Enter a mare's recorded reproductive date to estimate her foaling date and see the day 320 through day 360 planning span.
Table of contents
How to use our Mare Gestation Calculator
- Find the date in the mare's breeding log, cover record, insemination record, or veterinary reproductive record.
- Enter that date and click Calculate to get an estimated foaling date using the 340-day planning value.
- Open Advanced options if your record is specifically an insemination or ovulation date, or if you have a reliable mare-specific gestation length in whole days.
- Check that the displayed record basis and number of gestation days match the record you used before adding the dates to your foaling plan.
Definitions
Gestation length: The number of calendar days from the recorded reproductive event to foaling. This calculator uses 340 days as its planning default.
Recorded date basis: The event represented by the date entered: breeding or cover, insemination, or ovulation. It labels the result and does not change the calendar calculation.
Estimated foaling date: The recorded date plus the gestation length used. It is a planning estimate, not a promised foaling date.
Planning span: The calendar range from day 320 through day 360 after the recorded date.
Common mistakes and quick fixes
Mistake: Entering the date pregnancy was confirmed instead of the reproductive event date.
Fix: Use the dated breeding, cover, insemination, or ovulation record as the starting date.
Mistake: Leaving the record basis as breeding or cover when the entered date is an insemination or ovulation date.
Fix: In Advanced options, choose the event named on the record. This changes the result wording, not the date calculation.
Mistake: Entering weeks or months for gestation length.
Fix: Enter a positive whole number of days, or leave the field blank to use the 340-day planning value.
Mistake: Entering a fraction such as 340.5 days.
Fix: Use a whole-day value because the calculator adds complete calendar days to the recorded date.
Mistake: Treating the estimated foaling date as a guaranteed delivery date.
Fix: Use the estimate and the day 320 through day 360 span for planning, then monitor the mare and follow veterinary guidance.
Limitations & Key Assumptions / Boundary Conditions
- This calculation estimates calendar dates only. It cannot confirm pregnancy, fetal health, labor, or a need for veterinary care.
- The 340-day value is a planning default. An individual mare may foal earlier or later, so use reliable prior records and veterinary guidance when available.
- The day 320 through day 360 span shows planning boundaries, not clinical deadlines or an exact foaling prediction.
- The entered date must be the actual reproductive event date represented by the selected record basis.
- Calendar-day addition handles month ends, leap years, and year changes, but it does not use a time of day or time zone.
Methodology
Calendar calculation
The calculator adds a whole number of calendar days to the recorded reproductive date. If Gestation length used (days) is blank, it uses 340 days as the planning value. Foaling guidance describes average mare gestation as about 340 days. [1]
estimated foaling date = recorded date + gestation days used
The planning span always shows the dates that fall 320 days and 360 days after the recorded date. Changing a mare-specific gestation length changes the estimated foaling date only; it does not move these planning boundaries.
planning span = recorded date + 320 days through recorded date + 360 days
Worked example
For a breeding or cover date of January 1, 2024, adding 340 calendar days gives an estimated foaling date of December 6, 2024. Adding 320 and 360 days gives a planning span of November 16, 2024 through December 26, 2024.
Record wording
The selected date basis appears with the result so an insemination or ovulation record is not described as a breeding record. Selecting another basis does not convert, shift, or otherwise change the entered date.