Period calculator

Work out your next six periods, fertile windows and ovulation dates from one date.

Free, no account. By the Plum team · Updated September 6, 2026

This article is general information, not medical advice. For questions about your own cycle or health, talk to a doctor, nurse, or other clinician who knows you.

First day of your last period

Average cycle length20 to 45 days

Period length1 to 10 days

Pick the first day of your last period. This period calculator fills in your next six cycles.

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
Ovulation lands about 14 days before that next period. The count runs backwards from the period ahead of you
The fertile window is ovulation day plus the five days before it

Everything runs inside your browser. Nothing is sent anywhere. Nothing is saved. There is no account to make.

When is your next period?

Your next period is your last start date plus your cycle length. Say you bled on the 3rd and your cycle is 29 days. The next one is due on the 1st of the following month.

Cycle length is the number people get wrong. Count from the first day of bleeding to the first day of the next bleed. Do not 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.

When are you most fertile?

The fertile window is about six days long. It is the stretch of a cycle when sex can lead to pregnancy. Two clocks set its shape. 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 are not. 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. Day 14 only fits a 28-day cycle. Our fertile window guide shows the full working.

How accurate is a period calculator?

It is as accurate as its one assumption. That assumption 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 looser. By the sixth row of the table you are 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 then a single date is false precision, and 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

It cannot tell you that you are pregnant. It cannot confirm that you ovulated. It is not birth control. It does not know whether you have started perimenopause or whether something has changed. It counts days. That is all it does.

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 rather than a calculator.

Common questions

Is this period calculator free?

Yes. There is no account and no email box. The sum runs inside your browser. The dates you type are never sent anywhere. Close the tab and nothing is kept.

How do I calculate my period cycle?

Count from the first day of one period to the first day of the next. That gap is your cycle length. Do it for a few periods and take the average. Most people count from the day the bleeding stopped instead. That makes the cycle look about five days shorter than it is.

Is a 35 day cycle normal?

For most adults, yes. The NHS puts the usual range at 21 to 35 days. Mayo Clinic puts it at 24 to 38. How steady your cycle is matters more than the number. A 35-day cycle every month is fine. One that swings between 24 and 40 is worth raising with a clinician.

What if my cycle length is different every month?

An average will mislead you, because it hides the swing. Use the period calculator for irregular periods instead. It asks for several period start dates. It measures the real gaps and gives you a window.

Can a period calculator tell me if I am pregnant?

No. It only counts days forward from the date you typed. A late period has many causes. Stress, illness, weight change, birth control and perimenopause are all on the list. Only a pregnancy test can answer that question.

Can I use a period calculator as birth control?

No. The fertile window here is a guess from a calendar. It is not a measurement of ovulation, and ovulation moves. Ask a clinician about contraception that suits you.

From calculator to period trackerGet a period tracker that runs this every month.

This page does the sum once, from dates you type in. Plum is a period tracker for iPhone that keeps doing it. It learns your own cycle length as you log. It reminds you before your period and your fertile window arrive. You can also create a group with your friends. There are no ads. Nothing about you is ever sold.

Keep reading

Period calculator for irregular periodsMeasure the real gaps between your periods. Read your next one as a range you can trust.How long is a normal menstrual cycle?This guide explains what cycle length means, whether 28 or 35 days is normal, how many periods a year to expect, and what to get checked.What is the fertile window and how is it calculated?The fertile window is the few days each cycle when pregnancy is possible. Here is how apps and clinicians estimate it, and where the estimates fall short.