Working from your own cycle history rather than a guessed average
1. Enter the first day of your last period
The first day of real bleeding, which is cycle day 1. The field starts empty on purpose. The previous version pre-filled it with a date fourteen days ago and printed confident predictions for a date nobody had entered, which is a strange thing for a health tool to do.
2. Log your recent period start dates
This is the addition that changes what the page can tell you. Enter the last few start dates and the tool derives your actual average, shortest and longest cycle, instead of asking you to estimate a number most people do not track. Three or four entries are enough to be useful; six are better.
3. Add your typical flow duration
How many days bleeding usually lasts. It is assessed rather than merely recorded — there is a published normal limit, and knowing you are outside it is more useful than seeing the number echoed back.
4. Read your next few periods and the countdown
Projected from your own averaged cycle. If the date you entered is too old to describe a current cycle, the tool says so rather than labelling dates from last year as “next period”, which is exactly what the previous version did.
5. Check the fertile window and ovulation estimate
Six days ending on ovulation, and ovulation placed on the same cycle day our ovulation calculator uses. Those two tools used to disagree by a day on identical inputs, which is the sort of thing a visitor is entitled to notice.
6. Read the FIGO assessment underneath
Your frequency, regularity and flow duration are compared against the internationally agreed normal limits. This is the part that answers the question people actually arrive with, which is not “when is my next period” but “is my cycle normal”.
What counts as a normal cycle, according to the people who define it
“Normal” here is not our opinion. The International Federation of Gynecology and Obstetrics — FIGO — publishes agreed definitions so that clinicians and researchers worldwide describe menstrual bleeding the same way. The current version comes from Munro and colleagues in the International Journal of Gynecology & Obstetrics in 2018, the second revision of the two FIGO systems for normal and abnormal uterine bleeding.
Four parameters, each with a normal range:
- Frequency — normal is every 24 to 38 days. More often than every 24 days is frequent; less often than every 38 is infrequent.
- Regularity — measured as the difference between your shortest and longest cycle over a year. Normal variation is roughly 7 to 9 days depending on age, with a little more latitude in the years after menarche and approaching menopause.
- Duration of flow — normal is up to 8 days. Longer than 8 days is prolonged.
- Volume — defined by its effect on you rather than by measurement. Heavy menstrual bleeding is bleeding that interferes with your physical, social, emotional or material quality of life.
The last of those is worth pausing on, because it overturns the usual framing. NICE guideline NG88 in the UK takes the same position: heavy bleeding is defined by its impact on the woman, not by a volume in millilitres. You do not need to prove anything or measure anything for it to count as heavy. If it is disrupting your life, that is the definition met.
Note also that the FIGO frequency range is wider than the 21-to-35-day figure often quoted. A 23-day cycle sits outside the normal range under these definitions, which some older sources would have called fine.
Two tools on one site should not disagree about ovulation
This calculator used to place ovulation at fourteen days before the next expected period. Our ovulation calculator places it at cycle day (cycle length minus luteal phase). On a 28-day cycle those give day 15 and day 14 respectively.
Same inputs, two answers, one day apart, on the same website. Neither convention is indefensible in isolation — the discrepancy comes from whether you count the first day of bleeding as day 0 or day 1 — but a visitor moving between two pages on one site is entitled to consistency, and one day is a sixth of the fertile window.
Both now use cycle day (cycle length minus luteal phase), counting the first day of bleeding as day 1, which is the standard convention.
The fertile window was corrected at the same time. It previously ran from five days before ovulation to one day after — seven days. Wilcox, Weinberg and Baird established in the New England Journal of Medicine in December 1995, following 221 women planning pregnancy with hormone-tracked ovulation, that the window is the six days ending on the day of ovulation, and that intercourse after ovulation produced no pregnancies in their cohort. Sperm survive up to about five days; the egg survives roughly 12 to 24 hours. The window is now six days, ending where the evidence says it ends.
Why logging real dates beats entering an average
Almost every period calculator asks for your cycle length as a single number. Most people answer 28 because it is the number they have been told, or they estimate.
Logging actual start dates removes the guess and produces something a single average cannot: your variation. Frequency and regularity are separate parameters in the FIGO system, and regularity is the one you cannot see from an average. Someone whose cycles run 26, 34, 27 and 33 days has an average of 30 — comfortably normal — and a shortest-to-longest spread of 8 days, which sits at the edge of normal variation and is worth knowing about.
That number is also what makes the fertile window honest for an irregular cycle. A single date computed from an average is precise-looking and potentially several days wrong; a range computed from your shortest and longest is wider and correct.
The dates that stopped making sense, and now say so
Entering a last period date from 2024, the previous version produced three cards reading “Next period: 28 Jan 2024”, “Following: 25 Feb 2024” and so on — all dates that had already passed, all labelled as upcoming. The countdown, having nothing sensible to display, simply hid itself and said nothing.
That is a mistyped year, which is a common slip, and the response was a screen full of confident wrong answers. The tool now recognises a date too old to describe a current cycle and says what is wrong rather than projecting forward from it.
Two smaller things were fixed alongside. The date arithmetic subtracted two local midnights, which across a daylight-saving boundary differ by 23 hours and floor to the wrong day — in a tool whose whole output is dates, being one day out matters. All differences now go through UTC-normalised calendar days. And the safety note — the single line on the page that most needs to render on every device — was carried by an emoji, which is precisely the character most likely to appear as a blank box or not at all.
What moves a cycle, and what is worth investigating
Cycles respond to a great deal. Stress, significant weight change in either direction, heavy training loads, travel across time zones, shift work, illness and starting or stopping hormonal contraception all shift timing, usually by delaying ovulation and therefore lengthening the cycle.
Some variation is entirely normal, particularly in the first years after periods begin and in the years approaching menopause, when cycles commonly become irregular before stopping.
What warrants a medical conversation rather than another month of tracking:
- Cycles consistently shorter than 24 days or longer than 38
- Bleeding lasting more than 8 days
- Bleeding heavy enough to affect your daily life — flooding through protection, passing large clots, needing to change hourly, or anaemia symptoms such as breathlessness and fatigue
- Bleeding between periods or after intercourse
- Periods stopping for three months or more when you are not pregnant
- Severe pain that stops you doing normal things, or pain that is getting worse
- Any bleeding after menopause — this always needs prompt assessment
Several common causes are straightforwardly treatable once identified: polycystic ovary syndrome, thyroid disorders, fibroids, endometriosis, polyps and iron deficiency among them. Endometriosis in particular is diagnosed late in many countries, and severe period pain being dismissed as normal is a recognised part of why.
What this page is not
It is a projection from dates you supply, compared against published definitions. It is not a diagnosis, not medical advice, and not a pregnancy test — a late period has many causes, and if pregnancy is possible, a test is the only thing that answers that question.
It is also not a contraceptive method. Calendar-based prediction is not reliable for avoiding pregnancy: ovulation timing shifts, sperm survive for days, and fertility awareness-based contraception is a trained discipline combining daily observations, not a projection from an app or a web page.
Nothing you enter is transmitted
Your dates, cycle history and flow duration 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. Menstrual data has become a genuine privacy concern in several jurisdictions, and the honest answer to “what do you do with it” is that it never leaves your device.
Questions people ask about menstrual cycles
What is a normal cycle length?
Under the FIGO 2018 definitions, every 24 to 38 days. That is wider than the 21-to-35 figure often quoted, and it means a 23-day cycle sits outside normal where older sources would have called it fine.
How much can my cycle vary and still be normal?
Roughly 7 to 9 days between your shortest and longest over a year, depending on age, with more latitude in the years after periods begin and approaching menopause. This is the parameter an average cycle length hides, which is why logging real dates is worth the trouble.
How long should a period last?
Up to 8 days is within normal limits. Longer than that is classed as prolonged and is worth raising with a doctor.
What counts as heavy bleeding?
Bleeding that interferes with your physical, social, emotional or material quality of life. Both FIGO and NICE define it by impact rather than by volume — you do not need to measure anything for it to count.
Why is my period late?
Stress, illness, weight change, travel, heavy training, shift work and changes to contraception are all common causes, and they work by delaying ovulation. If pregnancy is possible, take a test — no calculator can answer that.
Can I rely on this to avoid pregnancy?
No. Calendar prediction is not a reliable contraceptive method, and this is not a fertility awareness-based method.
Why did the ovulation date change from the other calculator?
It should not, and now it does not. The two tools used different conventions and disagreed by a day on identical inputs. Both now place ovulation at cycle day (cycle minus luteal phase), counting the first day of bleeding as day 1.
My cycles have suddenly become irregular. Should I be concerned?
A sudden change in a previously regular pattern is worth mentioning to a doctor, particularly with other symptoms. It is often benign — stress and weight change are common causes — and it can point to something treatable.
Do you store my period dates?
No. Nothing is saved or transmitted. It never leaves your browser.
Sources behind these numbers
- Munro MG, Critchley HOD, Fraser IS, “The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions”, International Journal of Gynecology & Obstetrics, 2018 (PMID 30198563) — the frequency, regularity, duration and volume definitions used here.
- 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) — the six-day fertile window.
- NICE NG88 — Heavy menstrual bleeding: assessment and management, which defines heavy bleeding by its impact on quality of life.
- International Federation of Gynecology and Obstetrics.
- NHS — periods, for when irregularity or pain warrants review.
- American College of Obstetricians and Gynecologists, whose guidance treats the menstrual cycle as a vital sign.
- CDC — contraception, on the effectiveness of fertility awareness-based methods.
Tools people open next to this one
- Ovulation Calculator — the fertile window in detail, with an adjustable luteal phase.
- Pregnancy Due Date Calculator — if a late period turns out to be the reason.
- Pregnancy Weight Gain Calculator — the IOM 2009 ranges by pre-pregnancy BMI.
- BMI Calculator — both very low and very high BMI disrupt cycles.
- Sleep Calculator — shift work and disrupted sleep affect cycle regularity.
- Water Intake Calculator — the everyday figure people track alongside.