Horse Gestation Calculator

Enter the mare's last breeding date. You get an estimated foaling date, the wide normal window and a prep calendar.

Pregnancy calendar and key dates

Check with your vet. This is an estimate from average gestation lengths. Your vet can confirm the pregnancy, date it more closely, and tell you when to call for help at birth.

How long is a horse pregnant?

A mare carries a foal for about 340 days, or 11 months. Purdue Extension gives 340 days with a range of 315 to 365. University of Nebraska puts the average at 335 to 340 days and says the range can run from 320 to 370 days.

Horse pregnancy varies more than most farm animals. Nebraska extension reports that colts tend to be carried 2 to 7 days longer than fillies. Mares due in late spring and summer tend to foal sooner than mares due in January or February. Many mares repeat their own pattern each year, so keep records.

How to use the foaling calculator

Enter the last date the mare was bred. The tool adds 340 days for the foaling date and shows a 320 to 370 day window. It also lists vaccine months and foaling prep dates that you can save to your calendar.

Many breeders plan around 11 months after the last breeding. Nebraska extension notes that most mares go longer than that, so planning early means fewer surprise foalings. Our window starts at day 320 so you can be ready.

Mare due date formula

Foaling date equals last breeding date plus 340 days. The normal window runs from day 320 to day 370. Foals born before day 326 count as premature and usually need a vet right away, according to University of Nebraska extension.

Foaling date = last breeding date + 340 days

Example: a mare bred on April 1, 2026 is due about March 7, 2027.

Foaling prep calendar

Purdue advises EHV (rhino) vaccines at months 5, 7 and 9, and core vaccines 30 to 45 days before foaling. Nebraska advises opening a Caslick at least 30 days before foaling and moving the mare to the foaling area 2 to 3 weeks ahead.

• First 60 days: keep stress low, with little or no hauling, per Purdue.
• Months 5, 7 and 9: rhinopneumonitis vaccine. Purdue says high risk farms may start at month 3.
• 30 to 45 days before: pre foaling vaccines to raise antibodies in colostrum. Your vet picks the list.
• Signs of foaling: the udder grows 2 to 6 weeks before. Teats fill 4 to 6 days before. Waxing shows 1 to 2 days before (Nebraska).

Questions people ask

Other animals

• Cow: 283 days
• Dog: 63 days
• Goat: 150 days
• Sheep: 147 days
• Pig: 114 days
• Cat: 65 days
• Rabbit: 31 days
• Alpaca: 342 days

References

1. What to Expect When Your Mare is Expecting (VA-26-W). Purdue University Extension.
2. Management of the Pregnant Mare. Extension Horses (K. Anderson, University of Nebraska).
3. Approximate Gestation Periods (table). Merck Veterinary Manual.

Written and checked by the Gestation Calculator team against the sources above. Last updated October 20, 2026. How we build these numbers.