Ovulation Calculator Formula

Understand the math behind the ovulation calculator. Each variable explained with a worked example.

Use the Ovulation Calculator

Formulas Used

Estimated Ovulation Day

ovulation = ovulation_day

Fertile Window Start

fertile_start = ovulation_day - 2

Fertile Window End

fertile_end = ovulation_day + 1

Fertile Window Length

fertile_days = 4

Variables

VariableDescriptionDefault
cycle_lengthCycle Length(days)28
ovulation_dayDerived value= cycle_length - 14calculated

How It Works

Ovulation Estimation

Ovulation Day = Cycle Length - 14

The luteal phase (post-ovulation) is typically 14 days. The fertile window spans from 2 days before ovulation to 1 day after.

Fertile Window

Sperm can survive up to 5 days, but the most fertile period is the 2-3 days before and the day of ovulation.

Worked Example

A woman with a 28-day cycle.

cycle_length = 28
  1. 01Ovulation day = 28 - 14 = day 14
  2. 02Fertile window start = 14 - 2 = day 12
  3. 03Fertile window end = 14 + 1 = day 15
  4. 04Fertile window = days 12-15 (4 days)

Ready to run the numbers?

Open Ovulation Calculator