About this calculator
Enter the breeding date and this calculator projects when a sow is due to farrow, using the notably consistent 114-day gestation length that pigs are known for — commonly remembered by generations of pig producers with the mnemonic "3 months, 3 weeks, and 3 days."
One of the most predictable gestation lengths in livestock
Compared to horses (a 50-day normal range) or even cattle (an 8-day range), swine gestation is remarkably tight, typically falling between 112 and 120 days with most sows farrowing within a day or two of 114. This consistency is a big part of why commercial pig operations can run tightly scheduled, batch-based farrowing systems, with groups of sows bred together and expected to farrow within the same narrow window weeks later.
Litter size: pigs are the outlier
Where cattle and horses almost always have a single offspring and sheep or goats commonly have twins, pigs are a different story entirely — litters of 10 to 14 piglets are routine in modern commercial breeds, and litters of over 16 aren't unheard of. As with the other litter-bearing species on this site, larger litters tend to shorten gestation slightly, and sow parity matters too: gilts (sows farrowing for the first time) often have smaller litters and, on average, slightly shorter gestation than mature sows on their third or later litter.
Why commercial timing matters so much
In large-scale pork production, an accurate farrowing date isn't just useful, it drives a whole logistics chain: sows are typically moved into individual farrowing crates about a week before their due date, staff schedules are built around expected farrowing windows, and heat lamps, processing supplies (for tasks like iodine navel dips, tail docking, or iron injections depending on the operation's protocols), and colostrum management plans are all timed off this one calculated date.
Feeding through gestation and into lactation
Sows are typically fed a restricted gestation diet for most of pregnancy to prevent excessive weight gain (which increases farrowing difficulty), then switched to a much higher-energy lactation diet in the final days before farrowing and throughout nursing, since milk production for a large litter is enormously energy-demanding. Getting this feed transition timed correctly around the calculated due date has a real, measurable effect on piglet birth weight and the sow's milk supply.
The importance of colostrum timing
Piglets are born with essentially no passive immunity of their own and depend entirely on colostrum consumed in the first 24 hours of life for disease protection — and because larger litters mean more competition for a limited number of functional teats, knowing when farrowing is likely to start lets caretakers be present to help weaker piglets nurse in that critical early window, which measurably improves survival in large litters.
Weaning and the sow's reproductive turnover
In commercial operations, sows are typically weaned from their piglets after 3 to 4 weeks of lactation, at which point they return to heat within about a week and are rebred — meaning a sow's entire reproductive cycle, from breeding to farrowing to weaning to rebreeding, can repeat roughly every 5 months. This tight, repeatable cycle is only manageable at scale because gestation length is so predictable; a farrowing date calculator that was routinely off by a week or more would ripple through every subsequent breeding group scheduled behind it.
Signs farrowing is imminent
In the final 24 hours, sows commonly show restlessness, udder development with milk that can sometimes be expressed by hand, and distinctive nest-building behavior (pawing and arranging bedding) even in confinement systems where there's little material to actually build with — a strong instinctive behavior inherited from wild boar ancestry. A drop in body temperature and reduced appetite in this same window are additional signs many producers watch for. Piglets are typically born roughly every 15 to 20 minutes once active farrowing starts; a gap significantly longer than that, especially with visible straining and no piglet delivered, is generally treated as a signal to check the sow or call for assistance.