Horse Gestation Calculator
Use this free horse gestation calculator to find your mare's estimated foaling due date and see how many days in foal she is right now. Enter the breeding or last cover date and you get the due date, the normal foaling window, and a month-by-month timeline of what to do before the foal arrives.
This tool is for owner education only. It does not diagnose, treat, or replace veterinary advice. Individual mares vary with season, age, nutrition, and management. Always work with your veterinarian on vaccine timing and foaling plans.
How long are horses pregnant?
A horse's gestation period averages about 340 days, or roughly 11 months. The normal range is wide, usually 320 to 370 days, so two mares bred on the same day can foal weeks apart. That is why a due date should be treated as the middle of a window, not a fixed day. A few things shift the length. Colts are often carried a few days longer than fillies on average. Mares bred earlier in the year tend to carry slightly longer than those bred in late spring or summer.
Breed also plays a role — if you are planning a breeding, the horse breeding color chart can help you map coat color outcomes alongside your timing decisions. Individual mares follow their own pattern, so a mare's own foaling history is the best predictor you have. If you are tracking multiple mares, the horse gestation period guide covers the biology behind these variations in more detail.
Mare pregnancy timeline: what to do and when
Months 5, 7, and 9
Give the equine herpesvirus (EHV-1) boosters at these three checkpoints to protect against abortion. Understanding what the 5-way equine vaccine covers helps you have a better conversation with your vet about which products fit your mare's full vaccination schedule.
Last trimester (about month 8 onward)
Most fetal growth happens in the final trimester. This is when the mare's energy and protein needs increase noticeably. The breakdown of orchard vs timothy vs alfalfa hay covers the NSC, protein, and calcium-to-phosphorus ratios you need to adjust for a mare in late gestation, and the alfalfa pellet feeding planner can help you dial in supplemental feeding if your hay quality is inconsistent.
4 to 6 weeks before the due date
Give the pre-foaling vaccine boosters so the foal receives antibodies through colostrum. Prepare a clean foaling area, review the horse's vital signs you will need to monitor, and have your foaling kit ready. The complete foaling kit guide covers the 24-hour newborn protocol and the 1-2-3 rule for assessing a foal's first hours.
Final 1 to 2 weeks
Watch for udder development, then waxing of the teats and softening of the pelvic ligaments. When you see those signs, foaling is usually within a few days. Keep your vet's number accessible and know how to tell if your horse is sick early so you can recognize anything that falls outside normal.
Frequently Asked Questions
How long are horses pregnant?
Horses are pregnant for about 340 days on average, which is roughly 11 months. The normal range is 320 to 370 days, so two mares bred on the same day can foal weeks apart.
How many days in foal is my mare?
Count from the breeding or last cover date to today. Enter that date above and the calculator shows exactly how many days in foal she is and how many days remain until her due date.
What is the gestation period for a horse in months?
About 11 months. Most mares foal between 10.5 and 12.5 months after breeding. The exact length varies by breed, season, and individual mare.
How do I calculate my mare's foaling due date?
Add 340 days to the breeding date for the average due date. The actual window runs from day 320 to day 370, so treat the result as the midpoint of a range, not a fixed day.
Why do some mares foal later than 340 days?
Colts are often carried a few days longer than fillies. Mares bred earlier in the year tend to carry longer than those bred in late spring. Individual mares also follow their own pattern.
What are the signs that a mare is about to foal?
Watch for udder development in the last few weeks, followed by waxing of the teats and softening of the pelvic ligaments. These signs usually appear within a few days of foaling.
When should a pregnant mare be vaccinated?
The standard schedule is EHV-1 boosters at months 5, 7, and 9, then pre-foaling boosters 4 to 6 weeks before the due date so the foal receives antibodies through colostrum. Confirm timing with your vet.
What is the earliest safe foaling date?
Foals born after day 320 are generally viable. Anything earlier is considered premature and requires close veterinary monitoring.
Can I use this calculator for miniature horses and ponies?
Yes. The 340-day average applies to most equines. Miniature horses sometimes run slightly shorter and ponies slightly longer, but this calculator gives a reliable baseline for any breed.