Ovulation Calculator Formula

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

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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)

Frequently Asked Questions

When do I ovulate?

Ovulation typically occurs 14 days before your next period. For a 28-day cycle, this is around day 14. For a 30-day cycle, it is around day 16.

How accurate is this calculator?

This provides an estimate. Actual ovulation can vary by a few days. For more accuracy, use ovulation test kits or track basal body temperature.

Learn More

Guide

Pregnancy Due Date Guide: How Your Due Date Is Calculated

Learn how pregnancy due dates are calculated using Naegele's rule, ultrasound dating, and conception date methods. Understand what your estimated due date really means.

Ready to run the numbers?

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