Period Calculator
Free period calculator: enter your last period date and cycle length to predict your next 6 periods, with a calendar, reminders and your fertile days.
How to calculate your next period
Add your cycle length to the first day of your last period. Cycle length is counted from the first day of one period to the first day of the next — most adults have cycles between 21 and 35 days, and 28 days is only the average. For example, if your last period started on 3 October and your cycle is 30 days, your next period should start around 2 November.
What is a normal menstrual cycle?
| Typical range | |
|---|---|
| Cycle length (adults) | 21–35 days |
| Cycle length (teens) | 21–45 days |
| Bleeding | 2–7 days |
| Month-to-month variation | up to 7–9 days is common |
Why is my period late?
- Pregnancy — take a test if your period is more than a few days late
- Stress, illness, travel or big changes in sleep
- Weight loss or gain, intense exercise
- PCOS or thyroid problems
- Breastfeeding, or stopping hormonal birth control
- Perimenopause (usually in your 40s)
See a doctor if your periods stop for 3 months (and you are not pregnant), come more often than every 21 days, last longer than 7 days, or are very heavy or painful. Irregular periods? Try our PCOS diet plan, and use the ovulation calculator if you are trying for a baby.
Frequently asked questions
How accurate is a period calculator?
It is accurate if your cycles are regular. If your cycle length changes by more than a week from month to month, treat the dates as a rough guide and update your last period date each month.
Is a 35-day cycle normal?
Yes, cycles from 21 to 35 days are considered normal for adults. Longer cycles, or cycles that change a lot, are worth mentioning to your doctor.
How late can a period be before I should worry?
A period up to about 7 days late is common. If it is later than that, take a pregnancy test; if negative and your period still does not come, speak to a doctor.
Sources: ACOG: Menstruation in girls and adolescents · NHS: Periods
Reviewed for accuracy against the sources above. This page is general information, not medical advice — talk to your doctor or a registered dietitian about your own needs.