Menstrual Cycle Calculator
Predict your next periods, ovulation day and fertile window — and see exactly where you are in your cycle today.
Your cycle
The cycle runs from day 1 of one period to day 1 of the next. 28 and 5 days are typical.
Available once you pick the first day of your last period.
At a glance
Estimates based on your average cycle — real cycles vary from month to month. Never rely on them as contraception.
About the Menstrual Cycle Calculator
This free menstrual cycle calculator (also known as a period calculator) predicts your next periods, your estimated ovulation day and your fertile window from just three inputs: the first day of your last period, your average cycle length and how long your period usually lasts. It also shows where you are today — your current cycle day and phase — plus a schedule of your next six cycles.
The estimates use the standard calendar method: ovulation is assumed to happen about 14 days before the next period starts, and the fertile window covers the five days before ovulation plus ovulation day itself. Real cycles vary from month to month, so treat every date as an estimate — especially if your cycles are irregular — and never rely on them as contraception.
Everything runs entirely in your browser: the dates you enter are never uploaded, stored or shared, which matters for something as personal as your cycle. Reload the page and they are gone.
Frequently asked questions
Answers to the questions we hear most about this tool.
How does the period calculator work?
It projects forward from the first day of your last period using your average cycle length: each new cycle starts that many days after the previous one. Ovulation is estimated at 14 days before each predicted period (the luteal phase), and the fertile window is the five days before ovulation plus ovulation day itself.
How accurate are the predicted dates?
They are calendar estimates based on averages. Stress, illness, travel or hormonal changes can shift ovulation and the next period by several days, and irregular cycles reduce accuracy further. The more regular your cycles and the better you know your true average length, the closer the predictions will be.
What is the fertile window?
The roughly six days per cycle when pregnancy is possible: the five days before ovulation plus the day of ovulation. Sperm can survive up to five days in the body, while the egg lives about 24 hours after it is released — together they define the days with the highest chance of conception.
Can I use this calculator to avoid pregnancy?
No. The calendar method is not a reliable form of contraception, because ovulation can shift from cycle to cycle even in people with regular periods. Use this tool to understand and plan around your cycle, and talk to a healthcare professional about proper contraception.
Is my menstrual data private?
Yes. The whole calculation runs locally in your browser with JavaScript — the dates you enter are never uploaded, stored or shared with anyone, and nothing is saved when you leave the page. The tool even keeps working offline once loaded.
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