How your next period is predicted
This calculator predicts your upcoming periods from the first day of your last period and your average cycle length. It counts forward by your cycle length to estimate each next start date. Tracking a few cycles makes the prediction more accurate.
Worked example
If your last period started on 1 January and your cycle is 28 days, your next period is predicted around 29 January, and the one after that around 26 February.
Good to know
Cycles naturally vary, and stress, travel and health can shift them. For irregular periods the prediction is a rough guide only, and it is not a substitute for medical advice if your cycle changes markedly.
Frequently asked questions
How do I predict my next period?
Count forward from the first day of your last period by your average cycle length. A 28-day cycle starting 1 January predicts the next around 29 January.
What if my periods are irregular?
The prediction assumes a regular cycle, so it is less accurate for irregular periods. Tracking several months gives a better average to work from.
What is a normal cycle length?
Anywhere from about 21 to 35 days is considered normal. Use your personal average for the best prediction.
Why is my period late?
Stress, travel, illness, weight change and hormones can all shift timing. Persistent changes are worth discussing with a doctor.
Does this track fertility?
For fertile days, use the ovulation calculator, which estimates your fertile window.