Period Calculator

How to use the period calculator

1. Enter the first day of your last period: the first day of real bleeding.
2. Enter your average cycle length (21 to 45 days), from the first day of one period to the first day of the next. The Office on Women's Health suggests marking the day each period starts on a calendar; an average of a few cycles beats a single one.
3. Enter how many days your period usually lasts (2 to 10).
4. If your cycles vary, tick My cycles vary in length and enter your shortest and longest recent cycles. The date range and regularity check appear only with this option.
5. Under Advanced options you can change the luteal phase (10 to 16 days) or the "as of" date.

How the predictions are calculated

• Next period = first day of last period + cycle length, repeated for each following period. The table lists the next six that start on or after today.
• Period end = start + period length − 1 day.
• Cycle day: day 1 is the first day of the last period you entered, and the count keeps going if your period is late. On the expected start date you see 0 days to go and, with a 28-day cycle, cycle day 29.
• Days late: once the expected date has passed, you see how many days late you are, and the next period is projected as if the late one had started on the expected day.
• Fertile window: ovulation = next period − luteal phase (14 days by default); the window runs from 5 days before to 1 day after it, the same rule as our ovulation calculator. Results show the current or next window that hasn't ended.
• Cycle length note: under 24, 24 to 38 or over 38 days, following the Office on Women's Health and FIGO 2018.
• Irregular cycles: the next period falls between last period + shortest cycle and last period + longest cycle, and the fertile range applies the −5/+1 rule to both. Regularity compares longest − shortest: 7 days or less is regular, 8 to 9 borderline (FIGO's limit is 7 to 9 days), 10 or more irregular.

Period calculator examples

1. Last period October 2, 2026, 28-day cycle, 5-day period

On October 12, 2026 you are on cycle day 11. The next period is due October 30, 2026, 18 days away, and should end November 3. The fertile window before it runs from October 11 to 17, with ovulation around October 16. The table continues with November 27 and December 25.

2. Same last period, cycles between 26 and 33 days

The next period could start any day from October 28 to November 4, 2026, and the widened fertile range is October 9 to 22. A 7-day spread counts as regular. With cycles from 24 to 36 days, the range becomes October 26 to November 7 and the 12-day spread is flagged as irregular.

3. Late period

Last period September 10, 2026, 28-day cycle: the period was expected on October 8, so on October 12 the calculator shows 4 days late and cycle day 33, still counted from September 10. It projects the next period for November 5, 24 days away, as if the late one had started on October 8. When your period starts, enter the new date and the count restarts at day 1.

What's normal, and when to talk to a doctor

The Office on Women's Health calls 28 days typical and cycles every 24 to 38 days regular, and notes that cycles longer than 38 days are common for a few years after a first period. FIGO 2018, as summarized in the Merck Manual, adds bleeding of up to 8 days.

Talk to your doctor or nurse if your cycles are shorter than 24 or longer than 38 days, if you have no period for 90 days and are not pregnant or breastfeeding, if bleeding often lasts longer than eight days, or if severe pain or bleeding makes you miss regular activities.

The calculator doesn't account for hormonal contraception, pregnancy, the months after birth, breastfeeding, perimenopause or conditions such as PCOS.