Key takeaways
- Predicts your next 6 menstrual cycles, ovulation, fertile window, and luteal phase from a single LMP plus your average cycle length.
- Three modes: Standard (regular 24–35 day cycles), Irregular (cycles vary by 5+ days), and PCOS (long luteal phase plus anovulation warnings).
- In Irregular and PCOS modes, you can paste your last 3–6 cycle lengths to get a confidence range — not a single point estimate.
- Calculations run entirely in your browser. Nothing is sent to a server, and we never ask for an email or sign-up.
- Results include shareable URLs so you can come back next month or send a screenshot to your clinician.
How it works
Every period calculator does one thing at its core: project your average cycle length forward from your last menstrual period (LMP). Where calculators differ is what they assume when your cycles aren't perfectly regular — and how honestly they tell you that the prediction is an estimate, not a calendar entry.
HerCalc's Standard mode uses the textbook luteal-phase rule: ovulation occurs 14 days before your next period, and your fertile window spans the five days before ovulation plus the day itself. We project your next six cycle starts as LMP + n × cycle length.
Irregular mode is for users whose cycles vary by 5+ days month to month. Instead of asking for a single average cycle length, it accepts the last 3–6 cycles you actually had. We compute the median cycle length (more robust to outliers than the mean) and the standard deviation. Predictions come with a ±n-day confidence range. The fertile window widens by ±2 days to absorb the uncertainty.
PCOS mode changes the assumptions further. PCOS users often have a longer luteal phase (we use 16 days), longer cycles, and anovulatory months — cycles where ovulation does not happen at all and the period is technically a "withdrawal bleed" rather than a true post-ovulatory period. When your cycle data has a standard deviation greater than 7 days, we flag the possibility of anovulation and recommend tracking BBT or talking to a clinician.
We also surface a luteal-phase view. Knowing whether your luteal phase is within the 10–16 day normal range matters for fertility planning and is a useful piece of clinical context if you are investigating cycle health.
The science behind it
The luteal-phase model
Modern menstrual-cycle math rests on a single observation: while the follicular phase (period through ovulation) is highly variable, the luteal phase (ovulation through next period) is relatively constant for a given person — usually within a one-day band. This makes ovulation a fixed offset from the next period, not a fixed offset from the last one. If your next period is on April 28 and your luteal phase is 14 days, you ovulated on April 14, give or take a day.
The classic 14-day luteal assumption comes from older textbooks. Real-world data — for example the Bull et al. analysis of 612,613 cycles tracked in mobile apps — shows that the median luteal phase is 13.4 days, with a standard deviation of about 1.8 days. The follicular phase varies far more: median 14.7 days but with a long right tail. This is why our Irregular and PCOS modes use median rather than mean cycle length.
Why we use median, not average
Imagine you are tracking five recent cycles: 28, 30, 27, 29, and 60 days. The mean is 34.8 days — wildly off because of the 60-day outlier. The median is 29 days, much closer to your typical pattern. PCOS and post-pill cycles produce these long-cycle outliers regularly; medians are the standard fix.
The fertile window
Sperm survive in the female reproductive tract for up to five days under ideal cervical-mucus conditions; an unfertilized egg lasts 12–24 hours. The fertile window is therefore the five days before ovulation plus the day of ovulation — a six-day window where conception is possible. Wilcox et al. (1995, NEJM) found that 95% of conceptions in their study occurred in this six-day window.
We display the window with a fertility heatmap on the Ovulation Calculator. On the Period Calculator, we show the bracket as a range and trust the user to extrapolate.
Anovulation: the case PCOS users care about
Anovulation is a cycle where no egg is released. The bleeding that follows is not a true post-ovulatory period — it is endometrial breakdown driven by estrogen withdrawal. From a calendar perspective, anovulatory cycles look like very long cycles or no cycles at all. From a fertility perspective, you cannot conceive that month.
We flag possible anovulation when your cycle data has a standard deviation above 7 days, when you skip a cycle entirely, or when cycles consistently exceed 45 days. PCOS mode shows this warning prominently and links to the PCOS section of our blog for ongoing reading.
Cycles as a vital sign
ACOG Committee Opinion 651 frames the menstrual cycle as a vital sign — a signal of overall health that clinicians should review at every preventive visit. Cycles consistently shorter than 21 days, longer than 35 days, or that change suddenly in pattern can be early signs of thyroid issues, endometriosis, fibroids, hormonal disorders, eating disorders, or PCOS. Tracking and bringing this data to a clinician is one of the highest-leverage things you can do for your gynecologic health.
References
- ACOG Committee Opinion 651: Menstruation in Girls and Adolescents — ACOG, 2015 (reaffirmed)
- Real-world menstrual cycle characteristics of more than 600,000 cycles — Bull et al., NPJ Digital Medicine 2019
- Clinical guideline: assessment and management of polycystic ovary syndrome — International PCOS Network 2023
- Contraceptive failure in the United States — Trussell J., Contraception 2011
If your cycles wander: PCOS & irregular mode
For PCOS and irregular cycles — the textbook 28-day model is rarely your reality.
Most period calculators were built around a phantom user with a perfectly clean 28-day cycle. That user is rarer than the assumption suggests — and if you have PCOS, post-pill irregularity, a thyroid issue, or post-pregnancy unpredictability, those calculators fail you.
- Switch to PCOS mode if you have a diagnosis or your cycles regularly exceed 35 days. The luteal-phase assumption changes from 14 to 16 days, and the calculator surfaces anovulation warnings when warranted.
- Switch to Irregular mode if your cycles fall in the 24–35 day range but vary by 5+ days. You'll get a confidence range and a wider fertile-window bracket.
- Paste your last 3–6 cycle lengths into either mode. Three is the minimum for a useful median; six gives you a meaningful standard deviation.
- Track for at least three months before drawing conclusions. Single anomalous cycles are extremely common — illness, travel, stress, sleep deprivation, and significant weight change all delay ovulation and shift period timing.
Our companion reading — PCOS-friendly cycle tracking — covers how to combine calendar tracking with basal body temperature charting and ovulation prediction kits for cases where calendar math alone can't keep up.
How to use this calculator
- Enter your last menstrual period (LMP). This is the first day of bleeding from your most recent period. We will use it as the anchor for every prediction.
- Enter your average cycle length. 21–35 days is normal for adults. If you are not sure, use 28 — but switch to Irregular mode if your cycles vary more than 5 days.
- Enter how many days your period typically lasts. 3–7 days is normal. Heavy bleeding or periods longer than 8 days are worth raising with a clinician.
- Optional: switch to Irregular or PCOS mode. Both modes accept a list of your most recent cycle lengths and produce a confidence range instead of one date.
- Read your prediction. You will see your next 6 cycles, your fertile window, your ovulation day, and (in PCOS mode) anovulation warnings if your data suggests cycles without ovulation.
Limitations & medical disclaimer
- Predictions are estimates, not certainties. Even regular cycles vary by ±2 days; irregular cycles vary much more.
- This calculator is not a contraceptive. Calendar-based "safe day" methods have a typical-use failure rate of about 24% per year. Pair fertility awareness with barriers or other methods.
- PCOS users with anovulatory cycles cannot reliably predict ovulation by calendar alone. Combine with BBT and OPK testing if conception timing matters.
- Sudden, large changes in cycle pattern (skipping periods, very heavy bleeding, severe pain, or any bleeding after menopause) are not normal and warrant a clinician's attention.
- HerCalc is for educational use. It is not a medical device and does not provide diagnosis or treatment. If you are experiencing symptoms, contact a qualified healthcare provider.
HerCalc tools are educational and do not replace professional medical advice. Always consult a qualified clinician for diagnosis or treatment decisions.
Frequently asked questions
How accurate is a period calculator if my cycles are irregular? +
For regular cycles (24–35 days, varying by less than 5 days), a calendar prediction is accurate within ±2 days about 85% of the time. For irregular cycles or PCOS, accuracy drops sharply — that is why our Irregular mode reports a confidence range instead of a single date, and our PCOS mode warns when cycle data suggests anovulation.
If you need a precise prediction, combine calendar tracking with basal body temperature (BBT) charting and an ovulation prediction kit (OPK).
What counts as the "first day" of my cycle? +
Day 1 is the first day of bright-red flow — not spotting. Spotting in the day or two before bleeding starts is common; do not count it as Day 1. If you bleed for less than a few hours and it stops, that is also probably spotting.
My period is late — am I pregnant? +
A late period is the most common early sign of pregnancy, but cycles can also be delayed by stress, illness, travel, intense exercise, weight change, hormonal contraception transitions, thyroid issues, and PCOS. A home pregnancy test 1–2 weeks after a missed period is the simplest answer; tests work earliest with first-morning urine. If you have repeated late periods without pregnancy, talk to a clinician about possible underlying causes.
What is the luteal phase, and why does it matter? +
The luteal phase is the time between ovulation and your next period — typically 12–14 days, though some PCOS users have a longer phase. Calculators predict ovulation by subtracting the luteal length from your next predicted period. A short luteal phase (under 10 days) can be associated with reduced fertility; a long phase can be associated with PCOS. If your charted luteal phase is consistently outside 10–16 days, it is worth a clinician's attention.
Can I use this calculator as birth control? +
No. Calendar-based fertility-awareness methods have a typical-use failure rate of roughly 24% per year (Trussell, 2011) — meaning about one in four users gets pregnant within a year. If you want to use periodic abstinence, the symptothermal method (basal body temperature + cervical mucus + LH testing) is significantly more reliable, with typical-use failure around 13%. Always pair calendar awareness with a barrier method during the fertile window.
Should I use Standard, Irregular, or PCOS mode? +
Use Standard if your cycles are 24–35 days and vary by less than 5 days month to month. Use Irregular if your cycles are within range but vary by 5–10 days, or if you have only 2–3 months of tracked data. Use PCOS if you have a PCOS diagnosis or if your cycles are over 35 days, vary by more than 10 days, or skip entirely. PCOS mode extends the luteal-phase assumption to 16 days, widens the fertile buffer, and surfaces anovulation warnings.
What does my cycle data tell my doctor? +
A 3–6 month tracking history is one of the most useful things you can bring to a gynecology appointment. The pattern matters more than any one number. Cycles consistently shorter than 21 days (polymenorrhea), longer than 35 days (oligomenorrhea), highly variable in length, accompanied by heavy bleeding, painful enough to disrupt daily life, or with abnormal bleeding between periods all warrant evaluation. Take a screenshot of your HerCalc results — none of it is stored on a server, and the URL contains your inputs so you can reload them later.
Why does the calculator show ±2 to ±5 days for my predictions? +
In Irregular and PCOS modes, we compute the standard deviation of your last few cycle lengths and present it as a confidence band. A small standard deviation means your cycles are reliable; a large one (5+ days) means any single prediction is just an educated guess. We round the confidence to the nearest day for readability.