How to Calculate Your Next Period Date

To calculate your next period date, start with the first day of your most recent period and add your usual cycle length. For example, if your last period started on March 1 and your average cycle is 28 days, your next period is likely around March 29. This is the same basic calendar method used by a period calculator, but a calculator can do it faster and also estimate ovulation day and fertile window from the same cycle history.

The short answer: when is my next period?

If your cycles are fairly consistent, your next period is commonly estimated by:

1. Finding the first day of your last period.
2. Working out your usual cycle length.
3. Adding that cycle length to your last start date.

High-trust health sources describe the menstrual cycle as running from the first day of one period to the day before the next one. That is why the most useful starting point is the first day of bleeding, not the last day.

The simple next period formula

Use this formula:

next period date = first day of last period + average cycle length

If you also want an estimated period end date, use your usual bleed length after that predicted start date.

Example calculation

• Last period start: March 1
• Average cycle length: 28 days
• Estimated next period: March 29

If your last few cycles were not all the same length, average several recent cycles instead of relying on one month alone.

How to calculate your period cycle length

Cycle length is counted from the first day of one period to the day before the next period starts. The most practical way to estimate it is to look at several recent period starts and find your usual pattern.

If you have only one recent cycle, you can make a rough estimate, but the prediction is more useful when you track a few months of starts.

Why predictions can shift

Even with consistent tracking, your next period can still arrive earlier or later than expected. Consumer-health sources commonly explain that timing can change with stress, illness, travel, medications, hormonal birth control, and other cycle disruptions.

If your cycles are irregular, prediction quality is usually lower, so it is better to treat the output as a range than one exact day. If your timing changes often, read the Irregular Period Calculator guide. If you also want the ovulation side of the estimate, read When Is My Ovulation Day?. If you want the fertile-window version of the estimate, read Fertility Period Calculator: What It Means and How to Use It.

Quick FAQ

How do I calculate when my next period will be?

Use the first day of your latest period and add your average cycle length. That gives you a likely next-period date, not a guarantee.

How do you calculate your next period if your cycle changes?

Use several recent cycles and treat the result as a likely range. If your cycle length varies a lot month to month, a single exact date is less reliable.

Is a 28-day cycle required?

No. A 28-day cycle is a common example, not a requirement.

How many past cycles should I track?

Track your cycle starts for a few months so you can understand your usual pattern instead of relying on one month alone.

Is this medical advice?

No. It is informational planning support only.

References

• NHS: Periods and fertility in the menstrual cycle
• Mayo Clinic: Menstrual cycle
• Office on Women's Health: Your menstrual cycle
• Cleveland Clinic: Ovulation Calendar and Calculator

Informational disclaimer

This page is for informational purposes only and is not medical advice.

Try it now

Use the live tool to calculate your next period and fertile window: Try the Period Calculator. You can also browse all practical guides in the Blog hub.