The farrowing-date reference
UNH Extension gives a pig gestation average of 114 days and a range of 111–117 days. These values are useful for arranging a farrowing calendar while recognizing that the average is not a guaranteed birth date. Open the livestock calculator with Pig selected to see both the central estimate and the source range for your breeding record.
Keep the service date in a form you can read later, alongside the sow's or gilt's tag. A date remembered from a general breeding period is less specific than an observed breeding date. If the animal was exposed over a period, use both the first and last days with the boar. That preserves uncertainty instead of hiding it in a single estimated date.
The familiar memory aid is not exact arithmetic
“3 months, 3 weeks, 3 days” is a familiar memory aid for pig gestation. It is helpful for recalling the approximate length, but calendar months do not all contain the same number of days. Adding named months and weeks to a breeding date can therefore produce a different result from the elapsed-day method used by this calculator.
GestationChart adds the 114-day reference directly to the breeding date. The breeding day is the starting point rather than an already completed day of gestation. When comparing a printed chart, handwritten note, or another calculator, first check whether it uses the same counting convention. A counting difference does not establish anything about the animal's pregnancy.
A farrowing example
Enter December 1, 2026 with Pig selected. The engine's average due date is March 25, 2027, and the displayed window is March 22–28, 2027. Those dates come from adding the 114-day average and 111–117-day range to the example breeding date. They are calculated dates, not dates quoted in the source.
| Record | What belongs in it |
|---|---|
| Identity | Sow or gilt tag, kept consistent across records |
| Breeding | Observed service date or the entire exposure period |
| Estimate | Due date using the 114-day average, plus the full window |
| Observation | Recorded heats and other notes, separate from predicted dates |
| Outcome | Actual farrowing date, retained for later comparison |
The herd planner puts each animal's estimate into a list and calendar. An exposure-period entry stays a range in both views. Print the calendar for a working copy, and export the list when you want a backup outside that browser. Retain the original service records as well; a calculated due date is not a substitute for them.
Watch for another heat after breeding
UNH recommends writing down the breeding date and watching for heat about 21 days later. Its table gives an 18–24-day cycle range, while its explanatory text uses 18–23 days. The calculator follows the specified 21-day average and 18–24-day range also given in the UC ANR reproduction material.
Keep the predicted heat-watch date separate from an observed heat. A return you see is useful information for the breeding record, while a return you do not see is not proof of pregnancy. The watch window tells you when to pay attention; it does not diagnose why an animal did or did not show the expected behavior. Ask your veterinarian about pregnancy confirmation.
The length of estrus is different from the length of the whole cycle. The UC ANR source describes estrus of about 36–48 hours in gilts and 48–72 hours or more in sows. Those figures describe the heat itself, not the interval used to calculate a later return. Keeping the terms distinct makes farm notes easier to interpret.
After weaning and across the season
Most sows show heat 3–7 days after weaning. That is a separate reference point from a breeding-date calculation. The current calculator accepts breeding or birth dates, not a weaning date, so its return-to-heat line should not be read as a weaning-based prediction.
For a practical record, label the event itself: breeding, observed heat, weaning, or farrowing. A date without its event can become confusing when several animals are managed together. The livestock heat-cycle guide compares the species references and explains the exposure-period watch window. If the timing or condition of an individual sow or gilt concerns you, call your veterinarian.