Period Calculator

Enter the first day of your last period and your usual cycle length, and this calculator projects the dates of your next periods, along with the ovulation estimate for each cycle.

Choose how many cycles to see. It is useful for planning around travel, events or appointments, and for spotting how much your cycles actually vary.

These calculators provide estimates for informational purposes only. They are not intended to diagnose, treat, prevent, or replace professional medical advice. Pregnancy and menstrual cycles can vary, so results may not be exact.

How this calculator works

What you need to enter:

The first day of your last period
Your average cycle length and how many days your period usually lasts
How many future cycles you want to see

How the projection works

Cycle length is counted from the first day of one period to the day before the next one starts. Adding that length to your last period gives the next expected start date, and repeating it projects further ahead.

Each date is calculated from your original last period rather than from the previous prediction, so small rounding differences cannot pile up over several cycles.

What counts as a normal cycle

Cycles between 21 and 35 days are generally considered within the typical range, and it is common for the same person's cycles to vary by a few days from month to month. This calculator accepts lengths from 21 to 45 days.

If your cycles are consistently shorter than 21 days or longer than 35, if they change substantially, or if periods stop, that is worth raising with a healthcare professional. It is a question about your health rather than about arithmetic, and a calculator cannot answer it.

Ovulation alongside each cycle

Each projected cycle also shows its estimated ovulation date, counted back about 14 days from the period start, because that is the more consistent half of the cycle.

If you are tracking cycles to try to conceive, the ovulation and fertile window calculators go into more detail on those days.

A worked example

A last period starting 1 March 2027, cycles averaging 28 days, periods lasting 5 days.

Next period: 1 March plus 28 days, so 29 March to 2 April 2027.
The cycle after that starts 26 April 2027.
The third starts 24 May 2027.
Ovulation for the first projected cycle is estimated around 15 March 2027.

On a 30-day cycle those dates land on 31 March, 30 April and 30 May instead, drifting further from the calendar month each time.

What this calculator can't tell you

The projection assumes your cycle length stays the same. Real cycles vary, and predictions further out are less dependable.
Stress, illness, travel, weight change, medication and hormonal contraception can all shift a cycle.
This is a planning tool, not a medical assessment of your cycle, and it cannot be used as contraception.
A missed or unexpectedly different period is worth discussing with a healthcare professional rather than a calculator.

What to do next

See the fertile days inside each projected cycle with the fertile window calculator.
Estimate ovulation for a single cycle with the ovulation calculator.

Common questions

How do I work out my cycle length?

Count from the first day of one period to the day before the next one begins. If that varies, take the average of the last few cycles and note the shortest and longest, since the spread tells you how much confidence to place in any prediction.

How accurate are period predictions?

They are as consistent as your cycles are. For someone with regular cycles the next period is often predicted within a day or two. For cycles that vary widely, treat the dates as a rough guide, and expect accuracy to fall off in the later projected cycles.

Why is my period late?

A cycle running a few days longer than usual is common and often has no identifiable cause. Stress, illness, travel, sleep disruption, exercise changes and some medications can all delay a period, and pregnancy is another possibility. A calculator cannot tell you which applies. If your period is significantly late or you are concerned, contact a healthcare professional.

Can I use this as birth control?

No. Calendar predictions are not a contraceptive method. Cycles shift, ovulation timing varies, and no day identified here is safe from pregnancy. Speak to a healthcare professional about contraception.

How many cycles ahead can I project?

You can display up to twelve. Bear in mind that each further cycle compounds the assumption that your length stays constant, so the first two or three are considerably more dependable than the twelfth.