Period Calculator
Estimate your upcoming period dates and fertile window with one entry. Add the first day of your last period, your average cycle length and usual period length — the calendar shows your next three predicted periods and ovulation estimates instantly.
How to use
- Enter the first day of your last period.
- Enter your average cycle length and usual period length.
- Your next three predicted periods appear instantly.
Note
Predictions add your average cycle length repeatedly, and ovulation is estimated as 14 days before the next period. Cycles naturally vary by a few days, and stress, illness and travel shift them — predictions get less accurate further ahead and with irregular cycles. Estimate only — not medical advice.
Frequently Asked Questions (FAQ)
What does the period calculator do?
Enter the first day of your last period, your average cycle length and usual period length to predict your next three period start dates.
Does it estimate ovulation?
Yes. Ovulation is estimated as 14 days before the next period.
How accurate are the predictions?
Predictions add your average cycle length repeatedly. Cycles vary naturally, and stress, illness and travel shift them, so accuracy drops further ahead.
Why are the further-ahead predictions less reliable?
Each predicted date adds your average cycle length again, so small natural variations in your cycle add up the further ahead you look.
Do I need to sign up or share any data?
No. It is free with no signup, and your dates never leave your browser.