A mare usually foals about 340 days after a successful breeding. Most foal between 330 and 345 days, but the normal range is wider, about 320 to 362 days. Enter your mare’s breeding date in the calculator above to get an expected date and a likely window.

How to use the horse pregnancy calculator

1. Enter the breeding date. If you bred more than once, use the date of the breeding that led to the pregnancy. Your vet can help confirm which one.
2. Pick a gestation length. The first option is 340 days, plus or minus 10. The second is a 331 to 346 day window. Choose “Custom value” to use your own numbers.
3. Leave the “count days up to” date as today, or pick another date.
4. Press Calculate due date.

You get the expected foaling date, the likely window, days in foal, weeks and days, and days to go. A short status line tells you if your mare is before, inside or past the window.

How the calculation works

The tool adds a set number of days to the breeding date. That gives the expected date. It then adds a shorter and a longer number of days to show the window.

• 340 days, plus or minus 10: this follows the estimator on the Snodgrass Veterinary page. Window: 330 to 350 days.
• 331 to 346 days: this follows The Horse. They take a 330 to 345 day range and add one day for ovulation. For the single “expected” date, this tool uses 340 days.
• Custom: you type the expected days and the plus or minus window.

Days in foal is the number of days from the breeding date to the date you choose. Weeks are that number divided by 7. Days to go is the expected days minus days in foal.

Worked example

A mare was bred on 10 May 2024. You want to know where she stands on 31 December 2024. You use the 340 day option.

• Expected date: 10 May 2024 plus 340 days is 15 April 2025.
• Window: plus or minus 10 days gives 5 April to 25 April 2025.
• Days in foal on 31 December 2024: 235 days. That is 33 weeks and 4 days.
• Days to go: 340 minus 235 is 105 days.

She is not yet in the foaling window. Notice that 2024 is a leap year. The calculator counts the extra day for you.

Reference table: gestation days and what they mean

Days from breeding	What it means
320	Low end of the average range cited by The Horse
330	Start of the most common foaling period (330 to 345 days)
331	Start of The Horse calculator window (adds one day for ovulation)
340	Typical estimate used by Snodgrass Veterinary
346	End of The Horse calculator window
350	End of the Snodgrass window (340 plus 10)
362	High end of the average range. Past this, call your vet.

Each mare has her own pattern. If she has foaled before, her past gestation lengths are a good guide. Ponies tend to carry for a shorter time than full-size horses. [CHECK: add a pony day range only if you have a source.]

Tips and common mistakes

• Using the wrong date. The calculator needs the breeding date that led to the pregnancy. A mare bred several times in one heat can be hard to date. Ask your vet.
• Treating the date as exact. Very few mares foal on day 340. Plan around the window, not one day.
• Waiting until the last week. Get your foaling kit, stall and vet contact ready well before the earliest date in the window.
• Ignoring the mare’s history. If she always foals late, use the custom option with her own number of days.
• Using it for donkeys. The presets are for horses. Donkeys and other equids carry for different lengths.
• Skipping the vet. A calculator cannot confirm pregnancy or spot problems. See the disclaimer for how to use this tool.

Breeding other animals? Try our cow pregnancy calculator or sheep pregnancy calculator. Each species has its own gestation length, so do not reuse the horse numbers.

Frequently asked questions

How far along is my mare?

Count the days from the breeding date (or the ovulation date, if you know it) to today. The calculator does this for you and shows days in foal, weeks and days, and days left. A vet exam or ultrasound is the only way to confirm pregnancy and check progress.

What is the 1-2-3 rule for horse birth?

It is a common guide for foaling. The foal should stand within about 1 hour, nurse within about 2 hours, and the mare should pass the placenta within about 3 hours. If any step runs late, call your vet. [CHECK: confirm the 1-2-3 timings against a veterinary source before publishing.]

What is the longest a horse can be pregnant?

TheHorse.com gives an average range of 320 to 362 days and notes that some mares have foaled outside it. There is no safe fixed upper limit for every mare. If your mare goes past 362 days, call your vet.

How long is too long for a mare to be pregnant?

Plan to involve your vet if your mare passes the end of her expected window without signs of foaling. Past 362 days, a vet check is a sensible step. Some mares carry longer, but only a vet can judge your mare.

How accurate is a horse due date calculator?

It gives an estimate, not a promise. Most mares foal within about 330 to 345 days of a successful breeding, but each mare has her own pattern. A past foaling history for the same mare is a useful guide.

Does the calculator work for ponies or donkeys?

The presets are for full-size horses. Ponies tend to have shorter gestations on average. Use the custom option if your vet gives you a different number of days. Donkeys and other equids are not covered by the presets.