How this calculator works
Ontario's sheep guidance gives an average of 147 days and a range of 144 to 151 days. That local, species-specific guidance supplies this page's planning values. Breed, litter size and uncertain service dates can shift the actual date. Date arithmetic uses whole calendar days in UTC, so daylight-saving changes do not shift the result. A negative elapsed value means the reference date is still ahead; a negative remaining value means the estimate has passed.
Formula
Due date = reference date + assumed gestation days. Reverse mode subtracts the same duration. The default is 147 days, with a 144 to 151 day planning window. Days remaining = due date - observation date. The window is not a guarantee.
Worked example
A ewe mated on 1 January 2026 has an estimated lambing date of 28 May, with earlier and later planning dates of 25 May and 1 June.
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Frequently asked questions
Does the estimated date confirm pregnancy?
No. A recorded mating may not result in pregnancy. Veterinary examination and appropriate imaging are needed for confirmation.
Can I work backwards from a due date?
Yes. Choose breeding-date mode. The answer is the date implied by the assumed duration, not proof of the actual conception day.
Why might birth occur outside the window?
Breeding and ovulation dates can differ, and litter size, breed and individual history influence timing. Arrange monitoring with your veterinarian instead of using the window as a safety test.