Period Calculator
Predict your next several period start and end dates from your last period, average cycle length, and typical period length. A simple way to plan ahead and spot when your cycle is running early or late.
How to Use
- Enter the start date of your last period (the first day of bleeding).
- Enter your average cycle length in days — the gap from the first day of one period to the first day of the next (28 is typical).
- Enter how many days your period usually lasts (commonly 3–7).
- Read the predicted start dates for your upcoming periods, each spaced one cycle apart.
- Try the short (21-day) and long (35-day) cycle presets to see how cycle length changes the schedule.
Show Work
Notes
About the Period Calculator
Need a hand with health and fitness tracking? The Period Calculator does the work for you — free, and right here in your browser. Predict your next several period start and end dates from your last period, average cycle length, and typical period length. A simple way to plan ahead and spot when your cycle is running early or late.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
How are the next period dates predicted?
Each predicted start date is simply your last period start plus your cycle length, repeated for each upcoming cycle. So with a 28-day cycle, the next period is estimated 28 days after the last, the one after that 56 days, and so on. The period-length input shades in roughly how many days of bleeding to expect for each predicted period.
What counts as a normal cycle length?
Most adult cycles run between 21 and 35 days, with 28 days often cited as the average. Cycles can naturally vary by a few days from month to month, and they tend to be longer and more irregular in the years just after first menstruation and approaching menopause. A cycle that's consistently much shorter or longer is worth discussing with a clinician.
Why are my real periods earlier or later than predicted?
Calendar prediction assumes your cycle stays a constant length, but real cycles fluctuate. Stress, illness, travel, significant changes in weight or exercise, and hormonal shifts can all move a period earlier or later. Tracking your actual start dates over several months and updating the average improves the predictions.
Can I use this to avoid or achieve pregnancy?
Not reliably. This tool predicts bleeding dates, not your precise fertile days, and the calendar (rhythm) approach is one of the least reliable methods of contraception. For family planning, combine cycle tracking with ovulation prediction and physical signs, and speak with a clinician about effective options. This calculator is informational and educational only, not medical advice.
How is this different from an ovulation calculator?
A period calculator projects when bleeding will start in future cycles, while an ovulation calculator estimates the fertile window around mid-cycle. They use the same inputs (last period and cycle length) but answer different questions. Many people use both together to get a fuller picture of their cycle.
How do I use the Period Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Planning ahead
Know when your next few periods are likely to fall when scheduling trips, events, or competitions.
Spotting irregularity
Compare predicted dates against what actually happens to notice when cycles drift early or late.
Packing and preparation
Make sure you have supplies on hand before an expected period, especially while traveling.
Tracking for a clinician
Build a record of expected versus actual start dates to share at a gynecology appointment.
Coordinating with ovulation timing
Pair period predictions with an ovulation estimate to understand the full cycle.
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