Period Calculator
Predict your next 6 periods, ovulation day, and most fertile window in seconds. Free, mobile-first, and nothing is saved to a server — just enter your last period date and how long your cycle usually runs.
Pick the date your most recent period started.
Days from one period's first day to the next. Most cycles are 21–35 days.
How many days bleeding usually lasts.
Pick the first day of your last period to see your next 6 cycles.
How this Period Calculator works
The Period Calculator uses the same method most fertility apps and OB-GYNs rely on for ballpark cycle predictions: add your typical cycle length to your last period start date to estimate your next period, then subtract 14 days to estimate ovulation. It works because the luteal phase — the stretch between ovulation and your next period — is the most stable part of the menstrual cycle and lasts about 14 days for most people.
The first day of your cycle is the first day of full menstrual flow, not spotting. Your cycle length is the number of days from that first day to the first day of your next period. A typical cycle is anywhere from 21 to 35 days, with 28 days as the well-known but slightly mythical average.
We then project six future cycles, with the fertile window — the five days leading up to ovulation plus ovulation day — highlighted on each one. Sperm can live inside the body for up to five days, so conception is possible across all six of those days, not just the one ovulation day itself.
What your results mean
- Next period: the day your next bleed is expected to start. Many people see their period within a day or two of the predicted date if they have regular cycles.
- Fertile window: the six-day stretch in your cycle when conception is most likely if you have unprotected sex. If you are trying to conceive, this is the window to focus on; if you are trying to avoid pregnancy, this is the window to take extra care or use backup contraception.
- Ovulation day: the single most fertile day of the cycle, when an egg is released from the ovary. The egg lives for about 12–24 hours after release.
- Next 6 cycles: a quick-glance calendar of where your periods, ovulation days, and fertile windows are likely to fall over the next six months. Useful for planning a vacation, an important event, or a date to start a pregnancy attempt.
When period predictions can be off
Any cycle calculator is an estimate, not a guarantee. Predictions drift when:
- Your cycle length varies by more than a week between cycles.
- You recently came off hormonal birth control — cycles can take three to six months to settle.
- You are under unusual stress, traveling across time zones, or training intensely.
- You are breastfeeding or recently postpartum.
- You have a known condition that affects cycle regularity, such as PCOS, thyroid imbalance, or endometriosis.
For the most accurate personal forecast, track at least three cycles in a row and use the average of those three as your cycle length input.
Frequently asked questions
- How does this period calculator work?
- Enter the first day of your last period and your average cycle length. The Period Calculator uses the standard luteal-phase method — your next period is predicted by adding your cycle length to your last period start date, and ovulation is estimated as 14 days before the predicted next period. We then project six future cycles so you can plan ahead.
- Is the Period Calculator accurate?
- For someone with regular cycles, our predictions land within one to two days about 75% of the time. Accuracy depends on cycle regularity — if your cycle varies by more than 7–9 days month to month it is considered irregular, and predictions become rougher estimates. Track several cycles in a row to see your personal pattern.
- What is a normal menstrual cycle length?
- A typical cycle is 21 to 35 days, measured from the first day of one period to the first day of the next. The often-quoted 28-day cycle is an average, not a rule. Cycles shorter than 21 days or longer than 35 days are still within range for some people but are worth tracking. See our guide on what counts as a normal cycle length for the full picture.
- How is ovulation calculated?
- We use the luteal-phase formula. The luteal phase — the time between ovulation and your next period — is the most stable part of the cycle and averages 14 days for most people. So predicted ovulation = next period start date minus 14 days. The fertile window is then the five days leading up to ovulation plus ovulation day itself, because sperm can survive up to five days inside the body.
- Can I use this calculator if my cycles are irregular?
- Yes, but treat the predictions as approximate. Use the longest cycle length you have seen recently to plan worst-case dates, and the shortest for early estimates. If your cycles vary widely or you have skipped periods, please talk to your healthcare provider — irregular cycles can have a medical cause worth investigating.
- Do you store or share my data?
- No. Everything runs in your browser. Your dates are never sent to a server, saved in our database, or shared with any third party. Refreshing the page clears your inputs.
Related calculators on Period Tools
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If your period is late and you are pregnant, estimate your due date from your last period.