Due Date Calculator
Due date calculator: work out your pregnancy due date and weeks along Free, no sign-up, and it runs in your browser.
Dedicated page: /en/due-date-calculator
Estimate only, not medical advice. Only about 5% of babies are born on their due date. Your doctor may adjust the date after an ultrasound.
Report an Issue
Found a mistake or a problem with this tool? Tell us below.
Quick answer
A due date calculator estimates when a baby will be born. The standard method is Naegele's rule: add 280 days, or 40 weeks, to the first day of the last period. This tool also works from a conception date, an ultrasound scan, or an IVF transfer date.
About this Due Date Calculator
Due date calculator: work out your pregnancy due date and weeks along It is one of the Health and Family on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a due date calculator?
A due date is an estimate of the day a pregnancy reaches 40 weeks, counted from the first day of the last menstrual period. It is not a deadline: only about 5% of babies are born on the due date itself, and most arrive within two weeks either side, which is entirely normal. The date still matters, because it sets the schedule for scans, tests, maternity leave and the hospital bag, and it is the reference point doctors use for every decision in the pregnancy.
Counting from the last period seems odd, because conception happens about two weeks later, but the period date is the one people usually remember, and it is the convention medicine uses. When the conception date is known, the due date is conception plus 266 days, which is the same 40 weeks counted differently. An early ultrasound measures the baby directly and is more accurate than either date, so doctors often adjust the due date after the first scan. IVF dates are the most precise of all, because the transfer date is known exactly.
How does due date maths work?
Naegele's rule adds 280 days to the first day of the last period: 1 March 2026 plus 280 days is 6 December 2026. From a conception date, add 266 days. From an ultrasound, subtract the gestational age at the scan from the scan date to find the last period, then add 280 days. From an IVF transfer, add 266 days and subtract the embryo age, so a day 5 transfer gives a due date 261 days after transfer. The tool also works out how far along the pregnancy is today, which trimester that is, and how many days remain, because those are the numbers people actually ask about every week.
When and why to use a due date calculator
- Right after a positive test: the due date turns a surprise into a plan, with a date for the first appointment.
- Planning maternity leave: employers and insurers work from the due date, not from the week of pregnancy.
- Booking scans and tests: the dating scan, the anatomy scan and screening tests each have a window measured in weeks from the last period.
Best practices
- Use the earliest ultrasound date if it differs from the period date by more than a few days: the scan measures the baby, the calendar only estimates.
- Remember the date is an estimate with a two week range either side. Pack the hospital bag by week 36, not by the due date.
Common mistakes to avoid
- Treating the due date as a deadline: going a week over is common and usually fine, and induction is normally discussed only after 41 weeks.
- Counting 40 weeks from conception: that gives a date two weeks too late, because pregnancy is counted from the period.
- Using the due date to judge the baby's size: growth varies widely, and only the scans measure it.
Who is the due date calculator for?
Expecting parents, their families, and anyone helping plan around a pregnancy. It is also used by people who have had fertility treatment, because the IVF mode uses the one date that is known exactly. The tool is an estimator, not medical advice: your doctor's dates, based on scans and examinations, always win over a calculator.
Learn: how to use the Due Date Calculator
- Pick what you know: last period date is the usual starting point, but conception, ultrasound and IVF dates all work.
- Enter the date, and for ultrasound also the gestational age at the scan.
- Read the due date, your current week and trimester, the days remaining, and the milestone table.
Features
- Four ways in: last period, conception, ultrasound or IVF transfer
- Naegele's rule (last period plus 280 days) with the working shown
- Current week and day, trimester, and days remaining
- Week-by-week milestone timeline with real dates
- Plain warning that only about 5% of babies arrive on the due date
Worked example
Last period 1 March 2026: due date 6 December 2026. On 10 November 2026 that is 36 weeks and 2 days, in the third trimester, with 26 days remaining.
Frequently asked questions
How is a due date calculated from the last period?
By Naegele's rule: add 280 days, or 40 weeks, to the first day of the last period. Pregnancy is counted from the period, not from conception, because the period date is usually the one people remember, and conception normally happens about two weeks later.
Why does my doctor give a different date?
An early ultrasound measures the baby directly and is more accurate than the period date, especially if cycles are irregular or the conception date is uncertain. Doctors usually switch to the ultrasound date when it differs by more than a few days.
What does 40 weeks mean if pregnancy is 38 weeks?
Conception to birth is about 38 weeks, but pregnancy is counted as 40 weeks from the last period, which is roughly two weeks before conception. Both numbers describe the same pregnancy from different starting points.
Is it normal to go past the due date?
Yes. Only about 5% of babies are born on the due date, and most arrive within two weeks either side. Going past 41 weeks is when doctors usually start talking about induction, so keep your appointments and call if you are worried.
What is Naegele's rule?
Add 280 days, or 40 weeks, to the first day of the last period. It is named after the German obstetrician Franz Naegele, who published it in the 1800s, and it is still the standard first estimate worldwide.
Which trimester am I in?
The first trimester is weeks 1 to 13, the second is weeks 14 to 27, and the third is week 28 to birth. The tool works this out from the same last period date as the due date.
Can the due date change?
Yes, and it often does after the first ultrasound, which measures the baby directly. A change of a few days is normal. A change of more than a week usually means the period date or the cycle length was not what you thought.
Related tools
More Health and Family like this one, plus two picks from other categories. Each opens its own dedicated page.