How the lambing date is worked out
The lambing date is the breeding date plus the gestation length. The Merck Veterinary Manual gives an average of 147 days for sheep, which is 21 weeks, so the calculator starts at 147 days. Working back from a lambing date you want, it subtracts the same number of days.
Both directions use real calendar days, so leap years and the different month lengths are handled for you.
Worked example: 1 November 2026
Ewes with a breeding date of 1 November 2026 are due to lamb on 28 March 2027, 147 days later. NADIS, the animal health information service for livestock vets, says that gestation lasts 143 to 147 days, which gives a lambing window of 24 March 2027 to 28 March 2027. Ewes can lamb outside that window, so start watching for signs a few days before it opens.
Working back from a lambing date
To have lambs on 15 April 2027, choose 'The breeding date, from the lambing date' and enter that date. With 147 days the breeding date is 19 November 2026 and the breeding window for the 143 to 147 day range is 19 November 2026 to 23 November 2026.
Why the date is an estimate
Merck gives 147 days as an average, not a fixed length, and the NADIS range of 143 to 147 days is for typical ewes. The window on this page is that published range, not a range around any length you type in. If your own flock records show a different average, type it into the gestation length box and the single date moves with it.
Sheep are seasonal breeders. Merck describes them as seasonally polyestrous, short-day breeders, with an oestrous cycle that averages 17 days, so ewes that return to the ram after an unsuccessful first cycle will lamb about 17 days later than the first date suggests. The calculator does not guess which cycle a ewe conceived in; enter the date she was actually mated.
What to watch for as lambing approaches
NADIS lists udder development and a build-up of colostrum among the signs that lambing is close, along with the ewe seeking a sheltered spot. Merck advises vaccinating ewes 4 to 6 weeks before lambing so that the colostrum carries more antibodies, which is a date you can plan from the lambing date this page gives.
Terms and dates on this page
This page uses US terms: the breeding date, when ewes are exposed to or bred by the ram, and the lambing date. Dates are shown with the month in words, so they cannot be misread as month/day or day/month. The date picker follows your browser's regional settings.
Other versions of this calculator
Choosing another country above the inputs opens its own page and resets the inputs.
For the UK wording, open the UK sheep gestation calculator.
For a version without country wording, open the sheep gestation calculator.
For cows, use the US cattle gestation calculator.
Inputs for the US sheep gestation calculator
What do you want to find?
The lambing date from the breeding date, or the breeding date from the lambing date.
Breeding date or lambing date
The breeding date, or the lambing date if you are working backwards.
Gestation length
In days. 147 is the usual average; change it if your flock records differ.
CALCULATOR QUESTIONS
US sheep gestation calculator questions
How long are sheep pregnant?
Merck gives an average of 147 days, which is 21 weeks. NADIS gives 143 to 147 days for gestation. Individual ewes can fall outside that.
What is the lambing date for a breeding date of 1 November 2026?
With 147 days it is 28 March 2027, and the 143 to 147 day window is 24 March 2027 to 28 March 2027.
Can I work back from a lambing date?
Yes. Choose the breeding date from the lambing date option and enter the date you want. The calculator subtracts the gestation length and shows the breeding window.
What if the ewe returned to the ram?
Merck gives the oestrous cycle as about 17 days on average. A ewe that was mated again at her next heat will lamb about 17 days after the first date suggests. Enter the date of the mating that worked.
Are leap years handled?
Yes. The calculator counts real calendar days, so a 29 February inside the gestation period is included.
Is this the same calculation as the UK page?
Yes. The pages differ in the words used for the dates, in the examples and in the notes.