Gestation Calculator

Sheep gestation calculator

A ewe carries lambs about 147 days, a shade under five months. Enter the day the raddle marked her and the lambing date falls out, with the shearing and booster dates alongside it.

147 days

Put in the date she was bred and you will get the due date, the normal window either side of it, and the dates worth writing on the calendar between now and then. The full chart is further down the page.

How far apart the breeds really are

Every dot is a breed average. The shaded band is the normal window for a single sheep — which is narrower than the gap between the shortest and longest breed, and that is the part worth noticing.

Finnsheep 145average 147Rambouillet 151

Gestation length by breed

Every figure below traces to a published study, and the ones we could not source are left out rather than guessed at. Pick a breed in the calculator and both the date and the window move with it.

BreedAverage gestationvs. sheep averageNotes
Finnsheep145 days-2144.9 days — the shortest documented.
Targhee150 days+3
Rambouillet151 days+4150–151. Fine-wool breeds run longest.

The detail, if you want it

It is the ram's breed that matters, not the ewe's

Gestation length is set by the lamb, and the lamb is half the ram, so it is the ram's breed that moves the date. Embryo transfer settled it: Finn embryos put into Targhee ewes came at the Finn length, not the Targhee one.

A Finn ewe bred to a Suffolk ram will carry longer than the same ewe bred to a Finn. If you are picking a breed in the calculator above, pick the one on the ram.

The spread itself is real but modest. Finn is the shortest anyone has measured at 145 days, the fine-wool breeds like Rambouillet the longest at 150 or 151. Individual ewes within one breed wander three to five days either way, which is wider than most of the breed differences people quote.

Marking harnesses beat memory

A raddle or a marking harness on the ram, with the crayon changed every 16 or 17 days, turns a vague tupping season into dated groups. That is what makes a lambing calculator worth anything — you get a lambing date per group rather than one long anxious month.

It also tells you which ewes returned, which is information you want in week three rather than in month five.

The last six weeks

Two thirds of lamb growth happens in the final six weeks, and that is where the management sits. Feed goes up, and ewes carrying twins and triplets need separating from the singles or the singles get fat and the triplets get twin lamb disease.

Clostridial boosters go in about four weeks out so the protection carries through the colostrum. Crutching around the same time makes the lambs' first hour easier and yours cleaner.

Full sheep gestation chart

Every day of the year, bred to due — built to print

Find the breeding date in the left column of the right month, read across for the due date. Hit print and it comes out clean.

Bred in January
BredDue
Jan 1May 28
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Bred in February
BredDue
Feb 1Jun 28
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Bred in March
BredDue
Mar 1Jul 26
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Bred in April
BredDue
Apr 1Aug 26
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Bred in May
BredDue
May 1Sep 25
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Bred in June
BredDue
Jun 1Oct 26
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Bred in July
BredDue
Jul 1Nov 25
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Bred in August
BredDue
Aug 1Dec 26
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Bred in September
BredDue
Sep 1Jan 26
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Bred in October
BredDue
Oct 1Feb 25
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Oct 4Feb 28
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Bred in November
BredDue
Nov 1Mar 27
Nov 2Mar 28
Nov 3Mar 29
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Bred in December
BredDue
Dec 1Apr 26
Dec 2Apr 27
Dec 3Apr 28
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Dec 31May 26

Built on a 147-day gestation. Dates are month and day only, so this chart works for any year — with one catch: if the pregnancy runs through 29 February, the real due date lands one day later than the chart says. The calculator at the top of the page already accounts for that.

Questions people actually ask

How long are sheep pregnant?

About 147 days, roughly five months. Normal runs from about 143 to 152 days.

How do you work out a lambing date?

Add 147 days to the tupping date. The old shepherd's version is five months less a week, which lands within a couple of days and needs no calculator at all.

Do ewes carrying twins lamb earlier?

Possibly, but not by enough to plan around. Montana State reports twins coming slightly earlier. The one study that tested it head-on found half a day of difference and could not call it significant.

Individual ewes in the same flock routinely vary by three to five days, which swamps it. Scan them so you can feed them properly, not so you can predict the date.

When should I scan the ewes?

Between about 45 and 90 days after the ram went in is the useful window, with 70 to 90 days the sweet spot for counting lambs accurately. Knowing singles from twins from triplets is what lets you feed three groups instead of one compromise ration.

What are the signs a ewe is about to lamb?

The udder fills and the teats stand out. She separates from the flock, paws at the ground and lies down and gets up repeatedly. Once she is straining properly you should see a lamb inside the hour, and if you do not, she needs help.

Where these numbers come from

Every figure on this page traces to a university extension publication or a veterinary reference. If you think one is wrong, the sources are right here so you can check it yourself.

Worth knowing

  • It is the lamb's breed that sets the length, not the ewe's. Embryo transfer proved it: put Finn embryos in Targhee ewes and they come at the Finn length. A Finn ewe carrying Suffolk-sired lambs will run longer than one carrying Finn lambs. If you pick a breed above, pick the ram's.
  • There is no defensible number for a twin adjustment. Montana State reports twins coming slightly earlier, but the one study that tested it directly found half a day at P=0.68. Do not plan around it.
  • Individual ewes routinely vary three to five days from each other within the same breed, which is larger than most of the breed differences claimed online.
  • Heat and a high plane of nutrition can shorten it two or three days.
  • We could not source per-breed figures for Dorset, Hampshire, Suffolk, Merino or Katahdin. What is documented is the pattern: fine-wool breeds longest, high-prolificacy breeds like Finn shortest. Hair sheep are not placed on that spectrum by any source we found.

Figures last checked August 2026.

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