Period calculator

How this period calculator works

It does the same sum a period tracker does. You give it three numbers you already know. It counts forward six cycles from there.

• Your next period is your last start date plus your cycle length
• The calculator puts ovulation 14 days before that next period. For a cycle under 25 days it counts back 12 instead, and for one over 32 it counts back 15, as Plum does
• The fertile window is ovulation day plus the five days before it

Everything runs inside your browser. Nothing you type is sent or saved.

When is my next period due?

Your next period date is the first day of your last period plus your cycle length. Say you bled on March 3 and your cycle is 29 days. Your next period is due on April 1. The calculator runs that sum for you and shows the five periods after it too.

Cycle length is the number people get wrong. Count from the first day of bleeding to the first day of the next bleed. Don’t count from the day the bleeding stopped. That mistake makes a cycle look about five days shorter than it is. Most adult cycles run 21 to 35 days. 28 is only an average.

I saw my period on the 27th. When is my next period?

With a 28-day cycle, about four weeks later, on a date two or three days earlier than the one you started on. If your period came on the 27th of a 30-day month, the next one is due around the 25th of next month. After a 31-day month it’s the 24th. Find the day yours started below.

February barely moves the date. 28 days after 5 February is 5 March, or 4 March in a leap year. If your cycle runs longer than 28 days, move the date later by the difference. With a 32-day cycle, add four days to the answer in the table. Put your own numbers in the calculator above and it does this for the next six periods.

If the date has passed and your period hasn’t come, use the late period calculator instead. It answers “am I late?” with the number of days past due, and it tells you when a pregnancy test will work.

When are you most fertile?

The fertile window is about six days long. It’s the stretch of a cycle when sex can lead to pregnancy. Two numbers set it. The NHS says an egg lives about 24 hours after ovulation. Sperm can live up to five days. So the window opens five days before ovulation and closes on ovulation day.

This page counts ovulation back from your next period. The days after ovulation are the steady part of a cycle. The days before it aren’t. A study of more than 600,000 cycles found the phase after ovulation sat near 12 days while the phase before it moved a lot. So someone with 33-day cycles ovulates nearer day 19 than the textbook day 14. Our fertile window guide shows the full working, and the guide to when you’ll ovulate lists the day for every cycle length from 21 to 35.

How accurate is a period calculator?

It’s only as accurate as its one assumption, which is that your next cycle will match your average. For a steady cycle the next period is a good guess. Every cycle after that is less certain. By the sixth row of the table you’re reading six guesses stacked on each other.

Two things widen the error. The first is a cycle that moves. If yours swings by a week, a single date will mislead you. The period calculator for irregular periods suits you better. The second is that ovulation itself shifts. Illness, travel, stress and poor sleep can all delay it. A late ovulation pushes the period back with it.

What a period calculator cannot do

A calculator can’t tell you that you’re pregnant or confirm that you ovulated. It isn’t birth control. It doesn’t know whether you’ve started perimenopause or whether something has changed. It only counts days. An app runs the same arithmetic and keeps the history for you. Our rundown of the top rated period tracker apps says which one suits which need.

Cycles under 21 days or over 35 days, bleeding longer than seven days, or a pattern that changes and stays changed are worth a call to a clinician.