Cow Gestation Calculator

How it works

The calculator adds the selected gestation length to the breeding or AI date to give the estimated calving date, then marks out a likely window from seven days before to seven days after that date. The gestation lengths are breed averages: 283 days is a widely used all-breed figure, Jerseys average around 279 days, Friesian Holsteins around 280, Herefords around 285 and Charolais around 287. The seven-day window reflects normal biological spread rather than a hard limit, because the sire, the calf's sex, whether the cow is carrying twins, and the cow's own age and condition all shift the actual date. Bull calves and calves by beef sires tend to be carried slightly longer, and heifers often calve a little earlier than mature cows. If the cow ran with a bull rather than being inseminated on a known date, work from the middle of the possible mating period and expect a wider spread either side of the result.

Worked example

A cow is inseminated on 1 August 2026 and you use the 283-day average. Adding 283 days to 1 August 2026 gives an estimated calving date of 11 May 2027, with a likely window from 4 May to 18 May 2027. If she is a Jersey and you select 279 days instead, the due date moves four days earlier to 7 May 2027, while a Charolais at 287 days pushes it out to 15 May 2027. That eight-day spread between breeds is why picking the right gestation length matters when you are drafting springers or planning the start of calving, and why the window either side matters more than the single date in the middle.

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