# Mare gestation: when will she foal?

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.

- Page: https://www.acalculator.org/biology/mare-gestation-calculator
- JSON spec: https://www.acalculator.org/biology/mare-gestation-calculator.json
- Version: 7a914dddc92d

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| d | Last breeding date | The last day the mare was bred (covered or inseminated). |
| on | Today’s date | The day to count from, to show how far along the mare is. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| due | Estimated foaling date | The last breeding date + 340 days. |
| earliest | Earliest normal foaling date | The last breeding date + 320 days. |
| latest | Latest normal foaling date | The last breeding date + 370 days. |
| days | Days along | Whole days from the last breeding date to today’s date. |
| left | Days to the estimated foaling date | Whole days from today’s date to the estimated foaling date; negative once it has passed. |
| stage | Where she is now | The stage on today’s date, in one line. |

## Method

Foaling date = last breeding date + 340 days; normal window = last breeding date + 320 to 370 days.

## Assumptions

- 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

1. d = 2026-05-01, on = 2026-12-01 gives due = 2027-04-06, earliest = 2027-03-17, latest = 2027-05-06, days = 214, left = 126, stage = 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.
2. d = 2027-04-10, on = 2028-03-20 gives due = 2028-03-15, earliest = 2028-02-24, latest = 2028-04-14, days = 345, left = -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).

## FAQ

### 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.

## Sources

- 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)
- Merck Veterinary Manual, Approximate Gestation Periods (horse: 11 months). https://www.merckvetmanual.com/multimedia/table/approximate-gestation-periods (retrieved 2026-10-03)