mare was covered on several days during one heat cycle, the last cover is the closest approximation to conception and gives the most reliable due-date estimate.
Why is the foaling window so wide?
Equine gestation length genuinely varies by 40+ days between healthy pregnancies, driven by breed, the individual mare, the season she was bred, and the foal's sex. That is why the calculator shows a 320–370 day window rather than promising an exact day. Use the 340-day estimate as your center point and begin foal watch a couple of weeks before it.
When should I start watching for foaling?
Begin close observation around 320 days (the earliest normal date shown by the calculator). Physical signs — udder development, waxing of the teats, softening of the tailhead and croup muscles, and relaxation of the vulva — are better short-term predictors than the calendar in the final two weeks.

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