Tim’s take:

The lambing calendar is one of those bits of kit that costs nothing but pays for itself a dozen times over, because almost every management decision in the run-up to lambing hangs off a date that someone, somewhere, should have written down. Plenty of flocks still run on the old “she’ll be ready when she’s ready” instinct, and honestly that works for the first few cycles of a small flock, but once you’re pushing 200+ ewes or juggling lambing groups across different fields, a written tupping date stops being admin and starts being the only way to keep the shed, the feed lorry and the vet on the same page.

The NADIS guidance that runs through this article is worth pinning above the desk: 143 to 147 days, with 147 as the working figure for anyone who wants to be set up and waiting rather than rushing about. Build backwards from your planned lambing window and the rest of the routine (Heptavac P boosters, the concentrate step-up in the final three weeks, scanning, housing decisions) drops into a tidy timeline instead of a January panic.

My one tip for anyone filling in the calculator: raddle the rams properly and swap the crayon at 17-day intervals. Three colour changes across tupping tells you which cycle settled the ewes, which separates the early-lambers from the late-lambers and saves a lot of guessing at scanning time.

Use this lambing calculator to turn any tupping date into an expected lambing date, then plan vaccination, feeding and housing around it with confidence.

How the lambing calculator works

A lambing calculator does one simple job: it takes the date a ewe was tupped and adds the gestation length to give an expected lambing date. Get that date right and the whole pre-lambing routine — booster vaccinations, the feed step-up, moving ewes inside — falls into place around it.

The number the calculator adds is the ewe’s gestation length. “Gestation length (pregnancy) lasts 143 to 147 days,” NADIS explained in its lambing guidance, which is roughly five months. Most UK shepherds work to 147 days as the planning figure, because it is better to be ready a few days early than to be caught out. If you turned the ram in with a raddle harness and changed the crayon colour each cycle, you can date groups of ewes to a single 17-day cycle and calculate each group separately.

To use the ready-reckoner below, find the date the ewes were tupped, then read across to the expected lambing date. If you scanned the flock, use the scanning result to split ewes carrying singles, twins and triplets — they need different feeding in the run-up, even though they share a due date. The maths never changes, but the value of writing the date down is that every other job in the calendar hangs off it.

Lambing date ready-reckoner

These worked examples add 145 and 147 days to common tupping dates, so you can see the window each group is likely to lamb in for the 2026 lambing season. The earlier figure is the “be ready by” date; the later one is the typical due date.

• Tupped 1 September: expect lambing between 24 and 26 January
• Tupped 1 October: expect lambing between 23 and 25 February
• Tupped 1 November: expect lambing between 26 and 28 March
• Tupped 15 November: expect lambing between 9 and 11 April
• Tupped 1 December: expect lambing between 25 and 27 April

Mark the earlier date of each pair on the calendar as the day the shed, heat lamps and lambing kit must be ready, not the day you start getting organised. A flock of 500 sheep can produce its first lambs across a fortnight, so the aim is to have everything in place before the leading ewes are due. Even a small flock of 50 sheep benefits from the same discipline: the early lamber always arrives on the night you were least prepared.

The pre-lambing countdown the date sets off

The value of a firm lambing date is that it anchors the jobs that have to happen at fixed points beforehand. The most important of these is the clostridial booster.

Ewes pass immunity to their lambs through colostrum, and that only works if the booster is timed correctly. NADIS advises that ewes are vaccinated against the clostridial diseases four weeks before lambing, so the antibodies peak in the colostrum the lambs drink in their first hours. Count back four weeks from the ready-reckoner date and put the vaccination day in the diary now, because a booster given too late leaves lambs exposed to diseases such as lamb dysentery and pulpy kidney.

The same countdown governs housing, bedding, and ordering colostrum substitute and stomach tubes. A calculator turns “sometime in spring” into a set of dated tasks, which is what keeps a busy lambing under control. Order vaccine and consumables well ahead of the vaccination date, not on the day, so a supply delay never pushes the booster past its window.

Feeding the in-lamb ewe

Most of a lamb’s growth in the womb happens right at the end of pregnancy, so late nutrition decides how strong lambs are born and how much colostrum the ewe produces. Guidance from AHDB notes that most foetal growth occurs in the last six weeks of pregnancy, which is exactly when the ewe’s rumen is being squeezed by the growing lambs and she can eat less.

Body condition is the practical measure to work to. Aim to hold lowland ewes at a body condition score of around 3.0 and hill ewes a little leaner, and avoid sudden feed changes. Ewe lambs carrying their first crop need extra care: AHDB reports that ewe lambs need about 20% more feed than mature ewes of similar weight, because they are still growing themselves while carrying a lamb.

Scanning earns its keep here. Knowing which ewes carry singles, twins or triplets lets you feed each group to its needs rather than over-feeding singles and under-feeding triplets on a flat ration. Where nutrition is in doubt, a vet can blood-sample a group of ewes three to four weeks before lambing to check energy and protein status while there is still time to act on the result.

Condition-score the flock at scanning and again a month before lambing, and manage thin and fit ewes as separate groups. A ewe that loses condition in late pregnancy is heading towards twin-lamb disease, while over-fat ewes carrying singles are prone to difficult births and prolapse. Neither problem is fixable in the last fortnight, which is the whole argument for working back from a known lambing date rather than reacting to ewes as they go.

Signs lambing is close

The calculator gives the diary date; the ewe gives the final confirmation. In the days before lambing, watch for udder development, the accumulation of colostrum, and slackening of the ligaments around the tail head. A ewe that separates herself from the group, paws at the bedding and goes off her feed is usually close.

Knowing the normal timings stops unnecessary interference. First-stage labour, when the cervix dilates, takes three to six hours and is quicker in older ewes. Second-stage labour, the actual delivery, typically takes about an hour, and a ewe making steady progress should be left to get on with it. The afterbirth is normally passed within two to three hours of delivery. If a ewe is straining hard for more than an hour with nothing showing, or you can see a malpresentation, that is the point to intervene or call the vet.

Having the date in front of you also tells you when to step up checks. Move ewes into individual pens as they lamb, make sure each lamb has taken colostrum quickly, and dip navels to guard against joint ill. None of this is complicated, but all of it is easier when the shed is set up and staffed before the first ewe is due rather than in a scramble on the night.

Dating groups with a raddle harness

A calculator is only as good as the tupping date you feed it. A raddle harness — a crayon block strapped to the ram’s chest — marks each ewe as she is served, and changing the crayon colour every 16 to 17 days sorts the flock into dated groups without any paperwork at tupping.

Record the date each colour went on, and note it somewhere you will still be able to read in five months’ time — a smudged crayon date on a gate is no use when the calculator needs it. A ewe marked in the first colour is due roughly 147 days after that colour started; a ewe marked in the second colour is a cycle later, and so on. Ewes re-marked in a later colour have returned to service, which flags a fertility problem worth discussing with your vet if numbers are high. Splitting a 200-head flock into two or three tupping groups this way means you can house, vaccinate and feed each group on its own timeline rather than treating a flock that lambs over six weeks as if it were all due on one day.

Colostrum and the newborn lamb

The calculated date matters right up to the moment the lamb hits the straw, because the first few hours decide survival. A newborn lamb has almost no energy reserve and cannot regulate its own temperature well, so getting a good feed of colostrum into it quickly is the single most important job at lambing. NADIS guidance on newborn lamb survival stresses warmth, a clean navel and prompt colostrum as the three things that keep losses down.

This is where the pre-lambing countdown pays off. Because the ewe was given her clostridial booster four weeks before her due date, the colostrum she produces is rich in the antibodies the lamb needs, provided the lamb actually drinks enough of it soon after birth. Have colostrum substitute, a stomach tube and a heat source ready before the first ewe lambs, so a slow or rejected lamb can be helped without a panic. Weak lambs chill fast, and a lamb that misses its early colostrum is far harder to save later.

Why the range is 143 to 147 days

No calculator is exact, because gestation length varies with breed, litter size and the age of the ewe. Hill breeds and ewes carrying multiples often lamb a day or two earlier than the 147-day figure, while single-bearing ewes can run slightly longer. That is why the ready-reckoner gives a window rather than a single day.

Treat the calculated date as the centre of a spread, not a deadline. Have the shed and the lambing kit ready by the earlier date in each pair, expect the bulk of the flock around the later date, and keep checking ewes night and morning across the whole window. Used that way, a lambing calculator is less about predicting one date and more about making sure nothing is left to the last minute.