How your due date is estimated
The standard way to estimate a due date is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This calculator does that for you and gives an estimated delivery date. It assumes a regular 28-day cycle, so it is an estimate, and only about 1 in 20 babies actually arrive on the exact date.
Worked example
If your last period began on 1 January 2026, your estimated due date is around 8 October 2026, which is 40 weeks later.
Good to know
A dating ultrasound is more accurate than the last-period method, especially for irregular cycles, and your provider may adjust the date. Use this as a friendly estimate, not a fixed appointment.
Frequently asked questions
How is a due date calculated?
By adding 280 days (40 weeks) to the first day of your last period, known as Naegele's rule. It assumes a regular 28-day cycle.
How accurate is it?
It is an estimate. Only around 5% of babies arrive on the exact due date, and a dating ultrasound is more precise, especially for irregular cycles.
What if my cycle is not 28 days?
Naegele's rule assumes a 28-day cycle. Longer or shorter cycles can move the date by a few days; your provider can adjust it.
Is an ultrasound more accurate?
Early ultrasound dating is generally more accurate than the last-period method, especially with irregular cycles.
Is this medical advice?
No. It is a general estimate. Your midwife or doctor gives the official due date.