What is a normal cycle length?

For adults, a normal menstrual cycle ranges from 21 to 35 days, measured from the first day of one period to the first day of the next. Cycles can vary month to month. Teenagers often have longer, more variable cycles in the first few years after starting menstruation.

How accurate is a period calculator?

It is most accurate for people with regular cycles, since it assumes your cycle length stays consistent. For those with irregular cycles, predictions are less reliable. Tracking several cycles improves accuracy. The estimates are a helpful guide, not a guarantee.

Can I use this to avoid pregnancy?

Calendar-based predictions are not reliable enough to use alone as contraception, especially with irregular cycles, because ovulation timing can shift. If you wish to avoid pregnancy, speak to a healthcare provider about effective contraceptive methods.

When should I see a doctor about my cycle?

Consider seeing a doctor if your periods are consistently irregular, unusually heavy or painful, absent, or if you notice a sudden change in your pattern. These can sometimes indicate conditions like PCOS or thyroid problems that benefit from assessment.

Women's HealthGlobal

Period & Menstrual Cycle Calculator

Predict your next period, fertile window and ovulation date based on your last period and cycle length. Track your menstrual cycle with confidence.

Written byAikins Apreku

Reviewed byDeborah Adotso, General Nurse

Updated June 2026

First day of your last period

Average cycle length (days)

From the first day of one period to the first day of the next (usually 21–35)

Understanding your menstrual cycle

The menstrual cycle is the monthly series of changes your body goes through to prepare for a possible pregnancy. It is counted from the first day of one period to the first day of the next. While the textbook cycle is 28 days, anywhere from 21 to 35 days is considered normal for adults, and cycles can vary from month to month. This calculator uses the first day of your last period and your typical cycle length to estimate when your next period, fertile window and ovulation are likely to occur.

The phases of your cycle

Your cycle has several phases. It begins with menstruation, the period itself. This is followed by the follicular phase, during which an egg matures. Around the middle of the cycle, ovulation occurs, when a mature egg is released. The luteal phase then follows, lasting about 14 days, ending either in pregnancy or the start of your next period. Ovulation typically happens about 14 days before your next period — which is why cycle length affects the timing of your fertile days.

Your fertile window

The fertile window is the span of days when conception is most likely: the day of ovulation and the five days before it, because sperm can survive several days in the body. This calculator estimates your fertile window so you can plan whether you are trying to conceive or wishing to avoid pregnancy. Keep in mind that calendar-based predictions are estimates and least reliable for those with irregular cycles. They should not be relied on alone as contraception.

When cycles are irregular

Occasional variation in cycle length is normal, influenced by stress, illness, travel, weight changes, exercise and approaching menopause. However, consistently irregular, very heavy, very painful or absent periods can sometimes signal an underlying condition such as polycystic ovary syndrome or a thyroid issue, and are worth discussing with a doctor. Tracking your cycles over several months, which this tool supports, gives useful information to share at any medical appointment.

Frequently Asked Questions

Add your average cycle length to the first day of your last period. For a 28-day cycle, your next period is expected about 28 days after the last one began. This calculator does the maths and also estimates your fertile window and ovulation date.

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Sources & References

LifeCheckIQ content is based on peer-reviewed research, recognised clinical guidelines, and authoritative medical sources, and is reviewed by Deborah Adotso, General Nurse, for clinical accuracy.

Medical Disclaimer: This tool is for informational and educational purposes only. Results are not a medical diagnosis. Always consult a qualified healthcare professional for medical advice, diagnosis, or treatment. In an emergency, call your local emergency number or visit the nearest hospital immediately.