What are the phases of the menstrual cycle?
The menstrual cycle is counted from the first day of bleeding to the day before the next period. It is usually divided into four phases: menstrual, follicular, ovulation and luteal. Health bodies, including the UK NHS, say a cycle often lasts around 28 days, but anything from 21 to 35 days is also entirely normal.
The menstrual phase is the time of bleeding, when the lining of the womb is shed. The follicular phase starts on the first day of the cycle and lasts until ovulation: a follicle matures in the ovary and oestrogen levels rise. In a cycle longer than 28 days, it is usually this part that is longer.
Ovulation is the release of an egg from the ovary. It is followed by the luteal phase, in which the corpus luteum forms and releases progesterone. If pregnancy does not happen, hormone levels fall towards the end of the cycle and a new period begins.
How the calculator works out the phases
The calculator counts the days from the first day of your last period. The menstrual phase lasts as many days as you enter. It places ovulation 14 days before the end of the cycle, because the luteal phase is fairly constant in most women and lasts about 14 days, and the ovulation window covers that day plus the day before and after. The follicular phase sits in between, and the luteal phase runs to the last day of the cycle.
If more days have passed since your last period than your average cycle has, the calculator repeats the same regular cycle going forward. That is why the display is not reliable when a period is late. If your cycles are not regular, the dates are only a rough guide.
Cycle phases and everyday habits
Hormone levels change through the cycle, so some women notice that their energy, sleep, appetite or mood differ from phase to phase. Research shows big differences between women, and even between cycles of the same person. What is true for one woman is therefore not necessarily true for another.
The practical use is simple: write down how you feel, and after a few cycles you will see your own pattern. On that basis you can plan commitments, training or rest. A balanced diet, enough sleep and regular movement matter in every phase. For severe pain, very heavy bleeding or sudden changes in your cycle, the right person to see is a doctor.
Frequently asked questions
How many phases does the menstrual cycle have?
It is usually divided into four phases: menstrual, follicular, ovulation and luteal. The boundaries between them are slightly different for every woman.
How long is the luteal phase?
In most women it lasts about 14 days, so the calculator places ovulation 14 days before the end of the cycle. The follicular phase is the one that differs in shorter or longer cycles.
Does the calculator tell me when I am fertile?
The calculator shows an ovulation window, but it is an estimate from an average cycle. It is not a method of contraception. For a more precise calculation of fertile days, use the fertile days calculator in Junai Lab.
Why does the calculator only accept cycles of 21 to 35 days?
That range covers typical cycles in adult women. If your cycle is often shorter or longer, talk to a doctor.
Does the calculator replace a medical check-up?
No. It is an informational tool. If your periods are irregular, or you have severe pain or other problems, talk to a doctor.
Sources
- NHS – Periods and the menstrual cycle
- Reed BG, Carr BR – The Normal Menstrual Cycle and the Control of Ovulation (Endotext)
- Bull JR et al. – Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019)
Updated: October 2026
For information only, not a substitute for medical advice.