Horse Gestation Calculator
Foaling due date and normal window from breeding date, at 340 days.
What it does
Horse pregnancy runs long enough to cross calendar years, which makes head math awkward. Count 340 days from cover and a March breeding lands in the next February. The tool above adds those days in UTC and prints the due date with the normal window beside it.
The default breeds Mar 15 2026 and lands on Thu Feb 18 2027, with routine foaling accepted from Fri Jan 29 2027 (day 320) through Fri Mar 12 2027 (day 362). Veterinary manuals pin the same average of about 340 days, with published ranges spanning roughly 320 to 362 for most mares.
Real mares scatter around the average for honest reasons. Maiden mares often run long, colts average a touch longer than fillies in several datasets, and fescue pasture infection can drag pregnancy far past term while cutting milk. Season, nutrition, and breed nudge the date by days each way.
Treat the result as a prep date, not an appointment. Move the mare to the foaling spot a month early, watch udder and wax signs in the final weeks, and keep the vet posted once day 320 passes. Anything past day 370 deserves a call rather than patience.
How to use
- Pick the breeding or cover date.
- Read the due date first: breeding plus 340 days.
- Read the window: plus 320 to plus 362 days.
- Plan foaling watch across the whole window.
- Confirm late or odd cases with a vet.
Formula
- 340
- Average mare gestation in days (Merck, about 11 months).
- 320 to 362
- Routine foaling range around the average.
- UTC
- Calendar math runs in UTC so zones never shift a day.
Averages describe herds; individuals foal healthy outside the window too. Tables round the same span to 11 months, which hides the 42 day spread this page shows outright.
Worked example
Example: bred Mar 15 2026
Default: breeding 2026-03-15.
- Add 340 days: Mar 15 2026 plus 340 = Feb 18 2027.
- Weekday check: Feb 18 2027 is a Thursday.
- Early edge: plus 320 days = Fri Jan 29 2027.
- Late edge: plus 362 days = Fri Mar 12 2027.
- Tool shows Thu Feb 18 2027, Fri Jan 29 2027 to Fri Mar 12 2027.
Reference table
| Bred | Due near | Watch window |
|---|---|---|
| Mar 15 2026 | Feb 18 2027 | Jan 29 to Mar 12 2027 |
| May 1 2026 | Apr 6 2027 | Mar 17 to Apr 28 2027 |
| Jun 10 2025 | May 16 2026 | Apr 26 to Jun 7 2026 |
Limits
- Average based planning, not a foaling guarantee.
- Cover date assumed exact; pasture breeding blurs day zero.
- Twins, illness, and fescue toxicity break the range.
- Vet checks beat arithmetic once the window opens.
Common mistakes
- Counting 11 calendar months instead of 340 days.
- Prepping only for the due day, not the 42 day window.
- Using the last heat date instead of the cover date.
- Ignoring fescue pasture risks late in pregnancy.
- Skipping vet contact when day 370 passes quietly.
Try it yourself
Quick checks. Answers sit tucked under each question.
Bred May 1 2026?
Due Tue Apr 06 2027, window Wed Mar 17 2027 to Wed Apr 28 2027.
Bred Jun 10 2025?
Due Sat May 16 2026, window Sun Apr 26 2026 to Sun Jun 07 2026.
How long is the normal window?
Day 320 to day 362, a 42 day span around the 340 average.
Questions
How long are horses pregnant?
About 340 days, roughly 11 months.
What is the normal foaling range?
Day 320 to day 362 from breeding for most mares.
Bred Mar 15, when due?
Feb 18 next year, watching Jan 29 to Mar 12.
Do colts run longer?
Often by a day or two in herd data, but individuals vary widely.
When to move to the foaling stall?
At least a month before the due date, sooner for maidens.
When to call the vet?
Past day 370, with odd discharge, or no udder near term.
Sources
- Merck Veterinary Manual: Management of Reproduction in Horses
- Merck Veterinary Manual: Approximate Gestation Periods