Free period & cycle calculator
When is my next period?
First day of your last period, cycle length, period duration – that's all it takes: The calculator shows you your next three periods, your current cycle day, and your current cycle phase – like a period calendar, just without the math. No account, no app, right here.
Calculator
When is my next period?
Cycle length runs from the first day of one period to the day before the next. If you do not know yours, 28 days is a starting estimate.
The calculation runs entirely in your browser. Your entries are not sent or saved.
Important: The result is an estimate, not a guarantee – and not a method of contraception. The calculator projects your cycle length into the future. A delayed ovulation also delays the period, and the "unfruchtbaren" days from a calendar calculation often are not. If you want reliable contraception, seek gynecological advice.
The method
How reliable is the calculation?
The calculation itself is simple: next period = start of last period + cycle length, projected over three cycles. For the cycle phase and the fertile window, the calculator uses the most important rule of thumb in cycle calculation: the luteal phase – the time from ovulation to the next period – is relatively constant at around 14 days. It follows that ovulation ≈ cycle length minus 14.
How good the result is depends almost entirely on your input: With a stable cycle, the first calculated period is often accurate to within a few days. Each subsequent one becomes more uncertain because small deviations add up from cycle to cycle – the third period is more of a month than a date. How the individual phases of your cycle are related is explained step by step under The 4 cycle phases explained simply; if you are specifically looking for your fertile days, use the ovulation calculator.
“Plan for a window of a few days – not an exact date.”
Influences
What shifts the period?
Almost always, the reason is delayed ovulation: The luteal phase afterwards remains fairly constant at 14 days – if your period arrives late, the first half of the cycle was usually longer. Typical triggers for this: Stress (Cortisol slows down the hormonal axis that triggers ovulation), Travel with time differences and a shifted sleep rhythm, Illness and fever, plus intense exercise, significant weight changes or stopping hormonal contraception.
Individual shifts of a few days are normal and no cause for concern. It only becomes interesting when it becomes a pattern – then it is worth noting the start dates for a few months so that you have concrete figures instead of a feeling at your doctor's appointment.
Irregular cycles
What about irregular cycles?
First, the classification: 21 to 35 days cycle lengths are considered normal, and hardly any cycle is exactly the same length to the day. "Irregular" in the relevant sense means: the length fluctuates significantly from cycle to cycle – for example by more than seven or eight days – or is permanently outside this range.
For the calculator, this means: Use as the cycle length the average of your last three to six cycles and read the result as a guide, not as a fixed date. Common reasons for strongly fluctuating cycles are periods of stress, the time after stopping hormonal contraception, menopause – or hormonal causes such as PCOS; in the case of the latter, nutrition can also play a role, more on this in the PCOS nutrition plan. How you generally eat in the rhythm of your cycle is shown by the Cycle Syncing Guide.
Factual, without alarm
When to see a doctor?
Most cycle fluctuations are harmless. It should be clarified at a gynecological practice if:
- the period is absent for three months or longer (and pregnancy has been ruled out by a test),
- your cycle is consistently shorter than 21 or longer than 35 days or fluctuates significantly over months,
- the bleeding is unusually heavy or lasts longer than seven days,
- spotting occurs or the pain is so severe that it limits your daily life.
None of this is a diagnosis – they are reasons to take a closer look. If you deal a lot with cramps and cravings around your period, you will find practical approaches in Nutrition during menstruation and PMS & Nutrition.
FAQ
Frequently asked questions about the period calculator
How accurate is a period calculator?
As accurate as your cycle is regular. The calculator simply projects your cycle length into the future – with a constant cycle, this is often accurate to within a few days; with fluctuating cycles, it is correspondingly less accurate. As a rule of thumb: The first calculated period is the most reliable, each subsequent one becomes more uncertain because small deviations add up. Therefore, it is better to plan for a window of a few days instead of an exact date.
Why does my period sometimes shift?
Because ovulation shifts – the time after that until the period (luteal phase) is relatively constant at around 14 days, the time before is not. Stress, lack of sleep, illness, travel with time zone changes, intense exercise, or weight changes can push ovulation back by days – and thus also the period. Individual shifts of a few days are completely normal and no cause for concern.
How long does a normal cycle last?
Between 21 and 35 days is considered normal – the often-cited 28 days are just the average, not the norm. The period itself also varies: two to seven days of bleeding are common. More important than comparing yourself with others is your own pattern: a cycle that is stable at 32 days is less conspicuous than one that jumps between 24 and 35 days.
What do I do in case of irregular cycles?
Use the average of your last three to six cycles as your cycle length and read the result as a rough guide, not as a fixed date. Note the start dates of your period for a few months – often there is a pattern after all. If your cycle fluctuates significantly over the long term, is regularly shorter than 21 or longer than 35 days, or if your period stops altogether, have the cause clarified by a gynecologist.
Period overdue – am I pregnant?
Overdue initially just means: Ovulation occurred later than usual – this happens due to stress, illness, or travel, even without pregnancy. If you had unprotected sex, a pregnancy test will provide clarity starting from the day of the missed period; the result is most reliable in the morning. If it is negative and your period still does not start, repeat the test after a few days.
Can I use a period calculator for contraception?
No. The calculator estimates calendar dates based on average values – it does not know when your ovulation actually takes place. Sperm can survive for up to five days, ovulation shifts easily, and just a single incorrectly estimated cycle is enough for an unintended pregnancy. If you want reliable contraception, talk to your gynecologist about suitable methods.
My period is missing – when should I see a doctor?
If pregnancy is possible: first, a pregnancy test. Beyond that, as a guide: if your period is absent for three months or longer, if your cycle shifts significantly over several months, if the bleeding is unusually heavy or lasts longer than seven days, or if severe pain is involved – then it should be checked at a gynecological practice. Not as a cause for alarm, but for clarification: usually, there is a harmless reason behind it.
So your period surprises you less often
The app reminds you before your period arrives – and also shows you which recipes match your current cycle phase. Try for free for 7 days.
Or continue calculating: The Ovulation calculator shows you your fertile days in detail.
Note: This page is for general information purposes and does not replace medical advice. The calculator provides an estimate based on average values and is explicitly not a method of contraception or family planning. In case of persistent cycle changes, please contact your gynecological practice.