Gestation Horses Calculator
Mare Gestation and Foaling Date Calculator works out Expected due date and Due date falls on from Breeding or service date, Gestation length override and Due window. The default figures on this page give expected due date of 2026-12-07; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
4 steps
- Horse (mare): published mean gestation of 340 days.
- 2026-01-01 + 340 days = 2026-12-07 (Monday), the expected foaling date.
- Normal range 320-370 days puts the window at 2026-11-17 to 2027-01-06.
- On 2026-09-14 the animal is 256 days into gestation, 84 to go.
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the Mare Gestation and Foaling Date Calculator calculates
Calculates the expected calving, foaling, whelping, kidding or lambing date from a breeding or service date using published species gestation lengths, with the normal range where one has been published.
How to use this Mare Gestation and Foaling Date Calculator
Enter Breeding or service date, Gestation length override, Due window and Today. Expected due date updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Expected due date, Due date falls on and What it is called.
The method follows Merck Veterinary Manual, The Reproductive System in Animals (gestation periods table), Merck Veterinary Manual, Pregnancy in Goats, Merck Veterinary Manual, Prolonged Gestation in Cattle and Sheep, Montana State University Extension, Beef Cattle Gestation Table and Kentucky Equine Research, Gestation in Mares: What Is Premature? What Is Overdue?.
What you can compare
Change one input at a time to see how it affects Expected due date, Due date falls on and What it is called. The inputs you can vary are Breeding or service date, Gestation length override, Due window and Today.
Understanding Expected due date, Due date falls on and What it is called
A planning estimate from published mean gestation lengths, not veterinary advice. Individual animals, breeds, litter size and sex of the offspring all shift the date; confirm pregnancy and timing with your veterinarian.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How can I calculate how many days my mare is pregnant?
Calculates the expected calving, foaling, whelping, kidding or lambing date from a breeding or service date using published species gestation lengths, with the normal range where one has been published. Open “How this is worked out” beside the result to see each step for your own figures.
What is the 1/2/3 rule in horses?
On the figures this page opens with, the expected due date is 2026-12-07. No single number fits everyone: it follows Breeding or service date, Gestation length override and Due window, so enter your own above and the answer updates as you type.
How long is too long for a mare to be pregnant?
It needs Breeding or service date, Gestation length override, Due window and Today. Nothing else is required, and the figures stay in your browser.
How can I calculate my gestation based on my due date?
The result gives Expected due date, Due date falls on, What it is called, Gestation length used, Earliest expected and Latest expected, each labelled with its unit so the figure can be read on its own.
What information do I need for the Mare Gestation and Foaling Date Calculator?
You need Breeding or service date, Gestation length override, Due window and Today. The page opens with default figures that give expected due date of 2026-12-07, so you can see the scale before entering your own.
What does this Mare Gestation and Foaling Date Calculator show?
It shows Expected due date, Due date falls on, What it is called, Gestation length used, Earliest expected, Latest expected, Shortest normal gestation and Longest normal gestation, with the working and any warnings for your entries.