About this calculator
Enter the first day of your last period and your usual cycle length, or a conception or IVF transfer date. The calculator also shows how far along you are today.
How to use it
- Choose how to calculate.
- Enter the date.
- For the period method, enter your usual cycle length.
The formula
- 280 days
- 40 weeks, Naegele’s rule
- IVF
- transfer date + 263 days (day-3) or 261 days (day-5)
Worked example
Last period started 1 June 2026, 28-day cycle
- Due date = 1 June 2026 + 280 days = 8 March 2027.
- On 1 October 2026 the pregnancy is 17 weeks 3 days, in the second trimester.
What the result means
Treat the date as the middle of a window: most babies arrive between 38 and 42 weeks.
Assumptions
- Regular cycles for the period method.
Limitations
- An early ultrasound gives a more accurate date; doctors usually go by that.
- This is general information, not medical advice.
Frequently asked questions
How accurate is a due date?
Only about 1 in 20 babies arrive on it; most come within two weeks either side.
How is the due date calculated?
By adding 280 days to the first day of the last period, adjusted for cycle length.
Which date do doctors use?
Usually the date from the first-trimester ultrasound.
Not medical advice. This calculator is a general screening tool and cannot diagnose any condition. Results are estimates. Please talk to a doctor or a qualified health professional about your health and before making changes to diet, exercise or treatment.
Last reviewed on 1 October 2026. Found a mistake? Tell us.