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Fertile window, ovulation date and next period.
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Due Date Calculator
An estimated due date, with trimesters and weeks so far.
Due Date Calculator tool
For the first option. 28 is the average.
For the IVF option.
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Estimated due date
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How far along
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Trimester
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Days to go
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Conception around
| Milestone | Date | Week |
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A due date is an estimate, not an appointment. About one baby in twenty arrives on it; most arrive in the two weeks either side. A dating scan is more accurate than any calculation, and it is the figure your midwife or doctor will use.
About the Due Date Calculator
Due Date Calculator estimates when a baby is due, from the first day of the last period, from a known conception date, or from an IVF transfer. The standard method is Naegele's rule: 280 days -- forty weeks -- from the first day of the last period. That counts from before conception, which is why a pregnancy is described as forty weeks when the baby has only been developing for about thirty-eight.
Cycle length matters and most calculators ignore it. The rule assumes a twenty-eight day cycle with ovulation on day fourteen. The second half of a cycle is fairly constant at about fourteen days, so a longer cycle means later ovulation and a later due date, by roughly the number of days the cycle differs. Entering your own cycle length applies that adjustment.
The due date is an estimate and should be treated as one. Only about five per cent of babies arrive on it; the great majority arrive in the two weeks either side, which is why full term is defined as a range rather than a day. An early dating scan measures the baby directly and is more accurate than any date arithmetic, so where the two disagree, the scan is the figure your midwife or doctor will work from. Nothing here is medical advice.
This is general information and not medical advice. The formula cannot know your medical history, your medication, your body composition or anything else specific to you. Speak to a doctor or another qualified professional about any question concerning your health, and never delay seeking advice because of a figure on this page.
Your measurements are processed inside your own browser. Height, weight, age and anything else you enter never leave your device, nothing is logged, and nothing survives closing the tab.
Looking for something this does not cover? FreeToolsHub.org is another hub of free online tools that need no account, and it is worth a look if due date calculator is only part of what you are working on.
How to use the Due Date Calculator
- Choose work from and embryo age at transferThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
- Enter date and usual cycle length (days)Decimals are fine, and thousands separators are accepted and ignored.
- Read the estimated due date, how far along and trimesterThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
- Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row.
Features
- Shows estimated due date, how far along, trimester, days to go and conception around together, so you are not running it twice
- A results table you can read across
- Updates as you type, with no button to press and no page reload
- Copy the result in one click
- Free, no sign-up, and it runs entirely in your browser
Who uses this tool
The due date calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:
- Estimating a due date from the last period
- Adjusting the estimate for a longer or shorter cycle
- Working out a due date from an IVF transfer date
- Seeing which week the scan and trimester milestones fall in
Frequently asked questions
How is a due date calculated?
Naegele's rule: 280 days from the first day of the last period, adjusted for cycle length when it differs from twenty-eight days.
Why count from the last period and not conception?
Because the date of the last period is usually known and the date of conception usually is not. It is why the forty weeks includes about two weeks before conception.
How accurate is it?
About one baby in twenty arrives on the estimated date. Most arrive within two weeks either side. A dating scan is more accurate than any calculation.
Does an irregular cycle change things?
Yes, substantially, because the method depends on knowing roughly when ovulation happened. With an irregular cycle a scan is the only reliable way to date a pregnancy.
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Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.