Period Calculator
Predict your next six periods, fertile windows and ovulation days on a calendar.
Period Calculator — your inputs
- How long it lasts
- 5 days
- Average cycle length
- 28 days
- Unit system
- Metric
Key takeaways
- Cycle length is counted from the first day of one period to the day before the next — not from the end of one to the start of the next, which is how most people accidentally get a number about five days too short.
- Only about 13% of cycles are exactly 28 days. The average across more than 600,000 real cycles was closer to 29, with wide individual variation.1
- Prediction error compounds. A two-day error in your average cycle length is two days out by next month and twelve days out by the sixth predicted period.
- The fertile window is lopsided: five days before ovulation and one after, because sperm outlive the egg by several days.2
- Calendar prediction is not contraception. Typical-use failure rates for calendar-based methods are far higher than most people assume.4
How to use this calculator
Enter the first day of your last period — the first day of actual bleeding, not spotting before it.
Set how many days the bleeding usually lasts and how long your cycle usually runs, counting from the first day of one period to the day before the next.
Read the calendar for the next three months, then open the table for six full cycles of dates.
Understanding your result
Cycle length is measured from the first day of one period to the day before the next — not between the end of one and the start of the next, which is the most common way people get the number wrong.
Ovulation is estimated by counting backwards from the next period, because the luteal phase is the stable part of the cycle and the phase before ovulation is what varies.
The fertile window is lopsided by design: five days before ovulation and one after, because sperm outlive the egg by several days.
How to count your cycle correctly
Before any prediction can be useful, the input has to be right — and this is the single input people most often get wrong. Your cycle length runs from the first day of bleeding in one period to the day before bleeding starts in the next. The period itself is part of the cycle, not a gap between cycles.
Counting instead from the last day of one period to the first day of the next produces a number roughly five days short, which then drags every predicted date earlier and earlier as the projection runs forward. If the dates this calculator gives you feel consistently early, this is the first thing to check.
Day 1 is the first day of proper flow
Light spotting in the day or two before bleeding properly starts does not count as day 1. Use the first day of full flow, and use the same rule every month — consistency matters more than which convention you pick.
| Event | Date | Cycle day |
|---|---|---|
| Period starts | 8 August | 1 |
| Period ends | 12 August | 5 |
| Most probable ovulation | 22 August | 15 |
| Next period starts | 5 September | 1 of the next cycle |
What a normal cycle actually looks like
The 28-day cycle is a textbook convention that has been mistaken for a norm. When Bull and colleagues analysed more than 600,000 real cycles from over 120,000 users of a tracking app, the mean length was 29.3 days, and only around 13% of cycles were exactly 28. Roughly two-thirds fell somewhere between 25 and 30 days.1
More usefully, the same data showed that variation within a person is normal too. Most people's cycles moved by a few days from month to month without anything being wrong. A cycle that is 27 days one month and 31 the next is unremarkable; it is sustained, large swings that carry information.
- 21–24 days8 % of cycles
- 25–27 days24 % of cycles
- 28–30 days39 % of cycles
Most common
- 31–34 days21 % of cycles
- 35+ days8 % of cycles
| Method | Value (% of cycles) |
|---|---|
| 21–24 days | 8 |
| 25–27 days | 24 |
| 28–30 days | 39 |
| 31–34 days | 21 |
| 35+ days | 8 |
Professional guidance treats the cycle as a vital sign for exactly this reason: a change in your own pattern is more informative than a comparison against anyone else's. Adult cycles between roughly 24 and 38 days are considered within the normal range.3
Why the fertile window is six days, and lopsided
The calendar shades six days, not one, because conception is possible from intercourse on any of them. Sperm can survive in fertile cervical mucus for up to about five days, while the released egg is viable for only around 12 to 24 hours. The window therefore opens well before ovulation and closes almost immediately after it.2
This has a practical consequence that surprises people: by the time you notice ovulation — through a temperature rise, say — the most fertile days have already passed. Prediction is useful precisely because the useful days are the ones before the event.
| Day relative to ovulation | Approximate chance |
|---|---|
| 5 days before | ~10% |
| 3 days before | ~14% |
| 2 days before (success) | ~27% |
| 1 day before (success) | ~31% |
| Day of ovulation | ~33% |
| 1 day after | close to zero |
How far ahead these predictions hold up
The forecast table gives six cycles because that is what people ask for, but the six rows are not equally trustworthy and it would be dishonest to present them as though they were. Every projected date is built by adding your average cycle length again and again, so any error in that average multiplies with each row.
| Predicted cycle | If your average is 2 days off | If it is 4 days off |
|---|---|---|
| 1st (next period) | ±2 days | ±4 days |
| 3rd | ±6 days | ±12 days |
| 6th (warning) | ±12 days | ±24 days |
This is not a contraceptive method
Ovulation can shift by several days in a single cycle because of stress, illness, travel or disrupted sleep, and the fertile window moves with it. Calendar-based fertility awareness methods have substantially higher typical-use failure rates than most people expect.4 If you are trying to avoid pregnancy, speak to a healthcare provider about methods with a known and documented failure rate.
What this calculator cannot tell you
It is calendar arithmetic. It knows your last period and two averages, and it assumes the pattern repeats forever. It has no access to the thing it is actually predicting.
- Whether you ovulated at all. Cycles that produce bleeding without ovulation are common in adolescence, perimenopause, PCOS and periods of high stress or low energy availability. The calendar cannot tell the difference.
- Whether you are pregnant. A late period has many causes. A home test from the day the period was due answers this; a calendar never can.
- Why your cycle changed. It will happily project a pattern that has already stopped being true.
- Anything at all while you are using hormonal contraception, which suppresses the natural cycle. Bleeding on the pill is a withdrawal bleed, not a period.
When to bring it up with a clinician
Cycles consistently shorter than 21 days or longer than 35, bleeding that soaks through protection every hour for several hours, bleeding between periods, periods stopping for three months or more without pregnancy, or pain that stops you doing normal things. None of these are automatically serious, and all are reasonable to ask about.5
Terms used on this page
- Cycle day 1
- The first day of full menstrual flow. Everything else in the cycle is counted from here.
- Fertile window
- The six days on which intercourse can result in conception: the five days before ovulation plus the day of ovulation itself.
- Luteal phase
- The stretch between ovulation and the next period. Close to fixed at 12–14 days, which is why ovulation is estimated by counting backwards from the next period rather than forwards from the last one.
- Anovulatory cycle
- A cycle in which no egg is released, though bleeding may still occur. Calendar prediction cannot detect one.
The formula
- Last period started 8 August, 28-day cycle, 5 days of bleeding
- Next period = 8 August + 28 = 5 September
- Ovulation = 8 August + 14 = 22 August
Source: Calendar projection with a 14-day luteal phase. Published clinical/scientific formula.
Limitations
- Prediction assumes the cycle repeats exactly. Real cycles vary month to month, and the error compounds — the sixth projected cycle is far less reliable than the first.
- This is not a contraceptive method. Ovulation shifts with stress, illness, travel and disrupted sleep, and the fertile window moves with it.
- Hormonal contraception, breastfeeding, perimenopause and conditions such as PCOS or thyroid disease all change cycle behaviour in ways calendar arithmetic cannot model.
References
- 1.Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Bull et al., npj Digital Medicine 2:83, 2019.
- 2.The timing of the 'fertile window' in the menstrual cycle: day specific estimates. Wilcox, Dunson & Baird, BMJ 321(7271):1259–1262, 2000.
- 3.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).
- 4.Fertility awareness-based methods of family planning (efficacy and typical use). Peragallo Urrutia et al., Obstetrics & Gynecology 132(3):591–604, 2018.
- 5.Periods and fertility in the menstrual cycle. National Health Service (NHS), United Kingdom, 2023.
Frequently asked questions
Related calculators
Menstrual Cycle Calculator
See which cycle day and phase you are in today, and whether your cycle counts as regular.
Open calculatorOvulation Calculator
Predict your ovulation date and fertile window for the next three cycles.
Open calculatorConception Calculator
Estimate the conception date and fertile window from a due date or birth date.
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.