Period Calculator

Enter the first day of your last period and your typical cycle length to estimate the next period start.

Health information notice: Results are estimates based on general formulas and may not reflect individual circumstances. This tool is not a diagnosis or substitute for personalized advice from a qualified healthcare professional.

How the period calculator works

Estimated next period = first day of the last period + usual cycle length. Estimated end date adds the typical period duration minus one day.

Formula and assumptions

Estimated next period = first day of the last period + usual cycle length. Estimated end date adds the typical period duration minus one day.

Example

Try the sample values, then change one input at a time to see how the result changes. Check that units match the labels.

Important limitations

This calendar estimate cannot diagnose pregnancy or a menstrual condition. If periods are unexpectedly late, unusually heavy, painful, or irregular, consider seeking medical advice.

Frequently asked questions

What does this calculator do?

What does this calculator estimate? Enter the first day of your last period and your typical cycle length to estimate the next period start.

What should I know about the result?

This calendar estimate cannot diagnose pregnancy or a menstrual condition. If periods are unexpectedly late, unusually heavy, painful, or irregular, consider seeking medical advice.

How does a period calculator estimate the next period?

It uses the start date of the last period and the cycle length entered to estimate a future start date.

Can this predict ovulation or prevent pregnancy?

Calendar estimates are not reliable contraception and cannot confirm ovulation. Consult a qualified healthcare professional for personal concerns.

Results are estimates for general information only. They are not a substitute for professional advice where applicable. Found a mistake? Report an error.