Mare Gestation Calculator - Horse Gestation

How to use this horse gestation calculator

What this calculator does. This tool turns one known Mating date into an estimated foaling date, an early-to-late planning window, the mare's current day in foal, the weekday of the central estimate, and a dated pregnancy-planning timeline. It uses the reference model of 338 days for the central estimate, day 331 for the earliest estimate, and day 346 for the latest estimate. It is a calendar estimator, not a pregnancy test, veterinary examination, or prediction of the exact hour a mare will foal. Equine gestation varies substantially among mares; the MSD Veterinary Manual notes an average near 340 days.

When to use it. Use the calculator after a recorded live cover or insemination, when preparing a broodmare calendar, when scheduling veterinary checks and late-pregnancy monitoring, or when comparing a mare's current progress with her expected foaling window. It is especially useful for creating a shareable worksheet for barn staff, but it should be paired with veterinary confirmation of pregnancy and ongoing care.

How to calculate. The calculator opens with a complete demonstration date and an immediately available example XLSX export.

1. Replace the example Mating date with the best-known date of successful breeding or ovulation. The accepted format is the browser's date format, stored as a real calendar date.
2. Read the Estimated foaling date as the central 338-day estimate. Then use Earliest estimated date and Latest estimated date as the calculator's planning window rather than treating one date as certain.
3. Review Current gestation day. A positive number means days elapsed since mating; “Not started” means the entered date is in the future; “Past estimated window” means today is later than day 346.
4. Use the Pregnancy planning timeline for dated reminders. Its columns show the milestone name, gestation day, calculated calendar date, and a short planning note.
5. Select Download Excel to export the current inputs and calculated results. Reset clears the demonstration date, results, table, and export state; Download Excel remains disabled until a complete valid date is entered again.

Input guide. Mating date is required and must be a valid calendar date in YYYY-MM-DD storage format, such as 2026-03-01. Enter the best-supported successful breeding date; do not enter today merely because the exact service date is unknown. Moving the date one day later moves every calculated date one day later. A future mating date is accepted for planning, but the current gestation result will say “Not started.” Invalid, incomplete, or impossible dates are rejected visibly and cannot be exported.

Output guide. Estimated foaling date is an estimate equal to mating date plus 338 days. Earliest estimated date and Latest estimated date are the model's day-331 and day-346 boundaries; they are planning limits rather than biological guarantees. Current gestation day is the whole-day calendar difference between today and the mating date. Estimated foaling weekday names the weekday of the 338-day estimate. The summary pills repeat the fixed model assumptions and current state. In the timeline, Gestation day is an exact offset used by this tool, while each Calendar date is computed from the entered date. Higher day numbers always occur later in pregnancy.

Worked example. With the startup mating date of March 1, 2026, adding 338 days gives February 2, 2027, the central estimated foaling date. Adding 331 days gives January 26, 2027, and adding 346 days gives February 10, 2027. The estimated central date falls on a Tuesday. The table also places the early pregnancy check at day 14 on March 15, 2026, the midpoint at day 170 on August 18, 2026, and the start of the calculator's foaling window at day 331 on January 26, 2027. The exported workbook preserves these dates as typed spreadsheet date values and includes the same assumptions.