Enter your last period and cycle length to get your ovulation day, fertile window, next period and due date at once!

The OgleOgle Ovulation Calculator estimates the key dates of your menstrual cycle from just two inputs — the first day of your last period and your average cycle length (21–35 days). In one screen it works out your estimated ovulation day, your fertile window, your next expected period, and your estimated due date if you conceive this cycle. Everything is calculated in your browser and nothing is sent to a server.

How it works: ovulation is estimated as 14 days before your next expected period, so it adapts to your cycle length rather than assuming a fixed day 14. The fertile window covers the five days before ovulation through one day after, reflecting how long sperm and the egg stay viable — these are the days conception is most likely. The estimated due date uses Naegele's rule (last period + 280 days). A simple cycle strip highlights your period days, fertile window and ovulation day so you can see the whole cycle at a glance.

Use it to plan for pregnancy, understand your cycle, or simply know when your next period is likely. It works identically on mobile and desktop and supports dark mode.

Important: these results are estimates based on a regular cycle and are for reference only. They are not medical advice and must not be used as a contraception or family-planning method. Ovulation can shift with stress, health and cycle irregularity, so consult a doctor or professional for anything important. FAQ — Q. My cycle is irregular, is this accurate? A. The estimate assumes a regular cycle; with irregular cycles the actual ovulation day can vary widely, so treat it as a rough guide only. Q. Can I use this to avoid pregnancy? A. No — it is not a reliable contraceptive method. OgleOgle also offers other free everyday tools such as the date calculator, D-day calculator and age calculator.