How does this predict cycle phases?

Using average cycle length and last period start date to estimate upcoming dates and fertile windows.

How accurate are these predictions?

Cycle length can vary month to month, so predictions are estimates that improve with more tracked cycles.

Should I use this for family planning?

It's a helpful reference, but consult a healthcare provider for precise fertility planning.

Menstrual Cycle Calculator - CalculatorLabs.net

Menstrual Cycle Calculator

Estimate your next period, ovulation day, and fertile window.

First Day of Last Period

Average Cycle Length (days)

Estimated Next Period -

Estimated Ovulation Day -

Estimated Fertile Window -

What Is the Menstrual Cycle Calculator?

The Menstrual Cycle Calculator estimates the date of your next period, your most likely ovulation day, and your fertile window, based on the start date of your last period and your typical cycle length.

Formula Used

Next Period = Last Period Start + Cycle Length
Ovulation ≈ Next Period − 14 days

These are statistical estimates based on average cycle patterns — actual ovulation timing varies by individual and cycle. This tool is for general awareness, not a medical or contraceptive method.

How to Use This Calculator

Select the first day of your last period.
Enter your average cycle length in days (28 is typical).
Click Calculate to see your estimated next period, ovulation day, and fertile window.

Worked Example

If your last period started on the 1st of the month with a 28-day cycle, your next period is estimated around the 29th. Ovulation is estimated 14 days before that (around day 15), with a fertile window roughly from day 10 to day 16.