Period Calculator

How this period calculator works

This period calculator, also called a menstrual calculator, predicts your next periods from three things: the first day of your last period, how many days your period usually lasts, and your average cycle length. It assumes each cycle repeats with the same length and projects the dates forward.

Next period start = first day of last period + cycle length
Period days = period start through period start + period length − 1
Most likely ovulation days = 16 to 12 days before the next period

The ovulation days are an estimate set by this calculator: a five-day window centered on 14 days before the next period, the usual timing of ovulation. Sources give the 14-day figure, not the five-day window itself.

For example, if your last period started on October 1, lasts 5 days, and your cycle is 28 days, your next period is expected October 29 through November 2, and your most likely ovulation days are October 13 through 17.

What the results show

• Next period. The start date and the days your next period is expected to last.
• Most likely ovulation days. The calculator's estimate of when you ovulate: two days either side of 14 days before your next period. The gap between ovulation and the next period can range from about 7 to 19 days, so your actual ovulation day may fall outside this window. These days are not your whole fertile window, which starts several days before ovulation because sperm can survive in the body.
• Period calendar. Three months starting with your last period, with period days and likely ovulation days highlighted. Move back or ahead three months at a time to see earlier or later cycles.
• Next 6 cycles. Period dates and ovulation days for the next six cycles, so you can plan ahead.

Menstrual cycle calculator basics

A menstrual cycle is counted from the first day of one period to the day before the next one starts. The typical cycle is 28 days, but cycles that come every 24 to 38 days are considered regular for adults. The calculator accepts cycles from 22 to 44 days and periods from 1 to 10 days.

Ovulation usually happens about two weeks before the next period, although the gap can range from about one week to more than two weeks. Because the calculator counts back from the next period, a longer cycle moves the ovulation days later in the cycle.

Limits of a period predictor

Any period predictor that uses the calendar method is only as good as the regularity of your cycle. Stress, illness, travel, weight change, breastfeeding, hormonal birth control, and perimenopause can all shift your cycle. If your cycles vary by more than a week, track a few months and use the average. This calculator is not a form of birth control and is not medical advice.

Frequently asked questions

When is my next period?

If you are wondering "when will I get my period," the calendar method gives a quick answer. Add your average cycle length to the first day of your last period. With a 28-day cycle and a last period that started on March 3, your next period is due on March 31. Enter your dates above to get the exact day and the following five periods.

Is a period calculator the same as a period tracker?

A period tracker records your actual period dates over many months. A period calculator takes your average cycle and projects future dates. Using a tracker to find your true average cycle length, then entering it here, gives the most accurate predictions.

How do I calculate my cycle length?

Count the days from the first day of one period up to, but not including, the first day of the next. If you have records for several months, average them.

Why is my period late?

A period can come a few days early or late without anything being wrong. Pregnancy, stress, illness, intense exercise, and changes in weight or medication are common causes of a late period. If your period is more than a week late, consider a pregnancy test, and talk to a doctor if your cycles are often irregular.

Can I use this calculator to avoid pregnancy?

No. Ovulation can shift from cycle to cycle, and sperm can survive several days, so calendar predictions are not reliable birth control.