How to Use This Period Calculator
Enter the first day of your most recent period and your average cycle length. Most adults have cycles between 21 and 35 days, according to the NHS, with about 28 days being typical. Cycle length is counted from the first day of one period to the first day of the next. If yours varies, track three to six cycles and use the average.
Period duration is how many days your period usually lasts. The NHS says a period can last between 2 and 7 days and usually lasts about 5. It sets the shaded days on the calendar; it does not change the ovulation estimate.
Ovulation is estimated to occur about 14 days before your next expected period, though this can vary by several days. The fertile window runs from about 5 days before ovulation to the day of ovulation, because sperm can survive in the reproductive tract for up to 5 days while the egg survives about a day. These predictions are estimates and can shift with stress, illness, travel, weight changes and other factors.
Worked example
Your last period started on May 2 and your cycle is 30 days. The next period is due on June 1 (May 2 + 30 days), then July 1 and July 31. Ovulation is estimated at June 1 − 14 days = May 18, with the fertile window from May 13 to May 18. A 5-day period starting June 1 ends on June 5.
What is a normal cycle?
| Measure | Normal range (FIGO 2018, adults) |
|---|---|
| How often | Every 24 to 38 days |
| How long bleeding lasts | Up to 8 days |
| Regularity | Shortest and longest cycle differ by 7 to 9 days or less |
Doctors use these FIGO limits to decide when bleeding is abnormal. The NHS gives a similar everyday range of 21 to 35 days. Real cycles are rarely exactly 28 days: in over 600,000 cycles recorded in an app, the average was 29.3 days (Bull and colleagues, 2019). In the first years after periods start, and in the years before menopause, cycles are often less regular.
When to see a doctor
The NHS advises getting checked if your periods change, if you bleed between periods, after sex or after the menopause, or if you have missed three periods in a row with a negative pregnancy test. Also see a doctor for very heavy bleeding (soaking a pad or tampon every hour or two, or passing large clots), periods that last longer than 7 or 8 days, severe pain, or cycles shorter than about 21 days or longer than 35 to 38 days. A missed period after sex can mean pregnancy, so take a test first.