Period Calculator

Knowing when your next period is due makes small things easier, from packing for a trip to planning a day out, and it helps you notice when something has changed. The arithmetic is a single addition. The useful part is knowing what the date means, how far it can move, and which changes are worth a conversation with a doctor. This page covers all three. It is general information rather than medical advice.

The calculation

Count your cycle from the first day of bleeding to the day before the next bleeding starts. To predict the next period:

next period = first day of last period + cycle length

If your last period began on 1 November and your cycle is 30 days, the next is expected on 1 December. The following ones fall 30 days apart: 31 December, 30 January, 1 March, and so on. The calculator lists six of them, with the likely end date from the period length you enter.

The first day of the period is the first day of proper bleeding, not spotting beforehand. Using spotting as the start can make your cycle look shorter than it is.

Finding your own cycle length

Twenty-eight days is the figure everyone quotes, but it is an average, and many people's cycles are longer or shorter. Commonly, adult cycles run between about 21 and 35 days. To find yours, write down the start date of your last several periods and count the days between them. Averaging the last few gives a better number than a single month.

For example, gaps of 29, 31, 28 and 32 days average 30. Enter 30 in the calculator, and treat the result as the middle of a range rather than a fixed date.

An app or a paper diary will do the job. What matters is that the record is kept for several months, because one cycle tells you very little.

Why the date moves

Your cycle has two halves. The first, from your period until ovulation, is the part that varies. The second, from ovulation to your next period, tends to be steadier, at about 14 days for many people. So if ovulation comes later than usual in a given month, your period comes later too.

Many everyday things can delay ovulation:

• Stress. A hard few weeks can shift a cycle.
• Illness. A fever or infection can delay a period.
• Travel and shift work. Disrupted sleep and time zones can have an effect.
• Weight change and heavy exercise. Rapid weight loss or intensive training can make periods irregular or stop them.
• Hormonal contraception and some medicines. These can change bleeding patterns, and the dates here do not apply while you are on the pill or similar methods.
• Life stage. Periods are often irregular in the first years after they begin, after childbirth and while breastfeeding, and in the years before menopause.

When is a period late?

Being a few days off your usual date is common. Many people treat a period as late when it has not arrived five days after the expected date, which is why the calculator shows that day. If you have had sex and could be pregnant, a home test is the first step. Most tests work from the first day of a missed period, and they are more reliable if taken a little later.

If the test is negative and your period still has not come, wait a few days and test again if you are unsure. One late period on its own, without other symptoms, is often nothing serious.

The fertile days shown here

The calculator also shows your likely fertile days. It takes ovulation as 14 days before the next period and marks the five days before it, the day itself and the day after. Sperm can survive for several days, which is why the window starts before ovulation.

With a 30-day cycle starting on 1 November, ovulation would be around 17 November and the fertile days from 12 to 18 November. The ovulation calculator gives more detail if you are trying to conceive, and the pregnancy due date calculator takes over once you are expecting.

These dates must not be used as contraception. Ovulation can come earlier or later than the calendar suggests in any given month, so counting days alone carries a real risk of pregnancy. Talk to a doctor about a reliable method if you want to avoid pregnancy.

When to see a doctor

Periods vary, and a slight change is seldom a concern. It is worth seeing a doctor if:

• Your periods stop for three months or more, and you are not pregnant.
• Your periods are very heavy, for example soaking through protection every hour or two, or lasting more than about seven days.
• Pain stops you from doing your usual activities, and ordinary painkillers do not help.
• You bleed between periods or after sex.
• Your cycles suddenly become much more irregular than they were.
• You have not had a first period by about 16, or you have bleeding after the menopause.

Conditions such as polycystic ovary syndrome, thyroid problems, fibroids and endometriosis can cause some of these, and they are treatable. A simple record of your dates is useful to take to the appointment, which is another good reason to keep one.

Using the calculator

Enter the first day of your last period, your usual cycle length and how many days your period normally lasts. The results show your next period, when it is likely to end, the fertile days before it and the day from which it counts as late. The table underneath lists the next six periods so that you can plan ahead for a trip or an event.

Each prediction uses the same cycle length, so uncertainty grows the further ahead you look. A one-day difference each month becomes six days by the sixth cycle. When your next period arrives, enter the new date to keep the forecast accurate.

If your cycle is irregular

A single date can be misleading when your cycles vary. A better approach is to work out a range. Note your shortest and longest recent cycles and run the calculator with each. If your last period started on 1 November and your cycles run from 26 to 34 days, enter 26 to get 27 November, then 34 to get 5 December. Your next period is likely to fall somewhere in those nine days.

This range is more useful than a single date for planning. If a trip or an exam falls inside it, carry what you need. If you often swing by more than a week or so from one cycle to the next, mention it at your next doctor's visit.

Keeping a simple note of symptoms alongside the dates can also help. Many people notice a pattern, such as breast tenderness, bloating, mood changes or tiredness in the days before bleeding starts. Once you know your own pattern, these signs are often a better guide to the exact day than any calculator.

What this calculator assumes

• Your cycle length stays the same from month to month. In practice, it rarely does.
• Ovulation happens 14 days before the next period, which is a population average.
• The fertile window runs from five days before ovulation to one day after it.
• You are not on hormonal contraception, pregnant or breastfeeding.
• Nothing is stored or sent anywhere. The dates are worked out in your browser.