Horse Gestation Calculator NZ
This estimates when a mare is due to foal from the date she was covered. Horses average about 340 days, roughly eleven months, but the honest part of this page is the window rather than the date. The normal range for a mare runs from about 320 days to about 370, a spread of nearly two months, which makes a single due date a far weaker guide in horses than in almost any other farmed species. Notice that the range is not centred on the average: it sits 20 days below and 30 days above, because mares are more likely to run over than to come early. Most foaling calculators draw a tidy margin evenly either side of the due date, which manages to be wrong in both directions at once, so this one uses the real offsets. What actually moves a mare within that range is fairly well understood. Season is the strongest influence, with mares foaling in late winter tending to carry longer than those foaling in late spring, and colts are commonly carried a little longer than fillies. Best of all, individual mares tend to be consistent year to year, so a mare's own history beats any average you can look up.
Estimate only, and a planning tool rather than veterinary advice. A foal arriving before about 320 days is generally considered premature and needs a vet. Talk to yours about foaling preparation and about anything unusual.
How it works
The calculator adds the gestation length to the covering date, then marks the two ends of the normal range: 20 days before the due date and 30 days after it, which is 320 and 370 days from covering. Those offsets are kept even if you change the gestation length, so a mare you know runs to 350 days still gets a window that reflects the shape of the species rather than a margin invented around her.
Why a two month window is still useful
Because the useful decisions sit at the edges rather than the middle. The earliest date tells you when to have the foaling paddock ready, the camera on and someone checking, and the latest tells you when a mare who has not foaled has moved from patience to a phone call. A due date on its own gives you neither. It is also why a mare's own record is worth keeping: three foalings at 335, 338 and 336 days tell you far more than any published average, and this page lets you type that in.
Worked example
A mare covered on 1 November 2026 and carrying the average 340 days is due on 7 October 2027. The normal range runs from 17 September 2027 at the early end to 6 November 2027 at the late end. That is a fifty day window, so having the foaling arrangements ready from mid September is realistic and being surprised in early November is not. If this mare has foaled at around 335 days in past seasons, entering 335 moves the estimate to 2 October and is a much better date to work to than the average.
Related calculators
- Cow Gestation: calving due dates.
- Goat Gestation: kidding due dates.
- Pig Gestation: farrowing due dates.
- Date Calculator: add or subtract days between dates.