Mare Gestation Calculator
Enter your mare's last breeding date and this free foaling calculator gives you her estimated due date, how many days in foal she is today, the normal foaling window, and every care date between now and delivery. Set a custom gestation length if she tends to foal early or late.
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 the mare gestation calculator works
Enter the last breeding or cover date. The calculator adds your chosen gestation length, 340 days by default, and returns the estimated foaling date, the day of gestation she is on right now, her trimester, and the percentage of the pregnancy complete. It also runs a reverse check, so if you already have a due date you can work backwards to the breeding date.
The default window runs from day 320 to day 370 because mare gestation varies widely. Two mares bred the same day can foal weeks apart. If your mare has foaled before and runs consistently short or long, change the average length field and every date recalculates around her. The horse gestation period guide covers the biology behind that variation.
Care dates the calculator sets for you
Months 5, 7, and 9
The calculator dates all three EHV-1 booster checkpoints from your breeding date, so you know when the abortion-prevention boosters are due without counting months by hand.
Last trimester (about month 8 onward)
You get the date the final trimester begins. Most fetal growth happens here and the mare's energy and protein needs rise, so it's the point to review her ration with your vet or your feed supplier.
4 to 6 weeks before the due date
The pre-foaling booster window is calculated for you. These boosters put antibodies into the colostrum, and the timing matters, so having the exact date range beats working from memory.
Final 1 to 2 weeks
The calculator flags when to start foal watch. Udder development comes first, then waxing of the teats and softening of the pelvic ligaments, usually within a few days of foaling.
Frequently Asked Questions
How do I calculate my mare's foaling due date?
Add 340 days to the breeding date. Enter that date above and the calculator does it for you, plus the full window. Treat the result as the midpoint of a range, not a fixed day.
How many days in foal is my mare?
Enter the breeding or last cover date and the calculator shows exactly how many days in foal she is today, which trimester she's in, and how many days remain.
What is the normal foaling window?
Day 320 to day 370 from breeding. Most mares foal between 330 and 345 days. The calculator shows the full window alongside the single estimated date.
Can I change the 340-day average?
Yes. If your mare has foaled before and runs short or long, enter her own average in the length field and every date, including the care milestones, recalculates around her.
How accurate is this foaling calculator?
It is as accurate as the breeding date you enter and the average you choose. Individual mares follow their own pattern, so a mare's own foaling history is the single best predictor you have.
Can I use it for ponies and miniature horses?
Yes. The 340-day average applies to most equines. Miniatures sometimes run slightly shorter and ponies slightly longer, so adjust the average length field if you know the tendency.
Which vaccination dates does the calculator give me?
EHV-1 boosters at months five, seven and nine, then the pre-foaling booster window four to six weeks before the due date. Confirm products and timing with your vet.
What are the signs a mare is about to foal?
Udder development in the final weeks, then waxing of the teats and softening of the pelvic ligaments. These usually appear within a few days of foaling.
Can I save this due date and get reminders?
Yes. Save the pregnancy to Horse Tracker, free for one horse, and you get a reminder before every care window instead of checking a calculator again.