Your next period is due one cycle length after the first day of your last period: 28 days later if your cycle is 28 days long. Most cycles last 21 to 35 days and most periods 2 to 7 days, according to the NHS.
How the period calculator works
Your menstrual cycle starts on the first day of your period and ends the day before your next period begins. The calculator takes the day your last period started and adds your usual cycle length, again and again, to estimate when your next periods will start. Each one then lasts your usual period length.
Ovulation usually happens about 14 days (in most women 10 to 16 days) before the next period starts (NHS), so the calculator counts back 14 days from each predicted start. You can get pregnant from 5 days before ovulation to 1 day after it, and the most fertile days are the 2 to 3 days before your body temperature rises at ovulation (ACOG). The calculator marks that whole fertile window and the ovulation day; the ovulation calculator also highlights the two days before ovulation and ovulation day as your most fertile days, for the next three cycles. Every date is also shown in the Hijri calendar (Umm al-Qura), so you can see where your period may fall in the Hijri month.
The formula
Next period = first day of last period + cycle length
Ovulation ≈ next period − 14 days
Fertile window = ovulation − 5 days to ovulation + 1 day
A worked example
Say your last period started on , your cycles are usually 30 days long and your period lasts 6 days.
- Next period
- – October 2026 ( + 30 days)
- Ovulation
- around ( − 14 days)
- Fertile window
- – September 2026
- The period after
- – November 2026 ( + 30 days)
- In the Hijri calendar
- your period started on , the next one is due on and ovulation is around
Hijri months last 29 or 30 days, so with a 30-day cycle each period starts on the same day of the Hijri month as the one before, or up to a day later. The calculator shows every date in both calendars, so you can plan by either.
If your cycles are irregular
Most cycles last between 21 and 35 days (NHS), and it is normal for yours to change by a few days from month to month. If your cycle length varies, enter your average: add up the lengths of your last few cycles and divide by how many there were. The further ahead a date is, the less precise it gets, so treat the later periods as a rough guide. If your next period would already be more than 90 days late, the calculator asks for a more recent date.
Stress, losing or gaining a lot of weight, exercising a lot, hormonal contraception and conditions such as polycystic ovary syndrome (PCOS) or thyroid problems can all make cycles irregular. In those months, an estimate based on dates alone can be several days out. The Period Calc app learns from your own cycles: it averages your last six logged cycles and tells you when they vary a lot.
When to see a doctor
Talk to a doctor or nurse if:
- your periods stop for three months or more and you are not pregnant (ACOG);
- the time between your periods is often shorter than 21 days or longer than 35 days (NHS);
- your period lasts longer than 7 days or is very heavy (NHS);
- your cycle length often changes by more than 7 to 9 days from one month to the next (ACOG);
- you bleed between periods or after sex, or have severe pain during your periods;
- your periods are irregular and you are struggling to get pregnant.
If your period is late and you might be pregnant, a pregnancy test can tell you. The due date calculator then works out your due date and how many weeks along you are. More answers about the app, privacy and the Hijri calendar are in the Period Calc FAQ.
This calculator gives estimates based on averages. It is not medical advice or a diagnosis, and it is not a method of contraception.
Questions about the period calculator
When will my next period come?
Add your usual cycle length to the first day of your last period. If your last period started on and your cycle is 28 days long, your next period is due around . The calculator above does this for your next six cycles, with every date in the Hijri calendar too.
How accurate is a period calculator?
It is as accurate as your cycles are regular. If your cycle is about the same length every month, the predicted dates are a good guide; if it varies, expect your period to start several days earlier or later. The Period Calc app learns from your own dates.
What cycle length should I enter?
Count the days from the first day of one period to the day before the next one starts. If you are not sure, start with 28 days — the average length — and adjust once you have followed a few cycles.
Can I use it to avoid pregnancy?
No. This calculator only gives estimates: ovulation can move from cycle to cycle and sperm can live for several days, so a calculator based on dates cannot tell you which days are safe. It is not a method of contraception.
Why are the dates also shown in the Hijri calendar?
Many Muslim women plan fasting, prayer and travel by the Hijri month. The calculator uses the Umm al-Qura calendar, so the Hijri date where you live may differ by a day.
Is what I type saved or sent anywhere?
No. The calculation runs in your browser. Nothing you enter is sent to us or anyone else, and this website uses no cookies or analytics.
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser, with the same rules as the Period Calc app. It works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.