Worked example
Last period on Thursday, October 1, 2026, a 28-day cycle and a 5-day period.
Next periods estimated October 29 to November 2, 2026, then November 26 to 30, December 24 to 28, 2026, January 21 to 25, 2027, February 18 to 22 and March 18 to 22, 2027.
The further ahead you look, the more uncertainty adds up: being 2 days off per cycle means almost two weeks after 6 cycles. Update the date with each new period.
How it is calculated
- Find the first day of your last period (first day of real bleeding).
- Add your cycle length in days: that is the estimated start of your next period.
- Add your period length minus one day to estimate when it ends.
- Repeat for the following cycles.
How to work out your cycle length
A cycle starts on the first day of your period and ends the day before the next one. Count the days between two “day 1s”: if your periods started on May 1 and May 29, your cycle was 28 days long.
For a better estimate, average your last 3 to 6 cycles. If they lasted 27, 30 and 29 days, use 29.
What is a normal cycle length?
- In adults, a cycle of 21 to 35 days is considered typical; in the first years after the first period, cycles up to 45 days are common.
- The “28-day cycle” is an average, not a rule: in an analysis of more than 600,000 cycles, the average was 29.3 days, and cycles get slightly shorter with age.
- Periods usually last 2 to 7 days.
- A few days of variation from month to month is normal.
Why your period can come early or late
- Stress, lack of sleep, travel or jet lag.
- Illness, significant weight loss or gain, intense training.
- Pregnancy, breastfeeding, recently stopping hormonal contraception.
- PCOS, thyroid problems, perimenopause.
- On the pill, the bleed during the break is not a real period: it follows your pack, not this calculation.
Late period: what to do
A period is usually called late when it has not started a few days after the expected date. If you had sex without contraception, a urine pregnancy test can be taken from the first day of a missed period, ideally with first-morning urine. If it is negative and your period still does not come, test again a few days later or ask for a blood test.
If you are not pregnant, see a doctor if you have had no period for more than 3 months, if your cycles change suddenly, or if the delay comes with pain or unusual bleeding.
When to see a doctor
- Cycles regularly shorter than 21 days or longer than 35 days.
- Periods lasting more than 7 days, very heavy (changing protection every hour) or very painful.
- Bleeding between periods, after sex or after menopause.
- No period for more than 3 months without pregnancy.