Finding your fertile window without pretending to more precision than exists
1. Enter the first day of your last period
The first day of actual bleeding, counted as cycle day 1. Spotting in the day or two before does not count as day 1, and getting this wrong shifts every date on the page by the same amount.
2. Enter your usual cycle length
From the first day of one period to the day before the next begins. If it varies, use the average and then fill in the shortest and longest as well — that is what turns a single guessed date into an honest range.
3. Set your luteal phase if you know it
This is the addition that matters most. The luteal phase — ovulation to the next period — is the more stable half of the cycle, but “more stable” is not “fourteen days for everyone”. It typically runs 12 to 14 days and can be shorter. Every calendar estimate that assumes 14 is carrying that assumption as its largest error.
4. Add your shortest and longest cycles if you are irregular
The tool then produces a fertile window range rather than a single window. For an irregular cycle this is the only honest output: a precise date derived from an imprecise average is false precision, and acting on it is how people miss the window entirely.
5. Read the six-day window, ending on ovulation
Not seven days, and not extending past ovulation. The window is the five days before ovulation plus the day itself, and the reason is biological rather than conventional — explained below.
6. Treat the day-by-day probabilities as population figures
They come from a specific published cohort and describe averages across it, not your personal chance in this cycle. They are most useful for showing you which days are worth prioritising.
The six days that end on ovulation
The fertile window is defined by two facts with very different timescales. Sperm can survive in the female reproductive tract for up to about five days in favourable cervical mucus. An egg, once released, survives roughly 12 to 24 hours.
So the window opens about five days before ovulation and closes on the day of ovulation itself. Six days in total. It does not extend into the day after, because by then the egg is usually already gone.
The definitive study is Wilcox, Weinberg and Baird in the New England Journal of Medicine on 7 December 1995, which followed 221 healthy women who were planning to become pregnant and who recorded intercourse daily while their ovulation was tracked with hormone measurements. Its central finding was precisely this: conception occurred only when intercourse took place in the six-day interval ending on the day of ovulation, and intercourse after ovulation produced no pregnancies at all in that cohort.
The previous version of this page ran the window from five days before ovulation to one day after — seven days — under a caption that read “6 days incl. ovulation”. The label and the calculation disagreed with each other, and the calculation disagreed with the evidence. It also assigned an 8 per cent conception chance to the day after ovulation, where the source found effectively zero. Three other day-specific figures were wrong as well, all understating the days that matter.
The published probabilities are now used. One of them is not smooth: the estimate at four days before ovulation sits above the day either side of it. That blip is in the data, and it has been left in rather than tidied away, because a curve that has been smoothed to look plausible is no longer the source it claims to be.
Why the luteal phase is the input that actually matters
A menstrual cycle has two phases, and they behave differently.
The follicular phase runs from the first day of bleeding to ovulation. It is the variable half. Stress, illness, travel, sleep disruption, weight change and exercise load all delay ovulation, and a cycle that is longer or shorter than usual is almost always longer or shorter here.
The luteal phase runs from ovulation to the next period. It is the more consistent half — usually 12 to 14 days, and reasonably stable within one person from cycle to cycle.
This is why counting backwards from the expected next period is more reliable than counting forwards from the last one, and it is why the luteal phase is adjustable here. A calculator that hardcodes 14 days is imposing an assumption on everyone; if yours is 11 or 12 days, every date it gives you is out by two or three — enough to miss the window.
A luteal phase consistently shorter than 10 days is sometimes described as a luteal phase defect and is worth raising with a doctor if you are trying to conceive.
The off-by-one that made the tool contradict itself
The previous build computed the ovulation cycle day as cycle length minus fourteen, and announced that number on screen. It then computed the ovulation date as the last period plus the same figure.
Those two are one day apart. With the first day of bleeding counted as cycle day 1 — the standard convention — adding 14 days to the start date lands on cycle day 15, not day 14. So the page said “cycle day 14” while pointing at a date that was cycle day 15.
One day is not a rounding error in this context. It is a sixth of the fertile window, and it is the kind of defect that is invisible unless you check the label against the date. Both now derive from the same calculation, counting the first day of bleeding as day 1.
A related problem: the cycle-day counter used a modulo that wrapped indefinitely, so an entry from a year earlier still produced a confident “cycle day 2”, and one from 2023 gave “cycle day 27”. The tool now recognises a date too old to describe a current cycle and says so rather than producing a plausible number from nothing.
What a calendar method can and cannot do
Calendar prediction is retrospective by nature. It tells you when you probably ovulated in past cycles and projects that forward. It cannot detect ovulation happening now, and it cannot know that this month you are ovulating three days late because of a stressful fortnight.
Several methods observe the current cycle instead, and they complement a calendar rather than replacing it:
Cervical mucus changes through the cycle, becoming clear, stretchy and slippery — often compared to raw egg white — in the days approaching ovulation. This is one of the better real-time indicators, and it identifies the window as it opens rather than after it has closed.
Ovulation predictor kits detect the surge in luteinising hormone that precedes ovulation by roughly 24 to 36 hours. Useful and specific; they can miss a surge if testing once daily, and can mislead in polycystic ovary syndrome, where LH may be persistently elevated.
Basal body temperature rises by around 0.3 °C after ovulation. It confirms that ovulation happened, which is useful for learning your own pattern, but it tells you only after the window has closed.
Used together over a few cycles, these turn a calendar estimate into something calibrated to you.
An irregular cycle deserves a range, not a date
If your cycles vary by more than a few days, a single predicted ovulation date is misleading in a specific way: it is precise, it looks authoritative, and it may well be wrong.
Entering your shortest and longest cycles produces a window range instead — the earliest ovulation could plausibly occur and the latest. That range may be wide, and a wide range is the correct answer rather than a failure of the tool. For someone whose cycles run from 24 to 38 days, the honest fertile window spans nearly two weeks.
Cycles that are highly irregular, absent, or consistently shorter than 21 or longer than 35 days are worth investigating regardless of whether you are trying to conceive. Polycystic ovary syndrome, thyroid disorders, high prolactin, significant stress, low body weight and heavy exercise loads all affect ovulation, and several are straightforwardly treatable once identified.
This is not a contraceptive
Stating it plainly because the arithmetic invites the opposite conclusion. Calendar-based ovulation prediction is not a reliable method of avoiding pregnancy. Ovulation timing shifts, sperm survive for days, and the window in any given cycle may not be where a calendar puts it.
Fertility awareness-based methods used for contraception are a different discipline entirely — they combine daily temperature, mucus and calendar observations under a defined protocol, require training, and even then have failure rates well above modern contraception. A prediction from a calculator is not one of those methods.
When to seek advice rather than another cycle of tracking
Guidance in the UK from NICE CG156 on fertility, and comparable guidance elsewhere, suggests seeking assessment after 12 months of regular unprotected intercourse without conception if you are under 35, and after 6 months if you are 35 or over, because the timeline matters more with age.
Seek advice sooner if your cycles are irregular or absent, if you have known endometriosis or polycystic ovary syndrome, if you have had pelvic inflammatory disease or pelvic surgery, if you have had two or more miscarriages, or if there is a known issue on your partner’s side. Roughly a third of difficulties conceiving relate to male factors, so investigation should generally involve both partners rather than only one.
Nothing on this page is medical advice or a fertility assessment. It is a calendar estimate with its source named and its limits stated.
Nothing you enter is transmitted
Your dates and cycle details are used locally in your browser. Nothing is sent, stored or logged; there is no account and no upload, and the page keeps working with the connection off once loaded. For reproductive health data that matters more than usual.
Questions people ask about ovulation timing
When am I most fertile?
The two days before ovulation and the day of ovulation carry the highest probabilities. The full window is the six days ending on ovulation — there is no fertile day after it.
Can I get pregnant the day after ovulation?
Very unlikely. The egg survives roughly 12 to 24 hours, and in the Wilcox cohort intercourse after ovulation produced no pregnancies. The previous version of this page gave that day an 8 per cent chance, which was simply wrong.
Does everyone ovulate on day 14?
No. Day 14 assumes a 28-day cycle with a 14-day luteal phase. Ovulation is better estimated by counting back from the next expected period than forward from the last one, which is why the luteal phase is adjustable here.
How accurate is a calendar prediction?
Reasonable for regular cycles, poor for irregular ones. It projects your past pattern forward and cannot see that ovulation is late this month. Cervical mucus observation and ovulation predictor kits track the current cycle and are worth combining with it.
My cycles are irregular. What should I do?
Enter your shortest and longest cycles to get a window range rather than a date. If your cycles are consistently outside 21 to 35 days, or highly variable, that is worth investigating with a doctor.
How often should we have intercourse to conceive?
Every one to two days through the fertile window is the usual guidance, and daily is not necessary. NICE suggests every two to three days throughout the cycle as an approach that removes the need to time anything at all.
What does egg-white cervical mucus mean?
Clear, stretchy, slippery mucus indicates the fertile window is open. It is one of the more useful real-time signs precisely because it appears before ovulation rather than after.
Can I use this to avoid pregnancy?
No. Calendar prediction is not a reliable contraceptive method. Ovulation shifts, sperm survive for days, and this tool is not a fertility awareness-based method.
Do you store my cycle data?
No. Nothing is saved or transmitted.
Sources behind these numbers
- Wilcox AJ, Weinberg CR, Baird DD, “Timing of sexual intercourse in relation to ovulation”, New England Journal of Medicine, 7 December 1995, 333(23):1517–1521 (PMID 7477165) — 221 women planning pregnancy; the six-day window and the day-specific probabilities used here.
- NICE CG156 — Fertility problems: assessment and treatment, for when to seek assessment and the intercourse-frequency guidance.
- American College of Obstetricians and Gynecologists, for cycle physiology and fertility evaluation.
- NHS — periods, for normal cycle ranges and when irregularity warrants review.
- CDC — contraception, for the effectiveness of fertility awareness-based methods compared with other options.