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Menstrual Cycle Tracker Calculator

Track your menstrual cycle, predict ovulation day, fertile window, and next period based on cycle length and variation.

Inputs

days
days
days

Results

Estimated Ovulation (cycle day)

14

Fertile Window Start (cycle day)9
Fertile Window End (cycle day)15
Fertile Window Length7 days
Next Period (est. day of month)29
Follicular Phase Length14 days
Luteal Phase Length14 days
Estimated Lower Fertility Window3 days
Cycle Regularity Score90/100
How to Use This Calculator
  1. Enter Average Cycle Length, Period Duration, and Last Period Start (day of month).
  2. Set Cycle Variation.
  3. Review the Estimated Ovulation (cycle day) result.
  4. Use Fertile Window Start (cycle day) and Fertile Window End (cycle day) to inform your decision.

How the result changes with Average Cycle Length

Average Cycle LengthEstimated Ovulation (cycle day)
239
2915
3622
4329

What each input means

Average Cycle Length
Number of days from the first day of one period to the first day of the next.
Period Duration
How many days your menstrual bleeding typically lasts.
Last Period Start (day of month)
Day of the month your last period started.
Cycle Variation
How many days your cycle typically varies from month to month.

What each result means

Estimated Ovulation (cycle day)
Predicted ovulation day within your cycle, counting from day 1 of your period.
Fertile Window Start (cycle day)
First day of your fertile window.
Fertile Window End (cycle day)
Last day of your fertile window.
Fertile Window Length
Total number of high-fertility days per cycle.
Next Period (est. day of month)
Estimated day of the month for your next period.
Follicular Phase Length
Days from period start to ovulation.
Luteal Phase Length
Days from ovulation to next period (typically ~14 days).
Estimated Lower Fertility Window
Estimated days between end of period and start of fertile window. This is not a guarantee of infertility.
Cycle Regularity Score
Score from 0-100 based on your cycle variation (higher = more regular).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Average Cycle Length = 28, Period Duration = 5, Last Period Start (day of month) = 1, Cycle Variation = 1 = 4 input(s) provided
  2. Calculate Estimated Ovulation
    Estimated Ovulation = cycleLength - lutealPhase
    14 = 14
  3. Calculate Fertile Window Start
    Fertile Window Start = max(1, ovulationDay - 5)
    9 = 9
  4. Calculate Fertile Window End
    Fertile Window End = ovulationDay + 1
    15 = 15

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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