Period Calculator

Predict your next several periods from your last period, cycle length, and period length — plus each cycle's ovulation and fertile window.

First day of your last period
Cycle length days
Period length days
Your next period
— Enter your dates

Forecasts assume a regular cycle. Real cycles vary by a few days month to month — track a few to sharpen the estimate. See a clinician if cycles are very irregular, very short, or very long.

Knowing when your next period is due helps with planning — trips, events, or just having supplies ready — and makes it easier to spot when something changes. Enter the first day of your last period, your usual cycle length, and how long your period lasts, and this calculator forecasts your next several periods along with each cycle’s fertile window.

How the prediction works

Your cycle is counted from the first day of one period to the first day of the next. To find your next period, add your cycle length to the day your last period started:

Next period = first day of last period + cycle length.

The calculator repeats this to project several cycles ahead, and uses your period length to show the expected end date of each one.

What’s a normal cycle?

  • Cycle length: usually 21–35 days in adults (a little longer for teenagers).
  • Period length: usually 2–7 days.
  • Blood loss: typically light to moderate; soaking a pad or tampon every hour for several hours is not typical and is worth checking.

Cycles are naturally more variable in the first years after your periods start and during perimenopause.

Ovulation and the fertile window

Each cycle also has a fertile window. Ovulation is estimated about 14 days before the next period, and the fertile window spans the five days before ovulation through the day after. The calculator shows this cycle’s ovulation day and fertile window — useful whether you’re trying to conceive or simply understanding your body.

When to see a clinician

Predictions assume fairly regular cycles. Talk to a doctor if you notice:

  • Periods that stop for three or more cycles (and you’re not pregnant or in menopause)
  • Cycles consistently shorter than 21 days or longer than 35 days
  • Bleeding between periods or after sex
  • Periods that are much heavier or more painful than usual

This tool is for general information and planning, not a diagnosis or a method of contraception.

Frequently asked questions

How do you calculate when your next period is due?

Add your average cycle length to the first day of your last period. For a 28-day cycle, your next period is due 28 days after the last one started. This calculator projects several periods ahead and marks each expected start and end date.

What is a normal cycle length?

A typical adult cycle is 21–35 days, counted from the first day of one period to the first day of the next; a period itself usually lasts 2–7 days. Cycles are often less regular in the first few years after periods begin and in the years before menopause.

Why is my period late or irregular?

Stress, big changes in weight or exercise, travel, illness, some medications, thyroid or hormonal conditions such as PCOS, perimenopause, and of course pregnancy can all shift your cycle. Occasional variation is normal; a persistent change is worth discussing with a clinician.

Can this calculator be used for birth control?

No. Calendar predictions assume regular cycles and can't account for the natural shift in ovulation, so they aren't reliable for avoiding pregnancy. Use a proven contraceptive method for that.

References