Period Calculator
Enter the first day of your last period, how long your cycle is and how many days your period lasts. We list your next six periods.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
Next period = first day of last period + cycle length. Repeat for each cycle.
How the prediction works
We add your usual cycle length to the first day of your last period to estimate the next one, and keep adding it for the cycles that follow. The further out you look, the less certain the dates become, because cycles drift.
Is my cycle normal?
A typical cycle is 21 to 35 days, and a period lasts 2 to 7 days. Cycle length can vary by a few days from month to month. Talk to a doctor if your cycles are often shorter than 21 days or longer than 35, if you miss periods, or if bleeding is very heavy or painful.
Make it more accurate
Use the average of your last three to six cycles rather than a single month. Many people also find a tracking app useful for spotting their own pattern.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| First day of your last period: 2026-09-20; Average cycle length (days): 28; Period length (days): 5 | Next period: 2026-10-18 to 2026-10-22 |
| First day of your last period: 2026-09-20; Average cycle length (days): 30; Period length (days): 4 | Next period: 2026-10-20 to 2026-10-23 |
Period Calculator: frequently asked questions
How do I know my cycle length?
Count from the first day of one period to the day before the next. Do that for three months and average it. Cycles between 21 and 35 days are considered normal.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026