Free cycle tool

Ovulation Calculator

Enter the first day of your last period and how long your cycle usually runs. This estimates the days ovulation is most likely and the window in which pregnancy is possible — and it prints the width of that estimate next to it, because the day itself is not something a calendar can know.

Runs entirely in your browser. No account, no email, and nothing you type is sent anywhere.

Your last period and your usual cycle
If your cycles are not all the same length

These two are optional, and they only ever make the answer wider. A window that has been narrowed by leaving them out is not a more accurate window — it is the same guess with the uncertainty hidden.

Your cycle drawn as a ring, with the estimated period days, fertile window and ovulation band marked.
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add a date above
Period Pregnancy possible Ovulation likely Rest of the cycle

One cycle, from your last period round to the next one estimated. The gold stretch is where ovulation is likely — it is a band on purpose.

Ovulation likely, estimated
Add the first day of your last period

Two numbers and this page fills itself in. Nothing you type is sent anywhere, and nothing here is a date you can hold anyone to.

Your window, and how wide it is

Both bands are counted backwards from your estimated next period. Counting forwards from your last one would put them in the wrong place on exactly the cycles that run long.

Ovulation likely between
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the NHS puts ovulation roughly 10 to 16 days before a period starts
Pregnancy possible
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opens seven days before the earliest possible ovulation and closes a day after the latest
Next period, estimated
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every band above hangs off this date, so it moves when this does

Fill the boxes above and this paragraph will tell you, in words, how many days each of those two windows covers.

Why this page will not name the day

When researchers checked measured ovulation against real cycles, this is the shape that came back.

Measured ovulation days: day 15 in 27% of cycles, day 16 in 21%, day 14 in 20%, and every other day in the remaining 32%. Four columns. The three named days are each a minority of cycles, and the tallest column is the one holding all the other days together. 20% Day 14 27% Day 15 21% Day 16 32% Every other day

The commonest single day was right in 27 cycles out of 100.

Day 15 was the most frequent, at 27%. Day 16 followed at 21%, and the famous day 14 at 20%. Whichever of the three you printed as the date, you would be describing a minority of cycles and missing the rest — and even inside a tidy 28-day cycle the observed ovulation days spread across ten of them.

The authors of that study concluded that accurate prediction from calendar input alone is not possible, and a regular cycle does not rescue it: ovulation time has been found to vary considerably even during regular 28-day cycles. That is the reason this page hands you a band and tells you how wide it is instead of a day you could put in a diary.

This is an estimate, and a calendar cannot find the day. What a calendar can do is tell you which days are worth paying attention to — and an ovulation test, which looks for the hormone surge, is the household method that sees something real.

Every date above came out of arithmetic on two numbers you typed. Nothing here has measured anything about you. The window is drawn backwards from a period that has not happened yet, so if that period arrives early or late — and the average person's own cycle length moves by about 2.6 days from one month to the next — the whole window slides with it.

This is not contraception, and no amount of widening the window would make it into contraception. Sperm can survive in the fallopian tubes for up to seven days, ovulation moves, and a calculator can see neither. 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 a pregnancy, that is a conversation with a doctor, nurse or pharmacist.

If you want to know when you actually ovulated, this is the wrong instrument. An ovulation predictor kit detects the LH surge, and the onset of that surge precedes ovulation by 35 to 44 hours — which is why a kit looks forwards and a calendar does not. Basal body temperature rises afterwards, so a chart confirms in hindsight rather than predicting, and it is a weak detector: measured against LH-confirmed ovulation across 193 cycles it caught under a quarter of them. A clinician can confirm it with a blood test or a scan. If you have been trying to conceive for a while, or your periods have changed, take that to them rather than to a calendar.

How this is worked out

  1. The window is worked backwards from your next predicted period rather than forwards from your last one. The NHS puts ovulation at roughly 10 to 16 days before a period starts, and a cycle that runs long runs long before ovulation — so counting forwards puts the window in the wrong place exactly when it matters most.
  2. The fertile window opens seven days before the earliest possible ovulation, because the NHS puts sperm survival in the fallopian tubes at up to seven days, and closes a day after the latest, because ACOG puts the egg's own survival at about 24 hours.
  3. No single ovulation day is printed anywhere on this page. Measured against real cycles, ovulation fell most often on day 15 at 27%, then day 16 at 21%, then the famous day 14 at 20% — and the same study concluded that accurate prediction from calendar input alone is not possible.
Questions

The short answers

It is arithmetic on your cycle length, and it is right about the neighbourhood rather than the day. In a study of real cycles, ovulation happened on day 15 in 27% of them, day 16 in 21% and day 14 in 20%; even within a tidy 28-day cycle the observed ovulation days spread across ten of them. The authors concluded that accurate prediction from calendar input alone was not possible. That is why this page hands you a window, tells you how many days wide it is, and points you at the tests that can see something real.
Most likely somewhere between day 13 and day 19, and day 14 only about a fifth of the time. The NHS puts ovulation at 10 to 16 days before the next period — and on a 28-day cycle the next period is day 29, so counting back gives that band rather than a point. And a 28-day cycle is itself less common than the diagrams suggest: across 612,613 cycles only 13% were exactly 28 days, with a mean of 29.3.
In the few days before ovulation and on the day itself. The window opens earlier than most people expect because sperm can survive in the fallopian tubes for up to seven days, and it closes quickly afterwards because the egg survives about 24 hours. That asymmetry is why this page draws the fertile window well before the ovulation band, and why waiting for a positive test to start trying is leaving days on the table.
No, and no calculator anywhere can — the ones offering it are drawing a calendar around a coin flip. The theory they run on is that the length of the follicular phase or the timing of sex tilts the odds; when researchers tested that against an independent database of 521 conception cycles they could not show an effect on the sex of the baby, and the sex ratio at birth stays close to a biological constant of about 106 boys to 100 girls. So the honest output is the same 51-to-49 you started with, and this page will not dress it up as a prediction however often it is searched for.
No, and this is the one answer here with no hedging in it. Ovulation moves, sperm survive up to seven days, and a calculator can see neither. 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 it. If you are trying to avoid a pregnancy, that is a conversation with a doctor, nurse or pharmacist rather than a web page.
Not from a calendar. An ovulation predictor kit detects the hormone surge that comes shortly before ovulation, which is the one household method that looks forwards; the ovulation test calculator on this site works out which days to start testing. Basal body temperature rises by roughly 0.2 to 0.5 °C afterwards, so a chart confirms in hindsight — and it is a weak detector, catching under a quarter of ovulations in one study against LH-confirmed dates. A clinician can confirm it with a blood test or a scan.

Where this comes from

  1. NHS (2025). Periods and fertility in the menstrual cycle. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
  2. 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/
  3. 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/
  4. 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
  5. NHS (2024). Natural family planning (fertility awareness). https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/
  6. PubMed Central (2021). Wrist skin temperature and basal body temperature compared against LH-confirmed ovulation. https://pmc.ncbi.nlm.nih.gov/articles/PMC8238491/
  7. Frontiers in Endocrinology (PubMed Central) (2022). Quantification of urinary total luteinizing hormone immunoreactivity may improve the prediction of ovulation time. https://pmc.ncbi.nlm.nih.gov/articles/PMC9581300/
  8. Frontiers in Public Health (PubMed Central) (2016). Bayesian Inference on Predictors of Sex of the Baby. https://pmc.ncbi.nlm.nih.gov/articles/PMC4877388/

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 · 7 min read

Ovulation Explained: The Signs, the Timing, and What the Evidence Shows

Ovulation is a distribution, not a date. What each sign can genuinely see, how far ahead it sees it, and how strong the evidence under it is.

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Cervical Mucus Tracking, Honestly: What It Shows and What It Cannot

The one sign that looks forwards, with both of its numbers. Beginners identified a peak day in 63% of cycles; the figure everybody quotes comes from women who had been taught. That gap is the article.

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The Luteal Phase: How Long It Really Lasts, and What Moves It

The textbook says fourteen days. The measurement says 12.4, with 18% of cycles under 11. What happens in the second half of a cycle, and how to work out your own.

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The app this site is for

Athena keeps all of this on your phone.

The calculators here forget you the moment you close the tab. 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.

In development for iPhone and Android. Not released yet — when it is, it will be on this site.

What Athena is