About Gestation Calculator

Gestation Calculator is a free, independent site for farmers, breeders and pet owners. It turns a breeding date into a due date, a normal birth window and a short list of dated tasks.

Who writes it

The site is written and kept up by a small independent team. We are not vets. We do not sell feed, drugs or breeding services, and no company pays for what we say. Every number on the site is taken from a source we link to.

Where the numbers come from

We only use university extension services, veterinary schools, the Merck Veterinary Manual and peer reviewed journals. When sources disagree, we show each one in a table and say which figure the tool uses. We do not use breed club blogs or other calculator sites as sources.

How the math works

Due date equals breeding date plus the average length in days. The window adds the shortest and longest normal lengths. Dates are counted in whole calendar days, so leap years are right. Our automated tests check the results against the printed gestation tables from North Dakota State University (cattle) and the University of Missouri (swine).

What this site does not do

It does not diagnose a pregnancy, judge the health of a mother or her young, or replace an exam. Ask your vet to confirm pregnancy and to plan for the birth.

Sources used

• Approximate Gestation Periods (table). Merck Veterinary Manual.
• Whelping and Queening in Bitches and Queens. Merck Veterinary Manual (A. P. Davidson, UC Davis), reviewed June 2025.
• Pregnancy in Goats. Merck Veterinary Manual (J. L. Stewart, Virginia Maryland College of Veterinary Medicine), reviewed June 2026.
• Breeding Management of Pigs. Merck Veterinary Manual (R. V. Knox, University of Illinois), reviewed July 2026.
• Reproduction of Llamas and Alpacas. Merck Veterinary Manual, updated August 2026.
• FAQ: Pregnant cows, timing of pregnancy. University of Nebraska Lincoln, UNL Beef.
• Calving Book (AS1954), gestation table based on 283 days. North Dakota State University Extension, revised March 2025.
• Options for Pregnancy Diagnosis in Beef Cattle. NDSU Agriculture (S. Zoca, University of Tennessee).
• How Long Should the Dry Period Be?. Penn State Extension, updated April 2025.
• Management of the Pregnant Mare. Extension Horses (K. Anderson, University of Nebraska).
• What to Expect When Your Mare is Expecting (VA-26-W). Purdue University Extension.
• The normal whelping process. Cornell University College of Veterinary Medicine, Riney Canine Health Center.
• Meat Goat Production, Chapter 8: Kidding. Oklahoma State University Extension.
• Sheep Production: Birth to Weaning (Circular 703). New Mexico State University, 2022.
• Care of Pigs From Farrowing to Weaning (G2500). University of Missouri Extension.
• Feline breeding and pregnancy management: What is normal and when to intervene. B. Ström Holst, Journal of Feline Medicine and Surgery, 2022 (PubMed Central).
• Rabbit Breeding and Management: A Guide for Producers. Utah State University Extension, July 2025.

Last reviewed October 20, 2026. Found an error? Tell us.