Free cycle tool

Period Calculator

Your next period is estimated by adding your usual cycle length to the first day of your last one. This page projects six of them, each as a window rather than a date: across 612,613 cycles the average person's own cycle length moved about 2.6 days from one month to the next. Print the six months as a calendar, or export the windows as an .ics file.

Enter the first day of your last period and how long your cycle usually runs. Athena estimates your next six periods and the window around ovulation, shows you how wide those estimates really are, and hands you the whole thing as a printable six-month calendar or a calendar file.

Free, and no account.

Your last period and your usual cycle
How much your cycles move

This is what turns a date into a range. Leave them alone and the calculator assumes three days either side, because almost nobody's cycle is the same length twice running.

Count the days between two dates

The gap between two period start dates is that month's cycle length — the number every other prediction on this site is built from. It also counts any two dates you like.

Your cycle drawn as a ring, with the estimated period days and fertile window marked.
—
cycle day
Period Pregnancy possible Ovulation likely Rest of the cycle
Your next period, estimated
Around 27 September 2026

Most likely between 24 September 2026 and 30 September 2026. That is a window of 7 days, and it is the honest width of this estimate.

The next 6 periods

Each one is the middle of a window. The window widens the further ahead you look, because every cycle in between can move and the movement adds up.

  • Next periodAround 27 September 2026
    Window: 24 Sep – 30 Sep · 7 days wide
  • The one afterAround 25 October 2026
    Window: 19 Oct – 31 Oct · 13 days wide
  • And the one after thatAround 22 November 2026
    Window: 13 Nov – 1 Dec · 19 days wide
  • Four cycles onAround 20 December 2026
    Window: 8 Dec – 1 Jan · 25 days wide
  • Five cycles onAround 17 January 2027
    Window: 2 Jan – 1 Feb · 31 days wide
  • Six cycles onAround 14 February 2027
    Window: 27 Jan – 4 Mar · 37 days wide

The fertile window, honestly

Worked backwards from your predicted period rather than forwards from your last one — which is the only direction the evidence supports.

Ovulation likely between
—
roughly 10 to 16 days before your period
Pregnancy possible
—
the NHS puts sperm survival at up to 7 days, so the window opens well before ovulation
—
days between those dates

Put a date in both boxes above.

Take it with you

Both are built from the numbers above and both keep the windows. A date copied into a phone outlives the page that made it, so every entry in the calendar file carries its own caveat rather than leaving it here.

Six months on one page

Six month grids on one page, A4 or US Letter, from 24 Sep onwards. Each predicted period is a pale run — every day it could be happening — with the arithmetic's own answer darker inside it. Watch the pale run swallow the last two months; that is what a calendar knows six cycles out.

Add the 6 windows to your calendar

An .ics file for Apple Calendar, Google Calendar or Outlook. Each period is a multi-day block spanning its whole window, never a single day — and there is no fertile-window entry, because a banner in a shared calendar cannot carry the sentence that has to go with it.

Read this before you trust a date on this page. Everything above is arithmetic on the numbers you typed. It cannot see your body. Real cycles move by a few days in either direction routinely, and a stressful month, an illness, travel or a change in sleep can move them further.

This is not contraception, and it cannot be made into contraception. Ovulation shifts, sperm can survive in the body for up to seven days, and no calculator can see either. Fertility awareness followed imperfectly is about 76% effective — 24 women in 100 get pregnant in a year — and the NHS does not officially recommend any app or device for it. If you are trying to avoid or plan a pregnancy, talk to a doctor, nurse or pharmacist.

It also cannot tell you whether you are pregnant, and it cannot diagnose anything. If your periods stop, if the gap between them keeps changing and that is new for you, or if something else about them worries you, that is a conversation with a clinician rather than a calculator.

How this is worked out

  1. A cycle is counted from the first day of one period to the day before the next. Your next period is estimated by adding your cycle length to the first day of your last one, and the one after that by adding it again.
  2. The fertile-window estimate works backwards from that predicted date rather than forwards from your last period. The NHS puts ovulation at roughly 10 to 16 days before the next period, so the window is drawn from the predicted date and never from "day 14".
  3. The range shown around every predicted date is your own: it comes from the shortest and longest cycle you enter. If you give a single cycle length, the calculator assumes a few days either side, because almost nobody's cycle is the same length twice running. The window compounds — the sixth is six times as wide as the first, because every cycle in between can move.
  4. The printable calendar and the calendar file are built by the server from the numbers in the address bar, so both work with JavaScript switched off and both carry the same windows the page shows. Both mark period windows and nothing else. The fertile estimate stays on this page, where its sources and the refusal that has to go with it are one line away — printed over six compounding windows it covers most of every month, and in a calendar event it travels alone.
Questions

The short answers

It is arithmetic, not a forecast. A calculator adds your cycle length to your last period and shows you the result. Real cycles move by a few days in either direction all the time — illness, travel, stress, a bad run of sleep and plenty of ordinary things shift them — so treat the date as the middle of a range rather than a promise. That is why this page always prints the range as well as the date.
Switch the calculator to "From date to date". Put in the first day of one period and the first day of the next, and it counts the days between them. That number is your cycle length for that month, which is the figure every other prediction on this site is built from.
That is normal, and the calculator is built for it. Enter your shortest and longest cycle and it will show a window instead of a single day. In a study of 612,613 cycles, the average person's own cycle length moved by about 2.6 days from one month to the next. The NHS suggests speaking to a GP if your periods are irregular or your usual pattern changes.
No, and this is the one answer on the page with no hedging in it. Sperm can survive in the body for up to seven days, ovulation moves around, and a calculator cannot see either. Fertility awareness followed imperfectly is about 76% effective, meaning 24 women in 100 get pregnant in a year — and the NHS does not officially recommend any app or device for this. If you are trying to avoid or plan a pregnancy, talk to a doctor, nurse or pharmacist.
The page keeps them in this browser's local storage so you do not have to type them again. They are held per browser, so another browser or another device starts empty, and the button under the form clears them.
Because nothing on a calendar knows that. When researchers checked real ovulation against real cycles, day 15 was the commonest day at 27%, then day 16 at 21%, then day 14 at 20% — and even within a tidy 28-day cycle the observed ovulation days spread across ten of them. Printing one day would be a made-up number dressed as a measurement, so this page prints the window and says how wide it is.
Because a day would be the same arithmetic wearing a better suit. A calculator adds your cycle length to your last period; what it cannot add is the two or three days your own cycle routinely moves. Across 612,613 cycles the mean movement in one person's own cycle length was 2.6 days from one cycle to the next, and researchers working from calendar data alone concluded that predicting ovulation from it was not possible. So every entry here is a run of days, and the runs get wider the further ahead you look, because the movement adds up.
Download the .ics file from this page and open it. On a phone, tapping it offers to add the events to whichever calendar you use; on a computer, Apple Calendar, Google Calendar and Outlook all import an .ics file from their own menus. Each of the six periods arrives as a multi-day block covering its whole window rather than a single day, and each one carries the working, the range and the caveat in its own notes — because once an event is in your phone, the page that made it is gone.
Deliberately. The page shows one and so does the printed sheet, and on both of them the estimate sits next to the sentence that has to go with it. A calendar event does not travel with that sentence: it arrives as a banner in a shared calendar weeks later, built from arithmetic that cannot see an ovulation, and it gets read as a green light. So the file carries period windows and says in every entry that it is not contraception. If you want the fertile estimate, it is on this page and on the printable calendar, with the sources under it.
Yes — the printable calendar builds six month grids on one page, at true A4 or US Letter size. Each predicted period is a pale run covering every day it could be happening, with the arithmetic's own answer shaded darker inside it, and the period you actually entered is darker still, because one block on that sheet is a record and the rest are sums. By the sixth month the pale run has swallowed most of the page, which is the honest picture of what a calendar knows that far out. Choose "Save as PDF" in the print dialog to keep it as a file instead.

Where this comes from

  1. NHS (2025). Periods. https://www.nhs.uk/conditions/periods/
  2. NHS (2025). Periods and fertility in the menstrual cycle. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
  3. NHS (2025). Irregular periods. https://www.nhs.uk/symptoms/irregular-periods/
  4. NHS (2024). Natural family planning (fertility awareness). https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/
  5. npj Digital Medicine (PubMed Central) (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/
  6. Human Reproduction Open (PubMed Central) (2020). Real-life insights on menstrual cycles and ovulation using big data. https://pmc.ncbi.nlm.nih.gov/articles/PMC7164578/
  7. American College of Obstetricians and Gynecologists (2023). Fertility Awareness-Based Methods of Family Planning. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning

Every link above was checked when this page was last updated. Athena is not affiliated with any of these organisations, and none of them has reviewed this page. Nothing here is medical advice.

Go deeper

The reading behind this page

Understanding your cycle · 6 min read

Cycle Length: What Is Normal, and What the Numbers Actually Say

Four respectable bodies, four different answers, all on one chart. Then the numbers from 612,613 real cycles, and what to do with your own.

Read it Understanding your cycle · 6 min read

Why Is My Period Late? The Ordinary Explanations, and the Ones to Check

A late period is nearly always a late ovulation, which is why the delay sits in the first half of the cycle. The ordinary explanations, in prose, for somebody who wants to understand rather than count.

Read it Understanding your cycle · 8 min read

When Will I Get My Period? What a Quiz Can and Can't Tell You

Every quiz that promises a date is guessing. Here is what the signs actually mean, what the timelines usually look like, and an estimator that gives you a range and admits it.

Read it
Next door

Period Late Calculator

How late is late, measured against your cycle rather than a textbook one — plus the ordinary explanations, and the day a test would actually mean something.

Open it

Cycle Length Calculator

Log a few period start dates and read your own numbers: average, shortest, longest, and how much the whole thing moves.

Open it

Ovulation Calculator

The days ovulation is most likely, worked backwards from your next period — printed as a window, because a single day would be invented.

Open it
The app this site is for

Athena keeps all of this on your phone.

A calculator answers the one question you brought it. Athena is the same arithmetic living on your device — the moon ring, an honest calendar with ranges instead of false precision, and a database that never leaves the phone because there is no account and no server behind it.

On the App Store and Google Play, for iPhone and Android.

What Athena is