What the Menstrual Period Calculator does
This period calculator predicts the start dates of your next six periods from the dates of your own past periods, works out your average cycle length and how much it varies, and gives a likely range for each prediction - so the forecast reflects your cycle rather than an assumed 28 days. You can add the predictions to your phone or computer calendar with one click.
Type or pick the first day of each recent period; adding the last day as well lets it work out your usual period length too. If you only know when your last period started, the quick estimate uses the cycle length you enter. It also marks the days around the estimated ovulation, but it is not contraception and cannot tell you when you can or cannot get pregnant. Your dates stay in this browser tab and are remembered only if you tick the box to say so.
How to use it
- Enter the first day of each period you remember, one per line as YYYY-MM-DD, or use Add a start date to pick each one from a calendar. Order does not matter. Three or more dates are useful; six or more are better.
- Optionally write the last day after the first, as in 2026-08-24 to 2026-08-28, so the calculator knows how long your periods usually last.
- Read the next predicted start at the top, with how many days away it is and the likely range, then the six-period table below it.
- Check How regular your cycle is: it shows your shortest and longest cycles and whether the difference counts as regular.
- Press Add to my calendar to download a calendar file with the predicted periods, and tick the box above it if you also want the estimated fertile days included.
Reading the results
Cycle length is counted from the first day of one period to the first day of the next. The average is the mean of your logged cycles; the median is shown too, because one unusually long cycle pulls the mean but not the median.
The likely range comes from how much your own cycles vary. If yours vary by a day or two, the range is a day or two either side; further ahead it widens, because small differences add up over several cycles. With only one date, no range is claimed.
A cycle is generally considered regular if the shortest and longest differ by 7 days or less (9 days or less for people aged 18 to 25 and 42 to 45), and normal in frequency if cycles are between 24 and 38 days. These are the definitions published by FIGO, the International Federation of Gynecology and Obstetrics.
The fertile days are a rough estimate: about 14 days before the predicted period, plus the five days before that. Ovulation does not happen on the same day every cycle, which is exactly why a calendar forecast cannot be used as contraception.
Worked example: seven period start dates
Periods started on 3 April, 1 May, 30 May, 29 June, 26 July, 24 August and 22 September. The gaps are 28, 29, 30, 27, 29 and 29 days, which add up to 172 days over 6 cycles: an average of 28.7 days, with a median of 29.
The shortest cycle was 27 days and the longest 30, a difference of 3 days, so the cycle is regular. The standard deviation is about 1.0 day.
The next period is predicted for 22 September + 29 days = 21 October, likely between 20 and 22 October. The one after is 22 September + 57 days (2 x 28.7, rounded) = 18 November, with a slightly wider range of 2 days either side. Estimated ovulation for the October cycle falls around 7 October.
Formulas and scoring rules
- Cycle length
days from one period's first day to the next period's first day- Average and variation
mean, median, sample standard deviation, shortest, longest, range = longest - shortest- Predicted start k cycles ahead
last start + round(k x average cycle)- Likely range
+/- max(1, round(SD x sqrt(k))) daysTreats cycles as independent, so uncertainty grows with the square root of the number of cycles ahead. Not shown without variation data.- Estimated ovulation
predicted next start - 14 daysThe luteal phase is roughly 14 days in many people but varies; this is an estimate only.- Estimated fertile days
ovulation - 5 days to ovulation day- Regularity (FIGO 2018)
range <= 7 days regular; 8-9 regular at ages 18-25 and 42-45; > 9 irregular
How this differs from the ovulation calculator
The Ovulation Calculator is built for trying to conceive and centres on estimated fertile days. This page is a period predictor: it learns your average cycle and its variation from a log of your own past periods instead of assuming 28 days, forecasts the next six periods with a likely range for each, and exports them to your calendar.
When to talk to a doctor
Cycle length changes naturally - in the first years after periods start, after pregnancy, with stress, illness, weight change, intense exercise and approaching menopause - so one unusual cycle is rarely a concern. Seeing a doctor is sensible if cycles are regularly shorter than 24 days or longer than 38 days, if they vary by more than 9 days, if periods stop for three months or more when you are not pregnant, if bleeding is very heavy or lasts longer than 8 days, or if you bleed between periods or after sex. A log of dates like the one on this page is useful to take with you.
Limitations: what the result does not prove
- It is not contraception and not a fertility test. Calendar predictions cannot reliably show when you can or cannot get pregnant.
- It predicts from the dates you enter. Hormonal contraception, pregnancy, breastfeeding, polycystic ovary syndrome, thyroid conditions and perimenopause can make past cycles a poor guide to future ones.
- With fewer than three cycles logged, the average and variation are only a first look; the forecast becomes more useful with six or more.
- It does not record symptoms, flow or temperature, and it is not medical advice or a diagnosis.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- Munro MG et al. The two FIGO systems for normal and abnormal uterine bleeding symptoms. Int J Gynecol Obstet 2018
- NHS - Periods
- ACOG - Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
Frequently asked questions
How do I calculate my next period date?
Add your average cycle length to the first day of your last period. If your last period started on 22 September and your cycles average 29 days, the next is due around 21 October. Entering several past start dates lets the calculator use your own average instead of a guess.
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, counting the first day but not the last. Do this for several months and average the results - the calculator does it automatically from the start dates you enter.
How much can a menstrual cycle vary and still be regular?
Under the FIGO definitions, a difference of up to 7 days between your shortest and longest cycles counts as regular for most adults, and up to 9 days for people aged 18 to 25 and 42 to 45. Cycles of 24 to 38 days count as normal frequency.
Can a period calculator be used as birth control?
No. Ovulation can shift by several days from cycle to cycle and sperm can survive for up to five days, so a calendar forecast cannot reliably identify safe days. Use a method of contraception and ask a doctor, nurse or pharmacist which suits you.
Why was my period late if my cycles are usually regular?
Stress, illness, travel, changes in weight or exercise, some medicines and pregnancy can all delay ovulation, which delays the period. A single late period is common; if you could be pregnant, take a test, and see a doctor if periods stay irregular.
Can I add my predicted periods to Google Calendar or my iPhone?
Yes. Press Add to my calendar to download an .ics file, then open it or import it into Google Calendar, Apple Calendar or Outlook. Each predicted period appears as an all-day event; the estimated fertile days are added only if you tick that option.
Does the period calculator save my dates?
Only if you tick Remember my dates, and then only in this browser on this device. Untick it and the saved dates are erased. Nothing is sent to a2z.tools or anyone else, and the calendar file is created on your device.
Last reviewed by the A2Z.Tools team against the sources listed above.
