Horse Gestation & Foaling Date Calculator
Put in your mare's breeding date and get her estimated foaling window — the day-320 safe date, the average 340-day due date, the day-350 normal upper bound, and the gestation checkpoints worth planning with your vet.
Enter the date your mare was bred (or the cover date). We'll estimate her foaling window from there.
How horse gestation is counted
A mare's pregnancy is counted from the day she was bred or covered. On average a horse carries a foal about 340 days — right around eleven months — which is why breeders talk in months rather than trimesters. This calculator adds 340 days to the breeding date you enter to land on the expected due date.
The single date is the easy part. The useful part is the window around it, because healthy foals arrive across a fairly wide spread.
The safe date, the due date, and the normal upper bound
Day 320 is the commonly medically accepted safe date — the earliest point a foal is generally considered safely carried to term. From there, the average lands at day 340, and most mares have foaled by around day 350, the commonly accepted upper edge of the normal window. A mare who foals at 325 days and one who goes to 348 can both be perfectly normal, and many individual mares are consistent year to year — if she went to 340 last time, she may well do it again.
Past day 350, gestation tails off quickly — but it's not medically impossible for a mare to carry a little longer, and in rare cases gestation can run as long as a full year. That's uncommon, not alarming. If your mare passes day 350, it's worth a call to your vet so they can check on her and the foal — a conversation, not a panic.
This is why the tool shows a safe date, an expected date, and a normal upper bound rather than a single answer. Plan for the front of the window, not the middle: it's far better to be ready a week early than to be caught off guard.
Gestation checkpoints to plan with your vet
Certain points in the pregnancy are common moments for a vet visit or a vaccination. The calculator marks generic versions of these — an early pregnancy and twin check, a heartbeat confirmation, the rhinopneumonitis (EHV-1) series many farms run through gestation, and a pre-foaling booster several weeks before the due date so the foal picks up protection through the mare's first milk.
Treat those as reminders to have the conversation, not a schedule to follow on your own. Vaccination and deworming timing varies by region, by farm, by product, and by mare. Your veterinarian sets the real plan — this tool just helps you see the dates coming.
Educational, not veterinary advice. Foaling is a medical event. Build your plan with your vet ahead of the season, and call them when anything looks off.
Common questions
How long is a horse pregnant?
A mare is pregnant for about 340 days on average — roughly eleven months. Day 320 is the commonly accepted "safe date," the earliest a foal is generally considered ready to arrive, and most mares have foaled by around day 350. It is best to plan around a window rather than a single date.
How is a horse gestation calculator counted?
This calculator counts forward from the last breeding (or cover) date. It marks day 320 as the safe date (earliest term), adds the average 340-day gestation for the expected due date, and marks day 350 as the normal upper bound most mares foal by.
How accurate is a foaling due-date estimate?
A due date from a breeding date is an estimate, not a guarantee. Individual mares foal earlier or later than average and can be consistent from year to year. Your veterinarian can refine the estimate with ultrasound and by watching udder development, milk changes, and other pre-foaling signs.
What if my mare goes past 350 days?
Most mares foal by around day 350, and gestation tails off quickly after that — but going a little longer is not medically impossible. In rare cases gestation can run as long as a full year. A mare past day 350 warrants a conversation with your vet rather than panic, so they can check on her and the foal.
When should I start foaling watch?
Many breeders begin closer observation and night checks as the mare approaches her safe date, around day 320, and watch for physical signs that foaling is near. Talk to your vet about the right time and what to watch for with your mare.
What vaccinations does a pregnant mare need?
Broodmare vaccination schedules vary by region and farm. Many programs include a rhinopneumonitis (EHV-1) series during gestation and a pre-foaling booster several weeks before the due date so the foal gets protection through colostrum. Your veterinarian sets the specific vaccines and timing for your mare.
Keep the whole breeding record, not just the date
A due date is one number. A breeding season is a pile of them — heat cycles, ultrasound findings, breeding dates, pregnancy checks, and eventually the foaling record itself. Two guides go deep on what to capture and why:
- Horse breeding documentation: what to record at every stage — heat cycles, ultrasounds, semen evaluation, stallion contracts, and registry rules.
- Foaling records and mare tracking: breeding to birth — pregnancy monitoring, the foaling record, and the neonatal details experienced breeders wish they'd written down.
Track the countdown for every mare
A calculator gives you the dates. HorseBook keeps them next to the mare they belong to — breeding date, ultrasound checks, a live due-date countdown, and the foaling record when the day finally comes. When your vet asks "when was she bred, and has she had her rhino boosters?" the answer's on your phone, not in a binder in the tack room.
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