Next Period Calculator: When Is My Next Period?

Enter the first day of your last period and your usual cycle length to see when your next period is due. It also shows your estimated ovulation day and fertile window. It runs in your browser, so nothing is sent or stored.

Written by the Cycella editorial team. See our editorial policy.

How the next period calculator works

A menstrual cycle is counted from the first day of one period to the day before the next period starts. The calculator takes the first day of your last period, adds your cycle length, and gives you the date your next period is expected. It then repeats the same step to show the two periods after that, so you can plan a trip, an exam or a festival around them.

Because it only needs two numbers, the date of the last period and the cycle length, it works the same way for any calendar. If you think in Nepali dates, use the Nepali (BS) period calculator, which accepts a Bikram Sambat date and shows the results in both BS and English dates.

The formula

The maths is short enough to do by hand:

• Next period = first day of your last period + cycle length (in days)
• Ovulation = next period date minus 14 days
• Fertile window = from 5 days before ovulation to 1 day after ovulation

For example, if your last period started on 1 March and your cycle is 30 days, your next period is expected on 31 March, ovulation is around 17 March, and the fertile window runs from about 12 to 18 March. Use the ovulation calculator or the fertile window calculator for more on those two dates.

Which cycle length should I use?

Use the number of days between the first day of one period and the first day of the next. Do this for your last three to six cycles and take the average. The NHS says the average cycle is about 28 days, and that anything from 21 to 35 days is within the usual range. If you do not know your cycle length yet, 28 is a sensible starting point, and you can correct it after a couple of months of tracking.

A single unusual cycle, for example after an illness or a stressful month, should not change your average much. That is why an average of several cycles beats the most recent one on its own.

If your cycles are irregular

A single date is only as good as the cycle behind it. If your cycles vary by more than about a week, the calculator cannot say the exact day. Open the box labelled "My cycles are irregular" and enter your shortest and longest recent cycle. You will see a range of dates in which the period is likely to start. Treat the early end of the range as the date to be prepared by.

Irregular cycles are common in the first few years after periods start, in the years before menopause, after pregnancy, and with conditions such as PCOS or thyroid problems. Our article on irregular cycles explains what is usual and what is worth checking. If you are in India, there is more on PCOD and PCOS on the Cycella in India page.

Why your period may come on a different day than predicted

The date is a prediction from a pattern, and bodies do not follow a timetable. Common reasons a period comes early or late include stress, poor sleep, illness, a big change in weight or exercise, travel across time zones, starting or stopping hormonal contraception, breastfeeding, and perimenopause. If a period is late and pregnancy is possible, take a pregnancy test. Our article on why a period can be late goes through other causes.

When to see a doctor

Talk to a doctor or health worker if your periods are regularly shorter than 21 days or longer than 35 days apart, if you have missed three periods in a row and are not pregnant, if bleeding lasts longer than 7 days, if you soak through a pad or tampon every hour for several hours, or if pain stops you from doing normal things. A calculator is no replacement for a medical opinion. The NHS and ACOG have clear guidance on when to get checked, and we list them on our editorial policy page.

This is not contraception

The fertile window shown here is an estimate based on an average cycle. Ovulation can shift from month to month, and sperm can survive for up to five days, so a calculator cannot tell you which days are safe. Do not use this tool to avoid pregnancy. Read the fertile window and why it is not contraception, and ask a doctor or health worker about methods that are proven to work.

Your privacy

This page works entirely in your browser. The dates you enter never leave your device, nothing is saved, and the site has no analytics or ads. The Cycella app follows the same idea: no account, logs kept on your phone, and optional backup that is encrypted before it leaves your phone. Read our privacy policy for the details.

Frequently asked questions