Free cycle calculators

Quick estimates from the dates you already know: your next periods, your fertile window, a due date, or your average cycle length.

• Period calculatorEstimate your next six periods, with start and end dates, from your last period, cycle length and period length.
• Ovulation calculatorEstimate your ovulation day (about 14 days before your next period) and six-day fertile window for the next three cycles.
• Due date calculatorEstimate a due date, current pregnancy week and trimester from your last period (280 days) or conception date (266 days).
• Cycle length calculatorWork out your average cycle length, shortest and longest cycle, and how much it varies from your period start dates.

How these calculators work

All four use simple calendar arithmetic, the same rules the Lunation app uses for its first predictions:

• Cycle day 1 is the first day of your period, and your cycle length is the number of days from one period start to the next.
• Next period: the first day of your last period plus your cycle length, repeated for each cycle after.
• Ovulation: about 14 days before your next period starts. The fertile window is the six days ending on ovulation day.
• Due date: 280 days after the first day of your last period, adjusted for cycles longer or shorter than 28 days, or 266 days after conception.

Every result is an estimate. Cycles vary by a few days for most people, and none of these tools is a method of contraception or a substitute for dating a pregnancy with your midwife or doctor. Each page explains its method, shows a worked example and lists its sources.

Nothing you enter leaves your browser: the calculations run on your device and nothing is stored.

Medical disclaimer. These calculators give general wellness estimates, not medical advice. They do not diagnose or treat any condition and are not a method of contraception. If something about your cycle or your health worries you, talk to a doctor or midwife.