Horse Gestation Calculator
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About Mare Gestation
Eleven months and change: a mare bred in spring foals the following spring, and the whole breeding calendar — nutrition changes, vaccination boosters, moving the mare to her foaling location, night watch — counts backward from the expected date. The catch is equine biology's spread: gestation length varies more in horses than most livestock.
Enter the cover or insemination date and a gestation length (340 is the standard planning number; use your mare's own history if she runs consistently early or late). You get the expected date, a ±10-day window, days in foal, and days to go — with the reminder that the window, not the date, is the real prediction.
The cattle version — tighter windows, different math — is the Cow Gestation Calculator
The Foaling-Date Math
One addition and honest uncertainty:
Foaling date = breeding date + gestation length (≈ 340 days) Window = date ± 10 days (mares normally span ~320–370)
Worked example: a mare covered May 1, 2026 at the 340-day average is due Tuesday, April 6, 2027, with a realistic window of late March to mid-April. Known tendencies: colts tend to run a day or two longer than fillies, and mares foaling early in the season often carry a bit longer.
Breeding Month → Foaling Month
First-of-month cover dates at the 340-day average:
| Bred | Foals (~340 d) | Bred | Foals (~340 d) |
|---|---|---|---|
| Feb 1 | Jan 7 | Jun 1 | May 7 |
| Mar 1 | Feb 4 | Jul 1 | Jun 6 |
| Apr 1 | Mar 7 | Aug 1 | Jul 7 |
| May 1 | Apr 6 | Sep 1 | Aug 7 |
The pattern: subtract about 25 days and go to the next year — an easy check on any date this calculator gives you.
Foaling Watch: the Signs
The calendar says when to start watching; the mare says when it's happening:
- 2–6 weeks out: udder begins filling; the belly drops as the foal positions.
- 1–2 weeks out: teats fill toward the ends; tailhead muscles soften.
- 24–48 hours: “waxing” — colostrum beads on the teats — is the classic short-notice sign; milk calcium testing narrows it further.
- Labor: mares foal fast and mostly at night — active labor should deliver within roughly 20–30 minutes, and trouble at that stage is an immediate vet call, not a wait.
- Post-foaling markers many barns use: foal standing within ~1 hour, nursing by ~2, placenta passed by ~3 — deviations warrant the vet.
These are husbandry conventions, not veterinary advice — your vet and breed association's guidance govern the real decisions.
Frequently Asked Questions
How long is a horse pregnant?
About 11 months — 340 days is the standard planning average, with normal pregnancies ranging roughly 320 to 370 days. Mares vary more than most livestock, which is why foaling watch leans on physical signs as the date approaches.
When will my mare foal if bred May 1?
Around April 6 the following year at the 340-day average, realistically anywhere from late March to mid-April — and a healthy mare can still go longer. Enter your date above for the window, and let her udder tell you the rest.
Is 350 or 360 days overdue for a mare?
Usually not — normal gestations reach 370 days and occasionally beyond, especially with early-season breedings. Past ~370 days, or with any concerning signs, involve your veterinarian rather than inducing panic (or labor) by calendar.
Do colts and fillies arrive at different times?
On average colts run a day or two longer — real but small compared to the overall spread. Mare-to-mare (and even year-to-year in the same mare) variation dominates; her own foaling history is the best predictor after the average.
What is waxing?
Beads of dried colostrum on the teat ends, typically 24–48 hours before foaling — the classic imminent sign. Combined with udder fill and tailhead softening, it's what turns calendar-watch into night-watch.
Can I use this for donkeys or minis?
Donkeys run longer — about 12 months (365+ days) — so shift the length accordingly; miniature horses sit near standard averages with wide variation. The date math is the same; the length input is what changes by species and breed.
Methodology. This calculator uses formulas and health categories recognized by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 11, 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.