How the lambing date is worked out
The lambing date is the mating 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: 24 December 2025
Ewes with a mating date of 24 December 2025 are due to lamb on 20 May 2026, 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 16 May 2026 to 20 May 2026. 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 1 April 2027, choose 'The mating date, from the lambing date' and enter that date. With 147 days the mating date is 5 November 2026 and the mating window for the 143 to 147 day range is 5 November 2026 to 9 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 the neutral terms mating date and lambing date. The UK and US pages use the words you will see on your own records.
Country pages
The arithmetic is the same everywhere, but the wording differs.
For tupping and lambing terms, open the UK sheep gestation calculator.
For breeding and exposure terms, open the US sheep gestation calculator.
Inputs for the sheep gestation calculator
What do you want to find?
The lambing date from the mating date, or the mating date from the lambing date.
Mating date or lambing date
The mating 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
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 mating date of 24 December 2025?
With 147 days it is 20 May 2026, and the 143 to 147 day window is 16 May 2026 to 20 May 2026.
Can I work back from a lambing date?
Yes. Choose the mating date from the lambing date option and enter the date you want. The calculator subtracts the gestation length and shows the mating 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.