How the Calculators Work
Every calculator on this site does the same simple thing: it counts calendar days forward (or backward) from a date you enter. The value is in using the right number of days and in showing how much real births vary. This page lists every number we use and where it comes from.
Calculation rules
- Dates are counted as whole calendar days, including leap days. Time zones and daylight saving never shift a date.
- Due date = breeding date + gestation length.
- Due window = breeding date + the low and high ends of the published range (the normal range for most species; for cattle, the shortest and longest gestations recorded in a 2024 study; for alpacas, the middle half of pregnancies).
- Breeding season (male in → male out): first birth = male-in date + gestation; last birth = male-out date + gestation.
- 21-day periods: the breeding season is split into 21-day blocks from the male-in date. Each block of conceptions maps to a block of births. In the herd planner, single-date animals are grouped into 21-day blocks from the first due date.
- Reverse planner: male-in date = target first birth − gestation.
- Return to heat: breeding date + 1, 2 and 3 estrous cycles, with the published range around each.
- Nothing you enter leaves your browser. The herd planner saves your list in your browser's local storage only.
Numbers used
| Species | Default gestation | Range shown | Heat cycle |
|---|---|---|---|
| Cattle | 283 days | 253 to 300 days | 21 days (18 to 24) |
| Sheep | 147 days | 144 to 152 days | 17 days (14 to 19) |
| Goat | 150 days | 144 to 155 days | 21 days (17 to 24) |
| Horse | 340 days | 320 to 370 days | — |
| Pig | 114 days | — | 21 days |
| Rabbit | 31 days | 28 to 35 days | — |
| Alpaca & Llama | 346 days | 339 to 353 days | — |
| Dog | 63 days | 58 to 72 days | — |
Cattle pregnancy checks: ultrasound from 28 days, blood test from 25 days (heifers) and 28 days (cows), rectal palpation from 35 to 50 days.
Cattle breed presets
Breed presets are rounded to whole days. Most come from the USDA Meat Animal Research Center Germplasm Evaluation Program (Table 3), which reports averages by sire breed of the calf in crossbred matings. Angus and Hereford come from a 1952 Journal of Animal Science study as reported by Oklahoma State University. The “modern beef cows” preset (280 days) comes from a 2024 study of 10,037 AI-bred Bos taurus beef cows. These are averages; your own herd, and your sire's gestation-length EPD, may differ, which is why every calculator accepts a custom length.
Sources by species
Cattle
- Drovers / Oklahoma State University: Is 283 days the accurate average gestation length? (2026)
- USDA-ARS Meat Animal Research Center, Germplasm Evaluation Program, Table 3 (sire breed means)
- Mississippi State University Extension P2616: The Estrous Cycle of Cattle
- Nebraska Extension: Pregnancy diagnosis in heifers and cows provides management options (2024)
Shortest and longest gestations recorded in a 10,037-record study of AI-bred beef cows (Journal of Animal Science abstract, 2024).
Sheep
Goat
Horse
Average is 335 to 340 days; colts tend to be carried 2 to 7 days longer than fillies.
Pig
Rabbit
Alpaca & Llama
- Ponthier et al. (2025), Pregnancy length in alpacas, Reproduction in Domestic Animals
- Merck Veterinary Manual: Approximate Gestation Periods
Middle half of 7,049 alpaca pregnancies; the full observed range was 270 to 428 days.
Dog
58 to 72 days from breeding when the stage of heat is unknown; 62 to 64 days from the day of ovulation.
Limits
- These are planning estimates. Individual animals deliver earlier or later than any calculator predicts.
- Nothing here is veterinary advice. Confirm pregnancies and discuss due dates and birthing problems with your veterinarian.
- Llama gestation uses the Merck Veterinary Manual's approximate 11.5 months; we did not find a stronger published average to cite.
Sources last checked 2026-10-21. Found an error? Tell us via the contact page.