Period Calculator
Predict your next period, ovulation day and fertile window from the first day of your last period and your average cycle length, with the next six cycles listed. Example: a period starting January 10 on a 28-day cycle predicts the next one on February 7.
Show steps
How to use the Period Calculator
- Enter the first day of your most recent period.
- Enter your average cycle length, counted from the first day of one period to the first day of the next.
- Add how many days your period usually lasts.
- Press Calculate to see your next period, ovulation and the following cycles.
Formula and method
Next period = first day of last period + cycle length. Ovulation is about 14 days before the next period, and the fertile window covers the five days before ovulation, ovulation day and the day after.
Worked example: period began January 10, 28-day cycle
| Event | Date |
|---|---|
| Last period starts | January 10 |
| Ovulation | January 24 |
| Fertile window | January 19 to January 25 |
| Next period starts | February 7 |
| Following period | March 7 |
Typical cycle facts
| Measure | Typical range |
|---|---|
| Cycle length (adults) | 21 to 35 days |
| Period length | 2 to 7 days |
| Average cycle | about 28 days |
| Luteal phase (ovulation to period) | about 12 to 14 days |
When to talk to a healthcare provider
Contact a provider if you have periods that are consistently shorter than 21 or longer than 35 days, skip three months in a row without being pregnant, bleed very heavily or for more than seven days, or have severe pain.
Frequently asked questions
How do I calculate my next period?
Add your average cycle length to the first day of your last period. For a 28-day cycle, that is 28 days later.
How is cycle length counted?
From the first day of one period to the day before the first day of the next.
Can I trust the prediction if my cycle is irregular?
Less so. Track several months and use the shortest and longest cycles to estimate a range.
When do I ovulate?
About 14 days before your next period, which is day 14 or 15 in a 28-day cycle.
Why is my period late?
Stress, illness, weight change, travel, hormonal changes and pregnancy are common causes. Take a pregnancy test if pregnancy is possible.
Does birth control affect this?
Yes. Hormonal birth control changes bleeding patterns, so predictions will not match.
Sources
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