Period Calculator
When your next periods are likely to come, on a three-month calendar. Add past start dates to use your own average; they stay on this device unless you say otherwise.
From the first day of one period to the first day of the next. 28 if you’re not sure.
Your dates stay on this device: nothing is sent, saved or added to the address bar.
ExampleA period that started 20 days ago, 28-day cycle
Wed, Oct 21
In 8 days, give or take a few. Then Nov 18 and Dec 16.
- Cycle used
- 28 days
- Next fertile days
- Oct 30 – Nov 4
- Day of your cycle today
- Day 21
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. If your periods change a lot, stop, or worry you, talk to a clinician. Terms
The next six periodsOct 21 to Mar 10
| Starts | Ends | Fertile window before it |
|---|---|---|
| Wed, Oct 21, 2026 | Sun, Oct 25, 2026 | Oct 2–7 |
| Wed, Nov 18, 2026 | Sun, Nov 22, 2026 | Oct 30 – Nov 4 |
| Wed, Dec 16, 2026 | Sun, Dec 20, 2026 | Nov 27 – Dec 2 |
| Wed, Jan 13, 2027 | Sun, Jan 17, 2027 | Dec 25–30 |
| Wed, Feb 10, 2027 | Sun, Feb 14, 2027 | Jan 22–27 |
| Wed, Mar 10, 2027 | Sun, Mar 14, 2027 | Feb 19–24 |
Every cycle the same length; the further ahead, the rougher.
How this works: Method, 8 sources, Checked against 2 worked examples,
How this works
Method
Each prediction adds your cycle length to the first day of the period before. The cycle length is counted from the first day of one period to the first day of the next, so a period that starts on the 5th with a 29-day cycle predicts the next on the 3rd of the following month. Full method
Sources
- Munro, Critchley and Fraser, The two FIGO systems for normal and abnormal uterine bleeding symptoms: 2018 revisions (Int J Gynaecol Obstet, 2018)
- Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digit Med, 2019)
- Committee Opinion No. 700: Methods for Estimating the Due Date (ACOG, reaffirmed 2025, 2017)
- Committee Opinion No. 579: Definition of Term Pregnancy (ACOG, 2013)
- Predicting date of birth and examining the best time to date a pregnancy (Khambalia et al.) (Int J Gynaecol Obstet, 2013)
- Length of human pregnancy and contributors to its natural variation (Jukic et al.) (Hum Reprod, 2013)
- Naegele’s rule: a reappraisal (Baskett and Nagele) (BJOG, 2000)
- Timing of sexual intercourse in relation to ovulation (Wilcox, Weinberg and Baird) (N Engl J Med, 1995)
How it’s tested
2 worked examples for this page are checked by automated tests before every release: given the inputs, the tool must show the expected answer.
Limits
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. If your periods change a lot, stop, or worry you, talk to a clinician.
Changes
- First published: a due date from your last period, conception, IVF or a scan, with ovulation, conception, period and weeks-pregnant pages; nothing you enter leaves your device.
How your next periods are predicted
Each prediction adds your cycle length to the first day of the period before. The cycle length is counted from the first day of one period to the first day of the next, so a period that starts on the 5th with a 29-day cycle predicts the next on the 3rd of the following month.
For example, starting on Tuesday, January 5, 2027 with a 29-day cycle, the next periods would start on Wednesday, February 3, 2027; Thursday, March 4, 2027; and Friday, April 2, 2027. The longer ahead you look, the more the small differences between real cycles add up, so treat dates months away as rough.
With two or more past start dates saved, the calculator can use your own average instead of a typed guess, and shows how much your cycles vary. Nothing about this is a diagnosis; it is a calendar.
next period = last period start + cycle length; period days = that day + your usual period length
- FIGO counts cycles from 24 to 38 days apart as normal frequency, periods of up to 8 days as normal length, and cycles as regular when the shortest and longest are within 7 to 9 days of each other. Source: Munro, Critchley and Fraser, The two FIGO systems for normal and abnormal uterine bleeding symptoms: 2018 revisions (Int J Gynaecol Obstet, 2018).
- Across 612,613 cycles from 124,648 app users the average cycle was 29.3 days, and it shortened by about 0.18 days a year between the ages of 25 and 45. Source: Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digit Med, 2019).
A year of predicted periods
From a period that started on Tuesday, January 5, 2027, with a 29-day cycle and 5-day periods.
| Period | Starts | Ends | Fertile window before the next |
|---|---|---|---|
| Last period | Tue, Jan 5, 2027 | Sat, Jan 9, 2027 | Jan 15–20 |
| 1 | Wed, Feb 3, 2027 | Sun, Feb 7, 2027 | Feb 13–18 |
| 2 | Thu, Mar 4, 2027 | Mon, Mar 8, 2027 | Mar 14–19 |
| 3 | Fri, Apr 2, 2027 | Tue, Apr 6, 2027 | Apr 12–17 |
| 4 | Sat, May 1, 2027 | Wed, May 5, 2027 | May 11–16 |
| 5 | Sun, May 30, 2027 | Thu, Jun 3, 2027 | Jun 9–14 |
| 6 | Mon, Jun 28, 2027 | Fri, Jul 2, 2027 | Jul 8–13 |
| 7 | Tue, Jul 27, 2027 | Sat, Jul 31, 2027 | Aug 6–11 |
| 8 | Wed, Aug 25, 2027 | Sun, Aug 29, 2027 | Sep 4–9 |
| 9 | Thu, Sep 23, 2027 | Mon, Sep 27, 2027 | Oct 3–8 |
Show all 13 rowsShow fewer
| 10 | Fri, Oct 22, 2027 | Tue, Oct 26, 2027 | Nov 1–6 |
| 11 | Sat, Nov 20, 2027 | Wed, Nov 24, 2027 | Nov 30 – Dec 5 |
| 12 | Sun, Dec 19, 2027 | Thu, Dec 23, 2027 | Dec 29, 2027 – Jan 3, 2028 |
Frequently Asked Questions
How do I work out my cycle length?
Count the days from the first day of one period to the first day of the next; the first day of bleeding is day 1. Do it for a few cycles and take the average, or add the start dates to your history here and let the calculator do it.
Where is my cycle history kept?
Only in this browser, and only if you turn it on. It is never sent to us, never synced between devices, and left out of a backup file unless you tick “Include sensitive data” when you make one. Clear it any time with one button.
My period is late. Does that mean I’m pregnant?
It can, but cycles also vary with stress, illness, travel, breastfeeding, some medicines and age. A pregnancy test from the day your period was due gives a clearer answer, and a clinician can help if your periods have stopped or changed.
What counts as an irregular cycle?
By FIGO’s definition, cycles are irregular when your shortest and longest differ by more than 7 to 9 days (the exact limit depends on age), and outside the usual range when they are often shorter than 24 days or longer than 38. Either is worth raising with a clinician, especially if it is new for you.