Enter the first day of your last period and your usual cycle length. See your estimated ovulation day, fertile window and next period, or switch to find your due date.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Ovulation and due date works
Both estimates count from the first day of your last period and your usual cycle length (20 to 45 days).
Ovulation mode – Ovulation is estimated at 14 days before your next period, so cycle length minus 14 days after the period starts. – The fertile window runs from five days before ovulation to one day after. – It also shows your next period date and the due date if you conceive that cycle.
Due date mode – The due date is 280 days (40 weeks) from the first day of the last period. This follows Naegele’s rule, which assumes a 28-day cycle, and the tool adjusts for a longer or shorter cycle. – It also shows how far along you are, trimester start dates and the estimated conception date.
These are estimates for a regular cycle. The tool is not a method of contraception, and a dating scan can change your due date. It does not cover IVF, where dating is based on the transfer date.
Tips and common mistakes
Track your cycle for a few months to find your usual length. If your cycles are irregular, ovulation dates here will be less reliable. Only a small minority of babies arrive on their due date, so treat it as a rough guide. Speak to a GP, midwife or pharmacist for advice on conception or pregnancy. General information, not medical advice.
Frequently asked questions
How is ovulation estimated?
About 14 days before the next period is due, so for a 28-day cycle that is around day 14. The fertile window is the five days before and one day after.
How is the due date calculated?
It is 280 days from the first day of your last period, adjusted if your cycle is longer or shorter than 28 days. A dating scan may give a different date.
Can I use this to avoid pregnancy?
No. It is an estimate based on a regular cycle and is not a form of contraception.