Period calculator
Your next period usually starts one cycle length after the first day of your last period: for a 28-day cycle, 28 days later. Enter your dates to see your next 6 periods and fertile windows.
How to work out your cycle length
Count the days from the first day of one period to the day before the next one starts. Do this for 3 to 6 months and take the average. Period tracking apps or a simple calendar note make this easy.
What's normal
| Typical range | |
|---|---|
| Cycle length | 21 to 35 days |
| Period length | 2 to 7 days |
| Variation month to month | Up to about 7 days |
| Ovulation | About 14 days before the next period |
When to see a doctor
- Your periods suddenly become irregular, or stop for 3 months and you're not pregnant
- Your cycle is often shorter than 21 days or longer than 35
- Periods are very heavy, last more than 7 days or are very painful
- You bleed between periods or after sex
Period questions
How do I calculate my next period?
Add your usual cycle length to the first day of your last period. For example, if your last period started on 1 March and your cycle is 28 days, your next period should start on 29 March. Day 1 of your cycle is the first day of bleeding.
What is a normal cycle length?
Anywhere from 21 to 35 days is normal for adults, and it can vary by a few days from month to month. Periods usually last 2 to 7 days.
Why is my period late?
The most common reasons are pregnancy, stress, illness, travel, weight change, breastfeeding, PCOS and thyroid problems. If you could be pregnant, take a test. See a doctor if your periods stop for 3 months or more and you're not pregnant.
Can I use a period calculator to avoid pregnancy?
No. Calendar methods are not reliable contraception, because ovulation can shift. Talk to a doctor about contraception.