Swine Gestation Calculator — Farrowing Date

Background.

A sow served on a Monday will, on average, farrow on a Wednesday sixteen weeks later. Pig pregnancy runs about 114 days, which is 16 weeks and 2 days, and that fixed weekday shift is why units that breed in weekly batches can plan their farrowing rooms months ahead. The MSD Veterinary Manual's table of gestation periods gives 114 days for the pig, and Penn State Extension's 2026 review of how gestation shapes piglet outcomes uses the same average.

Enter the service date and you get two dates back. Day 114 is the average farrowing date. Day 110, four days earlier, is a marker for the last stretch: the time to have the farrowing pen washed and ready, check heat lamps or creep areas, and start watching the sow more often. Penn State places late gestation at roughly day 70 to 114, the stage in which most fetal weight is gained, so the sow's feeding and body condition through those weeks matter as much as the date itself.

The average is not an appointment: individual sows farrow a few days either side of it, and your own herd records show how early or late your animals tend to run. Nor does a due date show that she is pregnant; a pregnancy check does that.

No induction, hormone or medication guidance is given here, and an average date is no basis for intervening. A sow that is unwell, has an abnormal discharge, strains without producing piglets or is visibly distressed needs a veterinarian promptly, whatever the calendar says.

What is swine gestation calculator?

Swine gestation is the time from conception to farrowing, the birth of the litter. Because a sow's exact moment of conception is not observed, farms count from the recorded service or insemination date instead, and the published average of about 114 days is applied from there. The result is a calendar date for planning: when to move the sow, when to staff for farrowing and when the litter should arrive.

How to use this calculator.

1. 01Enter the recorded breeding date from your service records. If she was inseminated more than once in the same heat, use whichever service your records normally count from, usually the first, and keep to that habit for every sow.
2. 02Read the day-110 date and use it to have the farrowing accommodation clean and ready before she is likely to need it.
3. 03Read the expected farrowing date, day 114, and plan observation and labour around the days either side of it.
4. 04If she comes back into heat about three weeks after service, she did not hold to that mating; enter the new service date and start again.

The formula.

Expected farrowing date = breeding date + 114 days, and observation date = breeding date + 110 days. Day 0 is the service date itself, so day 1 is the following morning. Whole days are added on the ordinary calendar, so month lengths and leap years are handled exactly: a service on 10 November 2027 runs through 29 February 2028 and lands on 3 March 2028. The 114 comes from the MSD Veterinary Manual's gestation table and agrees with Penn State Extension's stated average. No allowance is made for breed, parity or litter size: a gilt's first litter and a sow's sixth get the same 114 days.

A worked example.

Frequently asked questions.

References& sources.

1. [1]MSD Veterinary Manual's Approximate Gestation Periods table lists pig gestation at 114 days.
2. [2]Penn State Extension, updated June 2026, independently gives an approximately 114-day average.