How periods are predicted
Your cycle runs from the first day of one period to the first day of the next. Adding your cycle length to the start of your last period gives the next start date. Ovulation usually happens about 14 days before a period begins.
What's a normal cycle?
| Typical range for adults | |
|---|---|
| Cycle length | 21 to 35 days |
| Period length | 2 to 7 days |
Cycles are often longer and less regular in the first years after periods begin and in the years before menopause.
When to see a doctor
Talk with a healthcare provider if your periods stop for three months or more (and you're not pregnant), come more often than every 21 days, last longer than 7 days, or are very heavy or painful.
Frequently asked questions
How accurate is a period calculator?
It's a good estimate for regular cycles. Stress, illness, travel, and changes in weight or exercise can move a period by several days.
Can I use this to avoid pregnancy?
No. Calendar predictions aren't reliable birth control. Ask a healthcare provider about effective options.