Farrowing Date Calculator
Worked Examples
Two real calendars. One ordinary, one that crosses into the next year — which is where counting on your fingers goes wrong.
Why "three months, three weeks, three days" is not three months
Everybody in the Philippines learns gestation as 3-3-3, and it is right: 3 months + 3 weeks + 3 days. The trouble starts when somebody counts three calendar months first. Three months from 2 August is 2 November — but three months is 92 days here and 89 days if you start in December, so the answer moves depending on which months you crossed. 114 days does not move. Count days, not months.
The second thing worth knowing: 114 is the middle, not the deadline. Day 111 to day 117 is all normal. A sow farrowing on day 112 is not premature and a sow at 116 is not overdue, and the number of people who panic on day 112 because a calculator gave them one date is the reason this tool gives you a window.
Example 1 of 2
A sow served on a Sunday in August, weaned at 28 days
The ordinary case. A backyard sow served on August 2, 2026, weaned at four weeks — the commonest choice, and a fair balance between getting her back in pig and selling a weanling with some size on it.
| Input | Value |
|---|---|
| Date served | August 2, 2026 |
| Weaning age | 28 days |
| Result | Value | Notes |
|---|---|---|
| Due date | November 24, 2026 | Tuesday |
| Normal window | November 21, 2026 – November 27, 2026 | Day 111 to 117. All of it normal. |
| Watch for a return to heat | August 20, 2026 – August 26, 2026 | If she stands again here, she did not take. |
| Move to the farrowing pen | November 17, 2026 | About a week before due. |
| Weaning | December 22, 2026 | 28 days after farrowing. |
| Earliest sensible re-service | December 26, 2026 | Cycle length: 146 days. |
At 146 days a cycle, this sow is on course for 2.50 litters a year — if she takes first time, every time. She will not. That is what the return-to-heat date is for, and why it is the second row of the calendar rather than a footnote.
How to use this on your farm
The single date to write on the pen wall is not the due date. It is August 20, 2026 — three weeks after service. Catching an empty sow there instead of at day 114 saves you 93 days of feeding her for nothing: at 2 to 2.5 kg of gestation feed a day and ₱34 to ₱37 a kilo, that is roughly ₱6,300 to ₱8,600 of feed, on one sow, for five minutes of watching in one week.Bisaya / Cebuano
Ang 114 ka adlaw mao ang tunga sa window, dili ang deadline. Normal ra gyud kung manganak siya sa day 111 o sa day 117. Ayaw kabalaka kung sayo og tulo ka adlaw.
Ang labing importante nga petsa dinhi kay ang ikatulong semana human sa kasta. Kung mopakita siya og init pag-usab niadtong panahona, wala siya nabuntis. Kastaha dayon. Kung hulaton nimo ang day 114 aron mahibaw-an, upat ka bulan na ang nawala nimo.
Example 2 of 2
Served the Sunday before Christmas, weaned at 42 days
The case where mental arithmetic fails. A service on December 20, 2026 crosses New Year, crosses February, and lands in April. Weaning is set to 42 days here, which is common in backyard herds selling weanlings by size rather than by age.
| Input | Value |
|---|---|
| Date served | December 20, 2026 |
| Weaning age | 42 days |
| Result | Value | Notes |
|---|---|---|
| Due date | April 13, 2027 | Tuesday |
| Normal window | April 10, 2027 – April 16, 2027 | Seven days, all of it normal. |
| Watch for a return to heat | January 7, 2027 – January 13, 2027 | Second week of January. |
| Move to the farrowing pen | April 6, 2027 | A week before due. |
| Weaning | May 25, 2027 | 42 days on the sow. |
| Earliest sensible re-service | May 29, 2027 | Cycle length: 160 days. |
Look at what the longer lactation costs. This cycle is 160 days against 146 in example 1 — fourteen days longer, which is 0.22 of a litter a year. Whether that is a bad trade depends entirely on what a heavier weanling fetches where you sell, and it is a real decision rather than an obvious one.
How to use this on your farm
If you are planning services rather than recording them, run the date you are considering and look at where the due date lands. A farrowing in the middle of a fiesta week, or while the only person who can check on her at 2am is away, is a farrowing you want to move by a cycle — and moving it means moving the service by 21 days, not the due date by a week.What this calculator will not do
It gives you dates. It does not give you a vaccination schedule, a deworming plan or a dosage, and that is deliberate — those depend on your herd's disease history and on what is circulating in your barangay, and they belong with a vet or your municipal agriculture office. A calendar that printed a jab date next to a due date would be pretending to know something it does not.
It also assumes she took. Everything after the return-to-heat window is conditional on that window passing quietly, which is exactly why it is on the calendar at all.