Cycle guide
How the menstrual cycle works, and how this calculator reads it.
Your menstrual cycle starts on the first day of your period and ends the day before your next one begins. In adults, a typical cycle lasts anywhere from 21 to 35 days, and a period usually lasts 2 to 7 days. Only a minority of people have a textbook 28-day cycle, and it is normal for your own cycle to vary by a few days from month to month.
The four phases of the cycle
- Menstrual phase: the uterine lining sheds. Oestrogen and progesterone are at their lowest.
- Follicular phase: follicle-stimulating hormone (FSH) helps an egg mature and oestrogen rises, rebuilding the lining. This phase overlaps with your period and varies the most in length.
- Ovulation: a surge of luteinising hormone (LH) releases the egg, which survives for about 12 to 24 hours.
- Luteal phase: progesterone rises to prepare for a possible pregnancy. If the egg isn’t fertilised, hormones fall and your next period begins.
Read our full guide to the phases of the menstrual cycle to see how energy, mood and appetite often shift in each one.
How to calculate your next period
The method is simple: add your average cycle length to the first day of your last period. If your last period started on 1 March and your cycle is usually 30 days, your next period is due around 31 March. Your period length then tells you roughly which days to expect bleeding.
The estimate becomes more reliable when your average comes from several months of data instead of a guess. If your cycles regularly fall outside the 21 to 35 day range, or change by more than 7 to 9 days between cycles, read about causes of irregular periods and consider speaking with a doctor.
How to calculate ovulation
Ovulation doesn’t happen on day 14 for everyone. What is more consistent is the luteal phase, which typically lasts 12 to 14 days. That is why calculators count backwards from your next expected period: next period date minus 14 days gives an estimated ovulation day.
Your fertile window
Sperm can survive for up to five days, so your fertile window covers the five days before ovulation plus the day of ovulation itself, about six days in total. The two to three days just before ovulation are usually the most fertile.
Confirming ovulation with body signs
A calendar estimate is a starting point. You can confirm it with body signs such as clear, stretchy cervical mucus, a small rise in basal body temperature after ovulation, or a positive LH test. Learn more about the signs of ovulation.
Calendar calculations are not a reliable form of contraception, especially if your cycles are irregular.
Why track your cycle alongside daily habits?
Dates tell you when things happen. Your daily habits help explain why a cycle runs early, late or feels harder than usual. Hormones respond to sleep, light, stress, food and movement, so tracking them together shows patterns a period date alone can’t.
Morning sunlight and your body clock
Bright light in the first hour after waking helps set your circadian rhythm. A stable body clock supports a healthy morning cortisol rise and evening melatonin release, which in turn affect sleep quality. Research links disrupted light and sleep patterns, as seen in night-shift work, with more irregular cycles. Ten to twenty minutes of daylight outdoors each morning is an easy habit to start with.
Strength training and insulin sensitivity
Building muscle improves how well your body uses insulin. Steadier blood sugar matters for hormonal health, because high insulin can increase androgen production and is closely linked with PCOS and irregular ovulation. Two to three strength sessions a week, adjusted to how you feel through your cycle, is a practical goal.
Seeing the full picture
When you log your cycle next to sleep, sunlight, movement and symptoms, you can see how a stressful week, a late-night run of screens or a training block shows up in your next period. That is the idea behind the Nitya Health app: it tracks your cycle and turns your data into AI-guided holistic routines, so you don’t have to calculate anything by hand.
This calculator gives estimates for general information only. It is not a substitute for medical advice. See a doctor if your periods stop, become very painful or heavy, or change suddenly.