Period Calculator
Predict when your next 3 to 12 periods start and end from your last period, cycle length and period length, with a calendar view and a downloadable .ics file.
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About this toolWhat is the Period Calculator?
Tell the Period Calculator when your last period started, how long your cycle usually is and how many days you bleed, and it lists your next periods with start and end dates, counts down to the next one, and shades them on a month calendar. You can predict 3, 6, 9 or 12 periods ahead and download them as all-day events for any calendar app.
If you are not sure of your cycle length, paste a few past start dates and the calculator averages the gaps for you. A toggle adds estimated ovulation days to the calendar, placed 14 days before each predicted period, for anyone who wants a rough idea of where mid-cycle falls.
The predictions assume every cycle repeats the length you enter, so they drift if your cycle varies. The page shows a plus or minus three-day range around the next date as a reminder.
- Predicts 3 to 12 future periods with start and end dates
- Month calendar with last period, expected periods and optional ovulation days
- Average cycle length from past start dates
- .ics export as all-day events
- Nothing is stored or uploaded
How to use the Period Calculator
- Enter the date your last period started.
- Enter your cycle length, or open the past start dates panel and let the calculator average them.
- Enter how many days your period usually lasts.
- Pick how many future periods to predict, then read the next date, countdown and calendar.
- Download the .ics file to add every predicted period to your phone calendar, or copy the list as text.
Worked examples
Typical cycle
Input
Last period 1 October 2026, cycle 28 days, period 5 days
Output
29 Oct to 2 Nov, 26 to 30 Nov, 24 to 28 Dec 2026
Each start is 28 days after the previous one. Each end is start plus 4, because the first day counts as day 1.
Averaging past dates
Input
Past starts 3 July, 31 July and 29 August 2026
Output
Average cycle 28.5 days over 2 cycles, suggested 29
The gaps are 28 and 29 days. Three to six past cycles give a more stable average than one.
Longer cycle
Input
Last period 1 October 2026, cycle 34 days, period 4 days
Output
Next period 4 to 7 November 2026
Six extra days per cycle add up quickly: by the sixth prediction the dates are five weeks later than a 28-day count would give.
Background and insight
A menstrual cycle is counted from the first day of bleeding to the day before the next period starts. The often-quoted 28 days is an average, not a norm: ACOG and most gynaecology texts describe 21 to 35 days as typical for adults, and cycles of up to 45 days are common in the first years after periods begin. A study of over 600,000 cycles logged in a period-tracking app (Bull et al., npj Digital Medicine, 2019) found an average of 29.3 days, with only about 13% of cycles exactly 28 days long, and found that cycles shortened slightly with age.
Most cycle-to-cycle variation happens before ovulation, in the follicular phase. Stress, illness, travel, weight change, breastfeeding, hormonal contraception and the run-up to menopause can all lengthen or shorten a cycle. Predictions are therefore best read as a window. Calendar files written by this page use all-day dates with an exclusive end date, as RFC 5545 requires, so a five-day period from 29 October shows on 29 October to 2 November in every time zone.
Common use cases
Planning holidays and events
Check whether a trip, wedding or race is likely to coincide with your period.
Being prepared
Get a calendar reminder a few days before so supplies are packed.
Spotting changes
Compare predictions with what actually happens to notice cycles getting longer, shorter or less regular.
Doctor appointments
Bring accurate past dates and an average cycle length when discussing symptoms.
Tips and limitations
- Always enter the first day of full bleeding, not spotting beforehand.
- Re-enter the real start date each month; predictions further ahead accumulate error.
- A late period with possible pregnancy is best checked with a home test from the expected date.
- Periods that stop for three months or more, very heavy bleeding, or bleeding between periods are worth discussing with a clinician.
- Hormonal contraception produces withdrawal bleeds on the pack schedule, so use the pack rather than this calculator.
Frequently asked questions
How do I calculate when my next period is due?
Add your cycle length to the first day of your last period. With a 28-day cycle and a last period starting 1 October 2026, the next one is due on 29 October. The calculator repeats this for as many cycles as you choose.
What counts as a normal menstrual cycle length?
For adults, 21 to 35 days is considered typical, and a variation of a few days from month to month is normal. Teenagers often have longer, less regular cycles for the first few years.
How do I work out my average cycle length?
Count the days from the first day of one period to the first day of the next, for several cycles, and average them. Paste the start dates into the past dates panel and the calculator does it for you.
Why is my period late if I am not pregnant?
Stress, illness, travel across time zones, intense exercise, weight change, breastfeeding, some medicines and the years before menopause can all delay ovulation and with it the period. If a period is more than a week late and pregnancy is possible, a home test is the first step.
Can I add predicted periods to Google Calendar or iPhone?
Yes. Download the .ics file and open or import it. Each period is an all-day event, so it shows on the right dates wherever you are.
How many days does a period usually last?
Most periods last two to seven days. Bleeding that routinely lasts longer than seven days, or soaks through protection every hour or two, is worth mentioning to a doctor.
Is there an in-depth guide to the Period Calculator?
Yes. “Your Menstrual Cycle in Numbers: Periods, Ovulation and the Fertile Window” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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