Upcoming periods
The formula
How it works
Predict the start dates of your upcoming periods from your last period and your usual cycle length. The calculator lays out the next several months so you can plan ahead.
FAQ
What is a normal cycle length?
A typical cycle is 28 days, but anywhere from about 21 to 35 days is considered normal. Cycles can also vary a little month to month, so the predicted dates are a guide rather than a guarantee.
Why track my periods?
Knowing when your period is due helps with planning, spotting irregularities, and understanding your fertile window. A sudden change in cycle length or missed periods can be worth discussing with a doctor.
How do I find the first day of my last period?
It’s the first day you noticed bleeding in your most recent cycle, not the last day — that date is the starting point (LMP) the calculator counts forward from.
Why do the predicted dates drift further out the more cycles I look ahead?
Each prediction is built on the assumption of a perfectly steady cycle length, so any small real-world variation compounds a little more with every cycle projected forward.
What if my cycle length varies from month to month?
Use your average cycle length for the best estimate, and treat the predicted dates as a likely range rather than an exact day, especially if your cycles vary by more than a few days.
How is my fertile window related to these dates?
Ovulation typically happens about 14 days before your next period starts, so the days around the midpoint between two predicted periods are usually when you are most fertile.
Can this calculator be used if my periods are irregular?
It can still give a rough estimate using your average cycle length, but the more irregular your cycles are, the less reliable the predicted dates become.
About the period calculator
This calculator predicts when your future periods will start, based on the first day of your last period and how long your cycle usually runs. Periods follow a roughly regular rhythm, so once you know your cycle length you can project the coming months ahead. Whether you are planning around a holiday, an event or trying for a baby, having those dates mapped out takes the guesswork out of your calendar.
How to use it
Enter the first day of your last period, your average cycle length, and how many days your period usually lasts. The calculator shows the start date of your next period and several after that, along with how long each is expected to run. For example, a period starting 1 January with a 28-day cycle means the next one begins around 29 January, then 26 February, and so on.
The formula
Each period start is the previous one plus your cycle length: . Repeating this projects as many future cycles as you like. The period length simply marks how many days of bleeding to expect from each start date. Because real cycles vary by a day or two, the predicted dates drift slightly from reality over several months.
Where it is used
People use period predictions for everyday planning — packing for trips, scheduling events, and knowing when to expect symptoms. It underpins the cycle-tracking apps millions rely on, and it works hand in hand with ovulation tracking for those trying to conceive or avoid pregnancy. Keeping an eye on the pattern also helps flag irregular cycles that might be worth a medical check.