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Period calculator

Predict when your next period is due and plan ahead for the next six months. Enter the first day of your last period, your usual cycle length and how long your period lasts.

Important: These dates are predictions based on an average cycle, and real cycles vary, especially if yours are irregular. This is not a method of contraception. If you are trying to avoid pregnancy, speak to a doctor, nurse or pharmacist about reliable options.

Your dates are processed in this page and are not sent to our server or saved by this tool.

How the period calculator works

Your cycle starts on the first day of your period and ends the day before your next one begins. To predict your next period, we add your cycle length to the first day of your last period. We repeat that to predict later periods. We also work out ovulation, which usually happens about 14 days before the next period starts, and your fertile window, which is the five days before ovulation and the day after.

What is a normal cycle?

Many people have a cycle of about 28 days, but anything from about 21 to 35 days is common. Periods usually last between 2 and 7 days. Your cycle may change over time, and it is normal for it to vary by a few days from month to month. It is also common to have longer or less regular cycles in the first years after periods begin, after having a baby, while breastfeeding and in the years before menopause.

Why is my period late?

The most common reason for a late period is pregnancy, so it is worth testing if there is a chance you are pregnant. Other reasons include stress, illness, travel, a big change in weight or exercise, certain medicines and hormonal conditions such as polycystic ovary syndrome (PCOS) or thyroid problems. Our pregnancy test calculator and our guide to early signs of pregnancy may help.

When to talk to a doctor

Your periods have stopped for three months or more and you are not pregnant.
Your cycles are often shorter than 21 days or longer than 35 days.
You bleed between periods or after sex.
Your bleeding is very heavy, for example soaking through a pad or tampon every hour or two, or passing large clots.
You have severe pain that stops you from doing your normal activities.
You are pregnant or might be, and have bleeding with pain. Read about spotting and cramps in pregnancy and ectopic pregnancy symptoms.

Trying to conceive?

Use the ovulation calculator for a closer look at your fertile window, and read our guide to ovulation and the fertile window. If you already know you are pregnant, the due date calculator can work out your due date.

See all our pregnancy and baby tools.