How long is a normal cycle?

Cycles of 21 to 35 days are usually considered regular.

It helps you plan, spot changes and give useful information to your doctor.

When should I see a doctor?

If your periods stop, become very heavy or painful, or are often irregular, talk to a healthcare professional.

Period Calculator

Enter the first day of your last period, your usual cycle length and how long your period lasts to see your next three expected periods.

Cycle Details

Last period start

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Cycle length (days)

Period length (days)

Predicts the next three periods and fertile windows from your last period start, average cycle length and period length.

Formula

next period = last start + cycle length (repeated for each future cycle); fertile window = ovulation (next period − 14) − 5 to +1 days.

Reference Values

•Cycle lengths 21–35 days are regular
•Average period lasts 2–7 days
•Irregular cycles widen uncertainty

Related tools

Ovulation Due Date Days Between Dates

FAQ

Common questions about Period Calculator

How we calculate this

We use the standard published formula for this calculation, and we show the working so you can check it.

Spotted a mistake? Tell us and we'll check it within a few days. See how we check our calculators.

Medical disclaimer

This tool gives a general estimate for information only. It is not medical advice and cannot diagnose a condition. Talk to a doctor, midwife or other qualified professional about your own situation.