About this tool
This calculator estimates your next three periods, ovulation days and fertile windows from the first day of your last period, your cycle length (21 to 35 days) and your period length. Ovulation usually happens about 14 days before the next period, and the fertile window is counted from 5 days before ovulation to 1 day after. For example, with a last period starting on 2026-10-01 and a 28-day cycle, the next period is expected on 2026-10-29, ovulation around 2026-10-15 and the fertile window from 2026-10-10 to 2026-10-16. Real cycles move with stress, illness, travel, sleep and age, so treat the dates as a rough guide. This is not a medical diagnosis and is not a method of birth control; for irregular cycles, pain or planning a pregnancy, talk to an obstetrician-gynecologist. The dates you enter are not saved, not put in the page address and not sent anywhere; the calculation runs only in your browser.
How to use
- Pick the first day of your last period.
- Enter your usual cycle length (21-35 days) and period length.
- Read the next three expected periods, ovulation days and fertile windows.
- Remember it is an estimate and see a doctor for medical questions.