BYTETOOLS

Due Date Calculator

Estimate your pregnancy due date from your last period, conception date or IVF transfer, with gestational age today and trimester dates using Naegele's rule.

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What is the Due Date Calculator?

Naegele's rule estimates a due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a 28-day cycle. Only about 4 percent of babies arrive on that exact date.

  • Naegele's rule with an adjustment for cycle length
  • Conception-date and IVF transfer (day 3 / day 5) methods
  • Current gestational age in weeks and days
  • Trimester start dates and days remaining
  • Estimated conception date and full-term window
  • Completely private — computed in your browser, never uploaded

How to use the Due Date Calculator

  1. 1

    Choose your method: last menstrual period, conception date or IVF transfer.

  2. 2

    Enter the relevant date — for LMP, the first day of your last period.

  3. 3

    For the LMP method, set your average cycle length if it is not 28 days.

  4. 4

    For IVF, select a day-3 or day-5 transfer.

  5. 5

    Read your estimated due date, gestational age and trimester dates.

About the Due Date Calculator

The ByteTools Due Date Calculator estimates a pregnancy due date using Naegele's rule — 280 days from the first day of your last menstrual period, adjusted if your cycle is longer or shorter than 28 days. You can also calculate from a known conception date or from a day-3 or day-5 IVF embryo transfer.

Alongside the estimated due date it shows your current gestational age in weeks and days, the estimated conception date, the start of each trimester, and how long is left. Dates are read from your device and every calculation runs locally in your browser — nothing about a pregnancy is uploaded, logged or stored anywhere.

This is an estimate only and is not a medical assessment, a diagnosis or a substitute for antenatal care. A dating ultrasound in the first trimester is considerably more accurate than any date-based calculation, and your midwife or doctor may revise your due date because of it. Always follow the date your care team gives you.

Frequently asked questions

How is a pregnancy due date calculated?

Naegele's rule adds 280 days — 40 weeks — to the first day of your last menstrual period. The traditional shortcut is to subtract three months, add seven days and add one year. If your last period started on 1 January 2026, the estimated due date is 8 October 2026.

How accurate is a due date calculator?

It is an estimate, not a prediction, and definitely not a diagnosis. Only about 4 percent of babies are born on their estimated due date, and roughly 80 percent arrive within the two weeks either side of it. Naegele's rule also assumes a regular 28-day cycle with ovulation on day 14 — if your cycles are irregular the date can be off by a week or more. A first-trimester dating scan is substantially more accurate and should take precedence.

Why does pregnancy count from the last period rather than conception?

Because the date of your last period is something you can usually recall, while the date of conception is rarely known. It means the first two weeks of a '40-week' pregnancy occur before conception happens — actual gestation from conception is about 266 days.

How do I calculate a due date if my cycle is not 28 days?

Adjust by the difference between your cycle length and 28 days, because ovulation shifts with it. A 32-day cycle adds four days to the due date; a 25-day cycle subtracts three. This calculator applies that adjustment automatically when you set your cycle length.

How are IVF due dates calculated?

IVF dating is more precise because the fertilisation date is known. For a day-5 blastocyst transfer the due date is 261 days after transfer; for a day-3 embryo transfer it is 263 days. Your clinic's date always takes precedence over any calculator.

When do the trimesters start and end?

The first trimester runs from week 0 through the end of week 13, the second from week 14 through the end of week 27, and the third from week 28 to birth. Full term is defined as 39 weeks 0 days to 40 weeks 6 days.

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