Period Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

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The useful part of a fertility or pregnancy calculator is not just the answer; it is knowing the assumption behind the answer. Period Calculator keeps that assumption visible.

What this calculator does

Period Calculator uses Cycle duration, Period duration, and Last period start date to calculate period. It is built around the exact fields displayed in this plugin rather than a generic version of the topic. Where supported, it also shows Estimated period end and Following period so the primary result is easier to place in context. Pregnancy and fertility dates, hormone trends, and risk scores are estimates built around biologic averages. Cycle variation, ultrasound findings, assisted-reproduction details, and the individual pregnancy can matter more than a single calculated number.

How to use it

Enter or select Cycle duration, Period duration, and Last period start date. Use the units offered by the calculator and choose the option that matches the actual clinical or timing situation. Check date entries and laboratory units before relying on the output. For paired dates or serial measurements, keep them within the same menstrual cycle, pregnancy, or treatment episode as the calculator requires.

How the calculation works

The next period is estimated by adding the entered cycle length to the first day of the last period. Period end is calculated from the entered period duration, and one more cycle length is added to show the following expected period.

Example

For example, with a 28-day cycle starting June 1, the next period is estimated around June 29. If period duration is five days, the estimated end date is four days after that start date.

How to interpret the result

Treat the output as estimated next period based on the assumptions encoded in this calculator. It can help with planning or understanding a measurement, but pregnancy and reproductive biology vary enough that the calculated date, ratio, or probability should be seen as a range or estimate rather than a guarantee.

Limitations and notes

Cycle length can vary from month to month, so a calendar prediction becomes less reliable with irregular cycles, adolescence, postpartum changes, perimenopause, endocrine disorders, stress, or illness. It does not diagnose pregnancy or explain an unexpectedly late period.