How to use the period calculator
- Enter the first day of your last period.
- Enter your average cycle length — count from the first day of one period to the day before the next one starts. The average is about 28 days, and anything from 21 to 35 days is common.
- Enter how many days your period usually lasts.
- See when your next period should start, which day of your cycle you are on, and the dates of your next six periods.
How the next period is predicted
Example: if your last period started on March 3 and your cycle is 28 days, your next period is expected on March 31. With a 31-day cycle it would be April 3.
Phases of the menstrual cycle
| Phase | Typical days (28-day cycle) | What happens |
|---|---|---|
| Menstruation | 1–5 | The uterine lining sheds. |
| Follicular phase | 1–13 | Follicles in the ovary mature and estrogen rises. |
| Ovulation | About 14 | An egg is released. The fertile window is the 5 days before and the day of ovulation. |
| Luteal phase | 15–28 | Progesterone rises; premenstrual symptoms may appear near the end. |
Why is my period late?
Cycles naturally vary by a few days. Common reasons for a late or missed period include pregnancy, stress, illness, travel, changes in weight or exercise, breastfeeding, some medications and conditions such as PCOS or thyroid disorders. If your period is more than a week late and you could be pregnant, take a pregnancy test. See a doctor if your periods stop for three months or more, or are often shorter than 21 or longer than 35 days apart.
Tips for tracking your cycle
- Record the start date of every period for a few months to find your true average cycle length.
- Note symptoms such as cramps, mood changes or spotting — patterns can help your doctor.
- Use the ovulation calculator if you are trying to conceive.
Frequently asked questions
How do I calculate when my next period is due?
Add your average cycle length to the first day of your last period. If your last period started on the 1st and your cycle is 28 days, your next period should start on the 29th.
How do I find my cycle length?
Count the days from the first day of one period up to, but not including, the first day of the next. Track a few cycles and average them for the best prediction.
What is a normal cycle length?
For adults, cycles between 21 and 35 days are considered normal, and the average is about 28 days. Teenagers and people approaching menopause often have more variable cycles.
How late can a period be before I should worry?
A few days late is common. If you are more than a week late, consider a pregnancy test. If you miss three periods in a row and are not pregnant, talk to a doctor.
Can I use a period calculator to avoid pregnancy?
No. Period and ovulation predictions are estimates, and ovulation can shift, so calendar calculations alone are not a reliable form of contraception.
This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a doctor or other qualified health provider about your individual situation.
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