Pregnancy due date calculator
Choose how you are dating the pregnancy and enter the date. You get an estimated due date, the weeks pregnant today and the 37 to 41 week range.
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How do I work out my due date?
Count 280 days (40 weeks) from the first day of your last menstrual period. This is Naegele's rule, which is the same as adding one year, subtracting three months and adding seven days. For a period that began on March 18, 2026, 280 days later is December 23, 2026. If you know the conception date instead, add 266 days (38 weeks). The result is an estimate: only about 4 in 100 babies arrive on the due date, and about 80% are born between 37 and 41 weeks.
Why we built this calculator
Almost everyone who is pregnant, or planning to be, wants to know two things: when is the baby due, and how far along am I? A calendar and a few sums give the answer, but the sums are easy to get wrong across month ends and leap years. This calculator does them in one step and shows the full picture: the estimated due date, the dates that mark 37 and 41 weeks, an estimate of conception, and how many weeks and days you are today. It also says clearly how rough a due date is.
Everything is worked out in your browser. Nothing you enter is sent anywhere.
How it works
Pregnancy is dated from the first day of the last menstrual period (LMP), even though conception happens about two weeks later, because that date is known and the conception date usually is not. A full-term pregnancy is counted as 40 weeks, or 280 days, from the LMP. Naegele's rule estimates the due date by adding a year, taking away three months and adding seven days, which gives nearly the same result as adding 280 days. This tool adds 280 days exactly, using calendar dates so daylight saving cannot move a day. If you choose the conception date, the LMP is taken as 14 days earlier. If you already have a due date, the LMP is taken as 280 days before it, and the tool shows how far along you are. The rule assumes a regular cycle of about 28 days with ovulation around day 14; if your cycle is much longer or shorter, or your dates are uncertain, the estimate can be off.
How to use it
- Choose what you know: the first day of your last period, the conception date or a due date you have been given.
- Enter the date.
- Read the due date, the weeks and days pregnant today and the 37 to 41 week dates.
- Check against the date your doctor or midwife gives you, particularly if an early ultrasound has set a different date.
- Use the dates for planning only.
How reliable is a due date
What the numbers mean in practice.
| Point | What it means |
|---|---|
| Due date (40 weeks) | Only about 4% of babies are born on their expected due date |
| 37 to 41 weeks | About 80% of births happen in this range |
| Dating by ultrasound | A first-trimester ultrasound is generally the most accurate way to date a pregnancy, and if it differs from the LMP date, the ultrasound date is usually used |
| Irregular cycles | The LMP date can be off by days or weeks |
Use cases
- Finding an estimated due date soon after a positive test
- Checking how many weeks pregnant you are today
- Planning leave from work, appointments and a hospital bag
- Working back from a due date to the likely conception date
- Comparing your own count with what your clinic gave you
- Telling family and friends roughly when to expect news
Problems you may run into
Most surprises come from the starting date.
| What you see | Likely cause | What to do |
|---|---|---|
| My doctor's date is different | An ultrasound set the date, or the cycle is not 28 days | Follow your care provider's date; this one uses the last period |
| 'Choose a date' | The date box is empty | Pick the date from the calendar or type it as year-month-day |
| Weeks pregnant shows 'n/a' | The date is in the future or more than about 43 weeks ago | Check the date and the choice of what it stands for |
| I do not know the date of my last period | The estimate needs a starting point | See a health professional; an ultrasound can date the pregnancy |
| Twins or a high-risk pregnancy | Timing and plans may differ | Follow your care provider's advice |
| The conception date looks odd | It is simply 14 days after the period began, assuming ovulation on day 14 | It is only a guess, and sperm can survive for days |
Using the result
A due date is a useful guide, not a deadline. Most babies are born within a couple of weeks either side of it, and the first pregnancy often goes a little past it. Your doctor or midwife sets your official date and follows your own pregnancy. This is general information, not medical advice, and it cannot replace a check-up.
The formula
How the answer is worked out.
due date = first day of last period + 280 days (Naegele's rule: + 1 year − 3 months + 7 days)from conception: due date = conception date + 266 days; last period used = conception date − 14 daysfrom a due date: last period used = due date − 280 days37 weeks = last period + 259 days; 41 weeks = last period + 287 days
Dates are worked out as calendar days, so leap years and daylight saving are handled. The 28-day cycle assumption is built in. Only the dates you enter are used.
Example: last period began March 18, 2026
A regular 28-day cycle.
- 280 days laterDecember 23, 2026
- Naegele's rule+1 year, −3 months, +7 days: December 25, 2026
- Range37 weeks: December 2, 2026; 41 weeks: December 30, 2026
Due about December 23, 2026
The rule and 280 days differ by a day or two because months have different lengths.
Terms on this page
- LMP
- The first day of the last menstrual period.
- Gestational age
- Weeks and days since the first day of the last period.
Common questions
How is a due date calculated?
280 days (40 weeks) after the first day of the last menstrual period.
What is Naegele's rule?
Add one year, subtract three months and add seven days to the first day of the last period.
How accurate is a due date?
About 4% of babies arrive on it; about 80% arrive between 37 and 41 weeks.
Why does my doctor give a different date?
An early ultrasound may have set it.
How far along am I?
The calculator shows the weeks and days pregnant today.
Is this medical advice?
No. Follow your doctor or midwife.
Is anything I enter sent to a server?
No. The calculation happens in your browser and nothing you type leaves your device.
Where these numbers come from
- Wikipedia — Naegele's rule The rule, 280 days from the last period, and how often babies arrive on the due date (about 4%) and between 37 and 41 weeks (about 80%).
Last verified 2026-10-25 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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