How is my next period date calculated?

The first day of your last period plus your cycle length gives the next start date. Each following period adds the cycle length again.

How do I count my cycle length?

Count from the first day of one period to the day before the first day of the next. Our cycle length calculator can average several cycles for you.

Cycles often vary by a few days. The calculator shows one estimate, and the real date can land earlier or later.

Use 5 days as a starting point and change it once you check your own pattern.

No. The calculation happens in your browser and nothing is saved or tracked.

Enter the first day of your last period, your cycle length and how many days your period lasts.

First day of your last period

Cycle length (days)

Period length (days)

Each period start is the previous one plus your cycle length. Period days follow your period length.

Your dates never leave this page. Everything is calculated in your browser, and nothing is saved or tracked.

Estimates only. Not a method of contraception. Not medical advice.

How is my next period date calculated?: The first day of your last period plus your cycle length gives the next start date. Each following period adds the cycle length again.
How do I count my cycle length?: Count from the first day of one period to the day before the first day of the next. Our cycle length calculator can average several cycles for you.
Why did my period come on a different day?: Cycles often vary by a few days. The calculator shows one estimate, and the real date can land earlier or later.
What if I do not know my period length?: Use 5 days as a starting point and change it once you check your own pattern.
Do you store my dates?: No. The calculation happens in your browser and nothing is saved or tracked.