A useful date, not a deadline
Due Date Calculator UK: Estimate Your Baby’s Due Date
Quick answer: enter the first day of your last period and your usual cycle length—or a known conception date—to estimate your due date. It is a starting point, not an appointment with your baby. Pregnancy commonly lasts 37 to 42 weeks from the first day of the last period, and your NHS dating scan may give you a more accurate date.
Free UK pregnancy tool
Calculate your estimated due date
Choose the date you know best. Your answer appears on this page; KidsCo does not send, save or share the dates you enter.
Your estimated due date is
Use these as planning estimates. Your midwife or doctor will use your history and scans. If they give you a different estimated due date, follow the date recorded by your maternity team.
Privacy note: the calculation runs only in your browser. Nothing you type is submitted to KidsCo.
How the due date calculation works
Pregnancy dating has one slightly odd habit: the count normally starts before conception. Clinicians count from the first day of your last menstrual period because that date is usually easier to identify. A standard 28-day-cycle estimate adds 280 days, or 40 weeks, to that date.
If your usual cycle is longer or shorter, this calculator shifts the estimate by the difference. A 30-day cycle adds two days; a 26-day cycle takes two away. If you enter a conception date instead, it adds 266 days and treats two weeks earlier as the estimated start of pregnancy dating.
That arithmetic is useful, but bodies do not run like calendar software. Ovulation can move, implantation happens after fertilisation, periods may be mistaken for other bleeding, and cycle length can vary. If you are still trying, our ovulation calculator explains why even a neat-looking fertile-window date remains an estimate.
How to work out your average cycle length
Day one is the first day of proper period bleeding. Count through to the day before the next period begins; that total is one cycle. If you have tracked several recent cycles, add their lengths and divide by the number of cycles. For example, 27, 29 and 28 days average 28 days.
An average can hide a wobbly pattern. If those cycles were 22, 35 and 27 days, typing 28 would make the answer look more certain than it is. You can still see a rough estimate, but tell your midwife that your cycles vary and let scan dating take the lead. If you have only one recent cycle, use its length or the 28-day default and treat the result as provisional.
How accurate is a due date calculator?
It is best thought of as an early estimate. It may be fairly close when you know the first day of a normal last period and have regular cycles. It is less reliable if your periods vary, you recently stopped hormonal contraception, you have been pregnant recently, you were breastfeeding without regular periods, or the bleeding date was uncertain.
The NHS dating scan, normally offered around 10 to 14 weeks, measures your baby and can adjust the estimated due date. That scan date is the one your maternity notes will generally use. Read our week-by-week pregnancy scan timeline for what the first and second routine scans are for, or our guide to an early pregnancy scan if you are wondering what can be seen before then.
What your result means
The estimated due date
This is the point at which the pregnancy reaches an estimated 40 weeks. It is not a prediction of the exact day labour will begin. A due date is more like the centre of a useful planning window than a booking your baby has promised to keep.
The 37–42 week range
The NHS explains that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period. The tool shows that whole span around your estimate. Your own care may use more precise terms and plans, particularly if you or your baby need additional monitoring.
Estimated scan windows
The tool also shows the calendar dates that correspond to 10–14 weeks and 18–21 weeks. These mirror the usual NHS windows for the dating scan and the screening scan commonly called the 20-week scan. They are not appointment dates. Local services arrange scans, and the timing can differ for individual care.
Estimated pregnancy age today
This is the number of completed weeks and extra days since the calculated start. If a clinician has already dated your pregnancy, use the gestation in your notes rather than recalculating it from memory.
What to do after a positive pregnancy test
Contact your local NHS maternity service, midwife or GP to begin antenatal care; many areas let you self-refer. Do not wait for a calculator date before getting started. The NHS pregnancy guide covers antenatal care, food, medicines, symptoms and week-by-week information.
If the test is very new, our guide to the early signs of pregnancy separates common clues from things a test or professional needs to confirm. At around the end of the first trimester, 12 weeks pregnant explains the everyday changes many people notice without treating them as a checklist.
Later, dates become useful for practical planning. You can save our UK birth plan template and read the signs of labour, but follow the timings and contact advice from your own maternity unit.
What if you do not know your last period?
That is common. Periods may have been irregular, very light or absent, or early pregnancy bleeding may have looked like a period. Do not invent a precise date simply to make the calculator produce an answer. Tell your midwife what you do and do not know; the dating scan can help estimate gestation.
If you are bleeding now, the date is not the main question. Read our practical guide to bleeding in early pregnancy and contact the appropriate NHS service if you have pain, heavy bleeding, feel faint or are worried. A web calculator cannot assess symptoms or rule out an ectopic pregnancy or miscarriage.
How does this work after IVF or fertility treatment?
Fertility clinics know the treatment dates and may calculate gestation from egg collection, insemination, embryo age and transfer date. A simple conception-date field cannot represent every protocol. Use the estimated due date supplied by your clinic, and ask them which date to give your maternity service.
Common due date questions
Can the due date change?
Yes. A dating scan may replace the period-based estimate, particularly when the measurements and menstrual dates differ. Later scans look at growth and wellbeing but do not simply keep rewriting gestational age each time a measurement is a little above or below average.
Does a longer cycle mean a longer pregnancy?
Not necessarily. A longer cycle often means ovulation happened later, so the starting estimate needs shifting. It does not prove that the time from conception to birth will be longer.
What if the calculator says I am more than 42 weeks?
Check the date you entered. If you are currently pregnant and think you may be at or beyond the date in your maternity notes, contact your maternity team for their plan. Do not use an online result to decide whether to wait.
Can I plan leave or travel around the exact date?
Use the estimate to start a conversation, but leave room around it. Babies can arrive before or after 40 weeks, and individual health or pregnancy circumstances can change a care plan.
Is a due date calculator a diagnosis?
No. It cannot confirm pregnancy, show whether it is developing normally, locate a pregnancy or assess pain, bleeding or reduced movements. It provides calendar estimates only.
The bottom line
A period or conception date can produce a sensible first estimate. Use it to picture the weeks ahead, not to judge your body against an exact deadline. Begin maternity care, attend the scans you are offered and let the date recorded by your maternity team take priority.
Sources reviewed
Editorial check: KidsCo Editorial Team, 6 August 2026. This tool provides general calendar estimates for UK readers; it is not medical advice, a diagnosis or a substitute for your maternity record.
- NHS: current due date calculator guidance
- NHS: 12-week dating scan
- NHS: ultrasound scans in pregnancy
- NHS trying to get pregnant
- NHS due date calculator (original KidsCo reference retained; NHS redirects it to the current guidance)
- yourduedate (original independent reference retained for continuity; not used as the basis for NHS care advice)
