Sheep Gestation Calculator

How it works

The calculator adds the selected gestation length to the mating or tupping date to give the estimated lambing date, then marks out a likely window from five days before to five days after that date. The gestation lengths are breed averages: 147 days is a widely used all-breed figure, early-maturing breeds such as the Finn average around 144 days, Romneys around 146, Suffolks around 148 and fine-wool Merinos around 150. The five-day window reflects normal biological spread rather than a hard limit, because the ewe's age, her condition, the sire and the number of lambs she is carrying all shift the actual date. Ewes with twins or triplets tend to lamb a little earlier than ewes with singles, and hoggets often run slightly shorter than mature ewes. If the ram ran with the mob rather than mating on recorded dates, work from the middle of the joining period and expect the spread of lambing to be wider than the window shown, roughly one cycle of 17 days for each time the ram went around the mob.

Worked example

A ewe is mated on 1 April 2026 and you use the 147-day average. Adding 147 days gives an estimated lambing date of 26 August 2026, with a likely window from 21 August to 31 August 2026. If she is a Merino and you select 150 days, the due date moves out to 29 August 2026, while a Finn at 144 days brings it back to 23 August 2026. That six-day spread between breeds is why the gestation length you pick matters when you are planning set stocking and lambing beats, and why the window either side of the single date is the number to plan around rather than the date in the middle.

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