Ovulation Calculator

Ovulation Calculator estimates when ovulation happens in a cycle and marks the days around it when conception is possible. It counts backwards from the expected next period rather than forwards from the last one, which is the more accurate way round: the luteal phase, between ovulation and the next period, stays close to fourteen days, while the first half of a cycle is the part that varies.

The fertile window runs from about five days before ovulation to the day after. Sperm can survive in the reproductive tract for up to five days; an egg is viable for roughly a day after release. That asymmetry is why the window opens well before ovulation and closes almost immediately after it.

This is a calendar estimate and its limits are real. Ovulation moves between cycles even for people whose periods are regular, and stress, illness and travel all shift it. It is not a contraceptive method: calendar-based avoidance has a substantially higher failure rate than any modern contraception. If you are trying to conceive, an ovulation predictor test or basal body temperature tracking will tell you more than a calendar can, and persistent irregular cycles are worth discussing with a doctor.

This is general information and not medical advice. The formula cannot know your medical history, your medication, your body composition or anything else specific to you. Speak to a doctor or another qualified professional about any question concerning your health, and never delay seeking advice because of a figure on this page.

Your measurements are processed inside your own browser. Height, weight, age and anything else you enter never leave your device, nothing is logged, and nothing survives closing the tab.

• Shows estimated ovulation, fertile window, next period, cycle day today and phase now together, so you are not running it twice
• A results table you can read across
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The ovulation calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Estimating a fertile window from a regular cycle
• Working out when the next period is likely to start
• Seeing which cycle day today is
• Planning ovulation tests around the likely window