Period Calculator
Add the start dates of your recent periods to see your average cycle length, how regular it is, and predictions for your next 6 periods with ovulation and fertile days. Your dates stay in this browser only and you can delete them at any time.
Your period history
Typical cycle is used only when fewer than 2 dates are logged.
Private: dates are saved only in this browser on this device (never uploaded). Delete my data removes them.
Next 6 predicted periods
| # | Period | Fertile window | Ovulation | Cycle |
|---|
Predictions are estimates based on your past cycles. Not a contraceptive method or medical advice.
How to use the Period Calculator
- Add your period start dates: pick the first day of a period and press Add date. Add 3 to 6 recent periods for the best average.
- Set your period length (how many days bleeding usually lasts) and, if you only have one date, your typical cycle length.
- Press Predict next periods to see your average cycle, the countdown and the next 6 periods with fertile windows.
- Browse the calendar with the arrows to see predicted period, fertile and ovulation days month by month.
- Keep it up to date: add each new start date. Use Delete my data to wipe everything from this browser.
How the predictions are worked out
A menstrual cycle runs from the first day of one period to the day before the next. The calculator uses simple, transparent calendar arithmetic:
- Cycle lengths = the days between consecutive start dates. Gaps shorter than 15 or longer than 90 days are left out, as they usually mean a missed log or spotting rather than a real cycle.
- Average cycle = the mean of those cycle lengths, rounded to whole days for predictions.
- Next periods = last start date + average cycle × 1, 2, 3… Each lasts the period length you enter.
- Ovulation ≈ 14 days before the next predicted period; the fertile window is ovulation −5 to +1 day.
Example: periods starting on 1 January, 29 January and 28 February give cycles of 28 and 30 days, an average of 29 days, so the next period is predicted for 29 March, with ovulation around 15 March and fertile days from 10 to 16 March.
What is a normal cycle?
| Measure | Typical range (adults) |
|---|---|
| Cycle length | 21–35 days (about 24–38 days in the FIGO 2018 definitions) |
| Period (bleeding) length | 2–7 days (up to 8 days in FIGO 2018) |
| Variation between cycles | Up to about 7–9 days between shortest and longest |
| Teens, first years after menarche | 21–45 days is common (ACOG Committee Opinion 651, 2015) |
Sources: International Federation of Gynecology and Obstetrics (FIGO) System 1 for abnormal uterine bleeding, Munro et al., 2018; American College of Obstetricians and Gynecologists. A large app-based study (Bull et al., npj Digital Medicine, 2019, about 600,000 cycles) found an average cycle of 29.3 days and that only about 13% of cycles were exactly 28 days, so your own average is more useful than the textbook 28.
When to talk to a doctor
See a doctor or nurse if your periods stop for more than 90 days (and you are not pregnant), cycles are regularly shorter than 21 or longer than 35 days, bleeding lasts more than 7 days or soaks a pad or tampon every hour or two, or you have severe pain or bleeding between periods. A late period can be a sign of pregnancy; a home test is reliable from the first day of a missed period. If you are trying to conceive, the ovulation calculator focuses on fertile days, and the due date calculator works out a due date from your last period.
Frequently asked questions
How does the period calculator predict my next period?
It measures the number of days between each pair of consecutive period start dates you enter, averages them, and adds that average to your most recent start date to predict the next periods. With only one date it uses the typical cycle length you enter (28 days by default).
How is ovulation estimated?
Ovulation is estimated 14 days before the next predicted period, because the luteal phase (ovulation to next period) is fairly constant at about 12–14 days while the first half of the cycle varies more. The fertile window shown is the five days before ovulation, ovulation day and the day after.
What is a normal cycle length?
For adults, cycles from 21 to 35 days are generally considered normal, and the period itself usually lasts 2 to 7 days. Teenagers can have cycles of 21 to 45 days in the first years after their first period. Talk to a doctor if your cycles are regularly outside these ranges, vary a lot, or stop.
Is my data private?
Yes. The dates are saved only in your browser's local storage on this device so they are there next time. Nothing is uploaded. Press Delete my data to remove them completely, or use a private window if you do not want anything saved.
Can I use this as birth control?
No. Calendar predictions are not reliable contraception, because ovulation can shift from cycle to cycle. Fertility awareness methods that are used for contraception need daily body signs such as basal temperature and cervical mucus and proper training.
Why was my period not on the predicted date?
Stress, illness, travel, weight change, breastfeeding, hormonal contraception, thyroid problems and conditions such as PCOS can all change cycle length, and a period that is late can also mean pregnancy. Predictions are averages, so a few days either way is normal.