About

This tool isolates the mathematics of the textbook 28-day menstrual cycle. While biological variance is the norm, the 28-day model remains the standard for medical education and initial fertility tracking. In this specific rhythm, the Luteal Phase is strictly fixed at 14 days, placing ovulation exactly on Day 14. This calculator removes the friction of "Cycle Length" inputs, assuming the standard clinical mean to provide immediate phase breakdowns. It is particularly useful for women with highly regular clocks or students visualizing the standard hormonal timeline.

Formulas

The standard rhythm method relies on fixed constants relative to the First Day of the Last Menstrual Period (LMP). The mathematical model defines the dates as follows:

{

Date_ovulation = LMP + 13 daysWindow_start = LMP + 8 daysWindow_end = LMP + 14 days

Note: While sperm can survive up to 5 days, the egg survives only 12-24 hours. The fertile window closes rapidly after Day 14.

Reference Data

Cycle Phase	Day Range (Relative to Day 1)	Hormonal Activity	Fertility Status
Menstrual Phase	Day 1 - 5	Progesterone ↓, Estrogen ↓	Low
Follicular Phase	Day 1 - 13	FSH ↑, Estrogen ↑↑	Rising (after menses)
Ovulation	Day 14	LH Surge (Peak)	Peak
Luteal Phase	Day 15 - 28	Progesterone ↑↑	Low (Post-Egg Life)

Frequently Asked Questions

This tool is designed specifically for the "clinical standard" cycle. Approximately 13% of women have a strict 28-day cycle. By removing the variable input, we reduce user error for this specific demographic and provide a cleaner educational model.

A deviation of even 1 day shifts the ovulation window. If your cycle is not exactly 28 days, this calculator will be inaccurate by the difference in days. Use our Irregular Cycle Calculator for variable lengths.

In a perfect 28-day model, yes. In reality, the Luteal Phase (post-ovulation) is fairly constant (12-14 days) across women, whereas the Follicular Phase (pre-ovulation) causes most cycle length variations.