Mare gestation: when will she foal?
Type the last day the mare was bred. The mare gestation calculator gives the estimated foaling date, the normal foaling window, and how many days along she is today.
- Estimated foaling date
- February 20, 2027
Your mare is due to foal about February 20, 2027, between January 31, 2027 and March 22, 2027.
- Earliest normal foaling date
- January 31, 2027
- Latest normal foaling date
- March 22, 2027
- Days along
- 200
- Days to the estimated foaling date
- 140
- Where she is now
- Day 200 of about 340: 140 days to the estimated foaling date.
Answer for the example date Saturday, October 3, 2026. It changes to today's date when the page loads.
Estimated foaling date: February 20, 2027. Your mare is due to foal about February 20, 2027, between January 31, 2027 and March 22, 2027.
How to calculate
Computes a mare’s foaling date from the last breeding date (340 days), the normal foaling window of 320 to 370 days, and how many days along she is.
Example with the default inputs (Last breeding date March 17, 2026, Today’s date October 3, 2026) on the example date Saturday, October 3, 2026: Your mare is due to foal about February 20, 2027, between January 31, 2027 and March 22, 2027.
Method: Foaling date = last breeding date + 340 days; normal window = last breeding date + 320 to 370 days.
- The estimate of 340 days is the top of Nebraska Extension’s average of 335 to 340 days; the window of 320 to 370 days is its normal range.
- Days count from the last breeding date as day 0. The window is wide: a foal within 320 to 370 days is normal.
Worked examples
Each example is checked against the calculator on every build.
- Last breeding date 2026-05-01, Today’s date 2026-12-01 gives Estimated foaling date 2027-04-06, Earliest normal foaling date 2027-03-17, Latest normal foaling date 2027-05-06, Days along 214, Days to the estimated foaling date 126, Where she is now Day 214 of about 340: 126 days to the estimated foaling date..Source: Nebraska Extension, The Foaling Mare, G1874 (the average gestation length of mares is 335 to 340 days, but can range from 320 to 370 days), https://extensionpubs.unl.edu/publication/g1874/2008/html/view (retrieved 2026-10-03); 1 May 2026 + 320, 340 and 370 days
- Last breeding date 2027-04-10, Today’s date 2028-03-20 gives Estimated foaling date 2028-03-15, Earliest normal foaling date 2028-02-24, Latest normal foaling date 2028-04-14, Days along 345, Days to the estimated foaling date -5.Source: Nebraska Extension, The Foaling Mare, G1874 (the average gestation length of mares is 335 to 340 days, but can range from 320 to 370 days), https://extensionpubs.unl.edu/publication/g1874/2008/html/view (retrieved 2026-10-03)
How it works
The calculator adds whole days to the last breeding date:
| Output | Days after the last breeding | Source |
|---|---|---|
| Estimated foaling date | 340 | Nebraska Extension: average 335 to 340 days (the top of the range) |
| Earliest normal foaling date | 320 | Nebraska Extension |
| Latest normal foaling date | 370 | Nebraska Extension |
- Days along = whole days from the last breeding date to today’s date. The breeding date is day 0.
- Days to the estimated foaling date = 340 − days along. It is negative once the date has passed.
- Where she is now, by days along (d): past the latest normal date when d is more than 370 (“Day d: past the latest normal foaling date (day 370). Call your vet.”); in the foaling window when d is from 320 to 370 (“Day d: in the foaling window (days 320 to 370).”); otherwise “Day d of about 340: (340 − d) days to the estimated foaling date.”
Rules. Today’s date may not be before the last breeding date: then there is no answer and the page says so. Both dates are from 1 January 1900 to 31 December 2200. Calendar days count leap days (2028 has 29 February).
Output format. Dates show as month, day and year. Days along and days to the foaling date are whole numbers.
Worked examples by hand
Last bred on 1 May 2026, counted on 1 December 2026. + 320, 340 and 370 days = 17 March, 6 April and 6 May 2027. From 1 May to 1 December is 31 + 30 + 31 + 31 + 30 + 31 + 30 = 214 days, so 340 − 214 = 126 days to go.
Last bred on 10 April 2027, counted on 20 March 2028. 2028 is a leap year. + 320, 340 and 370 days = 24 February, 15 March and 14 April 2028. From 10 April 2027 to 20 March 2028 is 345 days, so 340 − 345 = −5 days: the estimate passed 5 days ago, and she is in the foaling window.
Other questions people ask
How long is a mare pregnant?
About 11 months. Nebraska Extension gives an average of 335 to 340 days, with a normal range of 320 to 370 days. The calculator uses 340 days for the estimate.
How is the foaling date worked out?
The calculator adds 340 days to the last breeding date. The earliest and latest normal dates add 320 and 370 days. A mare last bred on 1 May 2026 is due about 6 April 2027.
Why use the last breeding date?
A mare is often covered or inseminated more than once in a heat. The last date is closest to ovulation, so it gives the better estimate. If your vet scanned the ovulation date, use that.
Is a mare past 340 days overdue?
Not by itself. 340 days is an average, and the normal range runs to 370 days. Watch for signs of foaling, and ask your vet if she passes 370 days or shows any sign of illness or discharge.
How exact is the foaling date?
It is an estimate. The normal window is 50 days wide (320 to 370 days), so get the stall and the foaling kit ready by the earliest date, about 3 weeks before the estimate.