Horse Gestation Calculator

The day she was bred in — her foaling window out, as the range the vets and extension services publish rather than one day nobody can promise. Then where she is today, the checks a vet schedules, and what the studies found moves the date.

Several covers? Use the last, or the ovulation date from a scan. A date ahead works too.

Colt or filly? (if a scan has told you)

Her foal is due

Between two dates

The dates to know

    Signs she is close

      A window, not a promise: healthy foals arrive weeks either side of day 340. Your vet’s scans date the pregnancy better than any calculator, and your vet sets her vaccination and deworming plan.

      Sources checked 2026-09-26. The calendar file is built in your browser — nothing you type here is sent anywhere.

      Quick answer: a mare carries her foal for about 340 days, a little over 11 months. That is the average the Merck Veterinary Manual gives. Healthy foals are born weeks either side of it: the University of Nebraska’s extension puts the normal range at 320 to 370 days. So a mare bred on May 1, 2026 is due between March 17 and May 6, 2027, around April 6. Count from the last day she was covered, or from the ovulation date if your vet scanned her.

      How to use it

      Put in the day she was bred. If she was covered on several days, use the last one. If your vet scanned her and dated the ovulation, use that, because it is closer to the day the foal was conceived. The calculator gives her foaling window first, then the middle of it. Below that are the checks a vet usually schedules, each with its date, and the signs that she is getting close. If a scan has told you whether it is a colt or a filly, say so, and you will also see what one large study measured for that sex. The calendar button puts every date into your phone’s calendar. The file is made in your browser, so nothing is sent anywhere.

      Why it is a window and not a day

      Every source gives a spread, and they are not small. Nebraska’s extension says 320 to 370 days. The University of Wisconsin–Madison says 335 to 342 is normal, but that lengths from 320 to 367 occur. Merck’s chapter on newborn foals says normal gestation runs from 310 days to more than 370. The studies that timed real pregnancies found the same. Davies Morel and colleagues followed 433 Thoroughbred pregnancies in the UK, timed from conception by ultrasound. They averaged 344 days, and ran from 315 to 388, all with viable foals. A German study of 16,226 Thoroughbred pregnancies averaged 347 days, and ran from 306 to 390. A single due date looks tidy on a calendar, but no mare reads the calendar.

      What moves the date

      Colts are usually carried a little longer. Nebraska says 2 to 7 days longer than fillies. Davies Morel measured 346.2 days for colts and 342.4 for fillies. The season matters too, but the sources do not agree on which way. Wisconsin says mares due in late winter to early spring may carry 5 to 10 days longer than mares that foal later in the year, and Nebraska says the same. In Davies Morel’s Thoroughbreds, though, January foals had the shortest gestations and April foals the longest. Wisconsin adds that the light treatment used to bring mares into season early also shortens gestation by about 10 days. In the German study, gestation got longer as mares got older, and with stallions aged 17 or more. Davies Morel found no effect of either age. The calculator shows each of these views with its source, rather than choosing one for you.

      The checks your vet will talk about

      Merck’s chapter on pregnancy diagnosis lays out a usual schedule. The first scan is at days 14 to 18, to confirm she is in foal and check for twins. If she is open, she can be bred again on days 19 to 20. The second is at days 25 to 30, when there should be a heartbeat and twins are checked again. The third, at days 40 to 60, checks that the foal is developing normally. A fall check then confirms she is still in foal. For the last three months, Merck recommends a gradual, increasing plane of nutrition. Nebraska suggests moving her to where she will foal 2 to 3 weeks before the expected date. Her vaccination and deworming plan is your vet’s to set for her and your farm. This calculator will not write one.

      FAQ

      How long is a horse pregnant?
      About 340 days on average, a little over 11 months, according to the Merck Veterinary Manual. The University of Nebraska’s extension gives an average of 335 to 340 days and a normal range of 320 to 370. Large studies of Thoroughbreds found averages of 344 and 347 days.
      How do I work out my mare’s due date?
      Add 340 days to the day she was bred for the middle of the window, 320 days for its start and 370 for its end. For a mare bred on May 1, 2026, that is March 17 to May 6, 2027, around April 6. The calculator does the counting, and it handles the months and leap years for you.
      Do colts take longer than fillies?
      Usually, by a few days. Nebraska’s extension says colts are typically carried 2 to 7 days longer. In a study of 433 Thoroughbred pregnancies, colts averaged 346.2 days and fillies 342.4. A German study of 16,226 pregnancies also found gestation was longer with colts.
      When is a foal premature, and when is a mare overdue?
      Merck defines a foal born before day 320 as premature. There is no agreed day for overdue. Nebraska’s normal range ends at 370 days, and Davies Morel’s Thoroughbreds ran to 388 days with viable foals. The University of Wisconsin–Madison says gestations of up to a year appear to pose no problem. If she goes past the range, talk to your vet.
      What are the signs a mare is about to foal?
      The udder fills first. Merck says it starts 2 to 4 weeks before foaling, and Nebraska says 2 to 6 weeks. Waxing, the beads of colostrum on the teat ends, comes 6 to 48 hours before in almost all mares, according to Merck. Merck adds that in some mares it comes much earlier, or not at all. Nebraska says 1 to 2 days before. Her vulva also relaxes in the last days.
      Can I use this before she is bred?
      Yes. Put in the day you plan to breed her, and you will see the foaling window that date gives. It is a quick way to check whether a foal would arrive in spring or in the heat of summer.