Pregnancy Due Date Calculator
Enter the first day of your last period and your cycle length below to see your results. This calculator is designed for general guidance and planning only.
For official medical guidance on your pregnancy journey, please visit the NHS Pregnancy guide or speak directly with your midwife.
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How the Pregnancy Due Date Calculator Works
The calculator then displays:
The standard ‘Naegele’s Rule’ formula assumes every woman has a perfect 28-day cycle and ovulates on day 14. However, many women have cycles ranging from 21 to 35 days. If your cycle is longer than 28 days, you likely ovulate later in the month; if it is shorter, you ovulate earlier. By inputting your specific cycle length, this calculator adjusts the estimated date of conception, providing a much more personalised and accurate due date.
Example 1: The Standard 28-Day Cycle
If the first day of your last period was the 1st of January, and you have a standard 28-day cycle:
– The calculator adds 280 days (40 weeks) to your last period.
– Your estimated due date will be the 8th of October.
If today is the 1st of February, the calculator will show you are roughly 4 weeks pregnant, even though conception likely occurred only two weeks ago.
Example 2: How Longer Cycles Change the Date
If your last period was also the 1st of January, but your average cycle is 35 days (meaning you ovulate later in the month):
– The calculator adjusts for that extra week.
– Your estimated due date shifts back to the 15th of October.
Takeaway: This is why entering your exact cycle length provides a much more accurate estimate than generic calculators.
When Should You Use a Pregnancy Due Date Calculator?
About This Pregnancy Due Date Calculator
Frequently Asked Questions
It is an estimate, not a guarantee. In reality, only about 4% to 5% of babies are born exactly on their estimated due date. Most babies arrive anywhere between 37 and 41 weeks of pregnancy. Your midwife or doctor will use a dating ultrasound scan (usually around 10 to 14 weeks) to provide a much more precise medical due date based on the baby’s actual development.
Because ovulation timing dictates when conception actually occurs. A woman with a 32-day cycle conceives later in the month than a woman with a 24-day cycle, even if their last periods started on the exact same day. Adjusting for this prevents your due date from being artificially skewed.
Some versions of the calculator allow this. If your form includes a selected date, the results will reflect that date.
Yes. All calculators on Calchub are free to use. You’re welcome to bookmark any of your favourites and use them as much as you would like.
Disclaimer
Last checked/updated February 2026.