Menstrual Cycle Calculator
See which cycle day and phase you are in today, and whether your cycle counts as regular.
Menstrual Cycle Calculator — your inputs
- Average cycle length
- 28 days
- Period length
- 5 days
- Shortest recent cycle
- 27 days
- Longest recent cycle
- 30 days
- Unit system
- Metric
Key takeaways
- The cycle has four phases — menstrual, follicular, ovulatory and luteal — driven by the rise and fall of oestrogen and progesterone.2
- The luteal phase barely moves: 12–14 days in almost everyone. Nearly all the difference between a 24-day cycle and a 35-day one sits in the follicular phase before ovulation.
- 'Regular' does not mean 28 days. Clinically, a cycle is regular when your shortest and longest differ by fewer than 8 days — a steady 33-day cycle is regular; one swinging between 24 and 38 is not.4
- Professional guidance treats the cycle as a vital sign: a change in your own pattern carries more information than any comparison against an average.1
- Phase boundaries here are modelled from your cycle length, not measured. Only ovulation tests, bloodwork or ultrasound can confirm where you actually are.
How to use this calculator
Enter the first day of your most recent period and your usual cycle and period lengths.
Optionally add your shortest and longest cycle over the last six months to get a regularity assessment.
Read your current cycle day, the phase you are in, and what that phase is doing.
Understanding your result
The cycle has four phases: menstrual (bleeding), follicular (the lining rebuilds), ovulatory (the egg is released) and luteal (progesterone maintains the lining).
The luteal phase is close to fixed at about 14 days. Almost all the variation between a 24-day cycle and a 35-day one sits in the follicular phase.
'Regular' does not mean identical. Clinically, a cycle is regular when the shortest and longest over a year differ by fewer than 8 days — not when every cycle is 28 days.
The four phases, and what each one is doing
A menstrual cycle is one full turn of a hormonal loop whose purpose is to release an egg and prepare the uterus in case it is fertilised. Each phase is a stage in that preparation, and the symptoms people notice — energy, sleep, temperature, appetite, mood — track the hormones rather than the calendar.2
- Menstrual phase (days 1 to about 5)
- Oestrogen and progesterone are at their lowest and the uterine lining sheds. This is the only phase you can observe directly, which is why the whole cycle is numbered from its first day.
- Follicular phase (from the end of bleeding to about two days before ovulation)
- Follicle-stimulating hormone matures a cohort of follicles in the ovary and one becomes dominant. Rising oestrogen rebuilds the uterine lining. This is the elastic phase — it stretches or shortens to make a cycle long or short.
- Ovulatory phase (roughly three days around ovulation)
- A surge in luteinising hormone triggers release of the egg, typically 24 to 36 hours after the surge begins. The egg itself is viable for only about 12 to 24 hours.
- Luteal phase (ovulation to the next period, about 14 days)
- The emptied follicle becomes the corpus luteum and produces progesterone, holding the lining in place. If no fertilised egg arrives, it breaks down, progesterone falls, and the lining sheds — which is the next period.
Why ovulation is counted backwards
Because the luteal phase is the reliable part. Estimating ovulation as 'cycle length minus 14' works far better than 'day 14', which is only correct on a 28-day cycle. On a 32-day cycle, ovulation is around day 18 — not day 16.
What 'regular' actually means
Almost everyone has been told that a normal cycle is 28 days, and almost nobody has one every month. The clinical definition is not about hitting a number — it is about consistency. Under the widely used FIGO terminology, a cycle is regular when the difference between your shortest and longest cycle over a year is under about 8 days.4
That reframes the question usefully. A person whose cycle runs 33, 34, 33, 35 days has a perfectly regular cycle that happens to be long. A person averaging 28 days but ranging from 22 to 38 does not, even though their average looks textbook. The average is the least informative number of the three.
| Shortest to longest | What it means | What to do |
|---|---|---|
| Under 8 days (success) | Regular by the standard clinical threshold, whatever the average length | Calendar prediction is reasonably reliable |
| 8 to 14 days (warning) | Irregular. Common in adolescence, perimenopause and periods of high stress | Worth mentioning at a routine appointment; ovulation tests beat the calendar |
| Over 14 days, or cycles absent (danger) | Substantial irregularity | Worth a dedicated conversation with a healthcare provider |
- Short — worth asking about (15–21)
- Normal range (21–38)
- Long — worth asking about (38–45)
| Category | Range |
|---|---|
| Short — worth asking about | 15 to 21 days |
| Normal range | 21 to 38 days |
| Long — worth asking about | 38 to 45 days |
Why the follicular phase is the one that moves
The single most useful structural fact about the cycle is that its two halves behave completely differently. The luteal phase — after ovulation — is governed by the lifespan of the corpus luteum, which is close to fixed. The follicular phase, before ovulation, depends on how long it takes a follicle to mature, and that is sensitive to almost everything.
Stress, illness, travel across time zones, disrupted sleep, sharp changes in training load or energy intake can all delay follicle maturation and push ovulation later. When that happens the cycle runs long — but the luteal phase that follows is still about fourteen days. In large-scale tracking data, this asymmetry shows up clearly: cycle-length variation is overwhelmingly variation in the first half.3
A short luteal phase is the exception worth knowing
If the stretch between ovulation and your next period is consistently shorter than about 10 days, that is one of the few patterns in the cycle that is genuinely worth investigating, particularly if you are trying to conceive. Confirming it needs ovulation tracking rather than calendar arithmetic.
What this calculator cannot tell you
Everything above the fold on this page is a model. It takes your cycle length and divides it into phases using the population rule that the luteal phase is fourteen days. It is a good model, and it is not a measurement.
- It cannot confirm you ovulated. Anovulatory cycles still produce bleeding, and the model will confidently label phases that did not happen.
- It cannot detect a short luteal phase, because it assumes the luteal phase is fourteen days by construction.
- It does not apply on hormonal contraception. Combined pills, hormonal IUDs, implants and injections suppress or override the natural cycle; there is no follicular or luteal phase to describe.
- It is not a diagnosis. Irregular cycles have many causes — thyroid disease, PCOS, perimenopause, stress, low energy availability, and others — and distinguishing between them requires a clinician.5
How to get real data instead of a model
Ovulation predictor kits detect the luteinising hormone surge a day or two before release. Basal body temperature taken at the same time each morning rises by around 0.3°C after ovulation and stays up — which confirms ovulation retrospectively and, over a few cycles, tells you your own luteal phase length rather than the assumed one.
Terms used on this page
- Follicular phase
- From the end of bleeding until just before ovulation. The variable half of the cycle — this is what stretches when a cycle runs long.
- Corpus luteum
- The structure the emptied follicle becomes after releasing the egg. It produces the progesterone that sustains the uterine lining, and its fixed lifespan is what makes the luteal phase so consistent.
- Anovulatory cycle
- A cycle in which no egg is released. Bleeding can still occur, so it is not detectable from a calendar alone.
- Basal body temperature
- Resting temperature taken immediately on waking. It rises slightly after ovulation and stays elevated through the luteal phase, confirming that ovulation happened.
The formula
- 30-day cycle, 5-day period, currently day 12
- Ovulation day = 30 − 14 = day 16
- Day 12 is after the period and before day 15
Source: Four-phase cycle model with an 8-day regularity threshold. Published clinical/scientific formula.
Limitations
- Phase boundaries are modelled from your cycle length, not measured. Only ovulation tests, blood work or ultrasound can confirm where you actually are.
- The four-phase model assumes ovulation happens. Anovulatory cycles — common in perimenopause, PCOS and periods of high stress or low energy availability — still produce bleeding without an ovulatory phase.
- Hormonal contraception suppresses the natural cycle entirely, so the phases described here do not apply while you are using it.
References
- 1.Committee Opinion No. 651: Menstruation in Girls and Adolescents — Using the Menstrual Cycle as a Vital Sign. American College of Obstetricians and Gynecologists, 2015 (reaffirmed 2023).
- 2.The normal menstrual cycle and the control of ovulation. Reed & Carr, Endotext (NCBI Bookshelf), MDText.com, 2018.
- 3.Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Bull et al., npj Digital Medicine 2:83, 2019.
- 4.The FIGO recommendations on terminologies and definitions for normal and abnormal uterine bleeding. Munro et al., International Journal of Gynecology & Obstetrics, 2018.
- 5.Irregular periods. National Health Service (NHS), United Kingdom, 2023.
Frequently asked questions
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Open calculatorRelated articles
How to Predict Your Next Period Accurately (and Why Apps Disagree)
Cycle prediction is simple arithmetic on a number most people measure wrong. How to count a cycle properly, how far ahead the maths holds up, and why two apps give you two different dates.
Read articleThe Four Phases of the Menstrual Cycle, Explained Properly
What each phase is actually doing, why only one of them varies in length, and what 'regular' means clinically — which is not 28 days.
Read articleNot medical advice. This calculator is for general informational purposes only and is not a substitute for professional medical guidance. Always consult a qualified healthcare provider before making decisions about your health. Read the full medical disclaimer.
Printed from mystatura.com — for general information only, not medical advice.