Horse gestation calculator

Type the date she was bred: the foaling date, the likely window and what to do before.
Horses carry about 340 days — about 11 months. Most have their foals between day 320 and day 370 after breeding.
Date bred
The day she was with the stallion.
Pick the date she was bred to see her due date.
Example: a mare bred on May 1 is due about April 6 — most likely between March 17 and May 6.

What to do and when

Day 14
First ultrasound
Days 14–16: confirms the pregnancy and checks for twins.
Day 25
Heartbeat check
A heartbeat can usually be seen by ultrasound about now.
42 days before she is due
Watch her udder
It starts to fill 2 to 6 weeks before foaling.
Day 326
Start watching her closely
A foal born before day 326 is premature and needs a vet at once.
Day 361
A long pregnancy
Past 360 days a pregnancy counts as prolonged — talk to your vet.
A common guide — your vet’s plan wins.

Questions

How long are horses pregnant?

Horses carry about 340 days — about 11 months. Most have their foals between day 320 and day 370 after breeding.

How do I work out a horse’s due date?

Add 340 days to the day she was bred. Example: a mare bred on May 1 is due about April 6 — most likely between March 17 and May 6.

Why a window and not one day?

No two horses are the same. Most give birth between day 320 and day 370, so start watching her at the beginning of the window. Most mares repeat their own length from year to year — her own average is the best guide.

What if she was with the stallion for weeks?

Enter the first and the last day they were together. The window then runs from the first day plus 320 days to the last day plus 370 days.

How do I know if she took?

Watch for her next heat about 21 days after breeding. If she does not come back into heat, she has probably settled — your vet can confirm it.

Sources

Prediction of Foaling and Normal Foaling
Colorado State University Equine Reproduction Laboratory
G1874: The Foaling Mare
University of Nebraska–Lincoln Extension
Table: Approximate Gestation Periods
Merck Veterinary Manual (MSD Veterinary Manual)
AFS-3974: Reproductive Management of the Mare
Oklahoma State University Extension
Pregnancy Examination
Colorado State University Equine Reproduction Laboratory
Last checked September 30, 2026. A common guide — your vet’s plan wins.

More calculators

DueBarn for iPhone
Remembers every animal’s due date and reminds you before each job — the pregnancy check, the pen, the watch. Coming to the App Store this winter.