Period Calculator
Estimate your next period start and end dates, and preview the following cycles.
How is my next period predicted?
This calculator adds your average cycle length to the first day of your last period to estimate when your next period will start, then adds your typical period length to estimate when it will end. It repeats this same math for the following one or two cycles to give you a short-term calendar preview, which can be useful for planning ahead.
Why might my actual period differ from the prediction?
Cycle length naturally varies for most people from month to month due to stress, travel, illness, changes in exercise or weight, certain medications, and hormonal shifts — even people with generally "regular" cycles often see some week-to-week variation. This calculator assumes your entered average cycle and period length will repeat exactly, which is a simplification rather than a guarantee.
Important disclaimer
This tool provides an estimate for planning and informational purposes only, and is not a medical diagnostic tool or a form of contraception. If your periods are consistently irregular, unusually painful, unusually heavy, or you have any other concerns, please consult a healthcare provider.
Frequently Asked Questions
How accurate is a period calculator?
It's a reasonable estimate for people with fairly consistent cycles, but actual timing can shift due to stress, illness, travel, or many other factors. Predictions for cycles further in the future tend to be less reliable than the very next one.
What is a normal menstrual cycle length?
Cycle length typically ranges from about 21 to 35 days for adults, with 28 days often cited as a common average, though many people fall outside that and still have a typical cycle. What matters most for most people is a pattern that stays relatively consistent for them over time.
Why is my period late even though the calculator predicted a date?
Many things can delay a period beyond the calculated estimate, including stress, changes in weight or exercise, illness, travel, certain medications, or hormonal fluctuations. If your period is significantly late and pregnancy is a possibility, consider taking a pregnancy test; if lateness is a recurring pattern, consider talking with a healthcare provider.
How is this different from an ovulation calculator?
This tool focuses on predicting when your period will start and end. If you're more interested in estimating ovulation timing and your fertile window, try our Ovulation Calculator instead.
Can I use this calculator if my cycles are irregular?
You can, but the prediction will be less reliable since it assumes your entered average cycle length repeats consistently. If your cycles vary widely, treat the result as a rough guide rather than a precise forecast, and consider discussing irregular cycles with a healthcare provider.
How far in advance can this calculator predict my periods?
This calculator shows a short preview of the next couple of cycles based on your average cycle length. Predictions further into the future carry more uncertainty, since small month-to-month variations in your actual cycle compound over time.
Is this calculator a form of birth control or pregnancy planning tool?
No — this tool is intended for general period planning and awareness only, and should not be relied on for contraception or to time conception. Talk with a healthcare provider about reliable methods for either goal.