Period Calculator
Estimate when your next period may start and see where you are in your current cycle. Enter the first day of your last period and your average cycle length.
Your cycle details
Your estimate
Enter your cycle details to see an estimate.
How this estimate is calculated
The calculator adds your average cycle length to the first day of your last period. If that date has already passed, it projects forward to the next expected period. It also shows your current cycle day.
Ovulation is estimated as about 14 days before the next expected period. The fertile window is shown as the five days before ovulation through the day after. These are calendar estimates, not measurements.
For a step-by-step explanation, read how to calculate your next period.
What this tool cannot tell you
- Whether or when ovulation actually occurred
- Whether your cycle is normal for you
- Whether you are pregnant or could become pregnant
- Whether you need medical care
Stress, illness, travel, sleep changes, and hormonal changes can shift your cycle. Calendar estimates are not a reliable method of contraception.
Questions about period estimates
How accurate is a period calculator?
It is a planning estimate, not a prediction of what will definitely happen. Accuracy depends on how regular your cycles are and whether the cycle length you enter reflects your usual pattern. Tracking several cycles can give you a more useful average.
Can I use this if my cycles are irregular?
You can enter an average, but the estimate may be less reliable. If your cycles vary widely, are very long or short, or have changed recently, talk with a healthcare professional. A calculator cannot assess the cause.
Why is this date different from my period app?
Apps may use logged history, different assumptions about the luteal phase, or a different average. The method here is transparent: last period start plus your average cycle length. Neither result is a guarantee.
General references
These organizations provide general health information. They are external sites with their own content and policies, and this site is not affiliated with them.