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Ovulation & Fertile Window Calculator

Estimate fertile days and ovulation dates with high precision. Supports irregular cycles, cycle history projection for 12 months, and iCal exports.

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About

Timing determines conception probability more than any other factor. Sperm survival rates within the reproductive tract range from 3 to 5 days, while the oocyte (egg) remains viable for only 12 to 24 hours post-ovulation. This creates a narrow "fertile window" of approximately 6 days. Missing this window delays conception by at least one full cycle.

This tool utilizes the standard calendar method, adjusting for cycle length variability. While typical cycles average 28 days, variance is common. The calculation shifts the ovulation estimate (O) based on the input cycle length (L) using the formula O = L − 14. Accuracy decreases with significant irregularity; in such cases, this calculator serves as a baseline estimate to be supplemented with basal body temperature (BBT) tracking or ovulation prediction kits (OPKs).

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Formulas

The core estimation relies on the fixed duration of the luteal phase (usually 14 days) in a standard cycle. The fertile window begins 5 days prior to ovulation.

Ovulation Day (do):

do = Lcycle − 14

Fertile Window Range:

{
Start: do − 5End: do

Where Lcycle is the total days in the menstrual cycle.

Reference Data

Cycle DayPhaseHormonal EventConception Probability
1-5MenstruationEstrogen/Progesterone Drop< 0.1%
6-9FollicularFSH Rise≈ 1%
10-12Pre-FertileEstrogen Surge10-15%
13-14OvulationLH Surge30-35% (Peak)
15Post-OvulationEgg Viability Decline10%
16-28LutealProgesterone Rise≈ 0%
VariableIrregularUnpredictable SurgesRequires Monitoring
28+Delayed MensesPossible hCG ProductionTest Required

Frequently Asked Questions

The calendar method assumes a consistent luteal phase. If your cycle varies by more than 3-4 days month-to-month, the estimated window may shift. We recommend tracking the shortest and longest cycles over 6 months to define a broader potential fertile range.
Sperm viability dictates the start of the window. Sperm can survive in cervical mucus for up to 5 days, waiting for the egg to be released. Therefore, intercourse days before ovulation can result in conception.
Reliability for contraception is low (76-88% typical use). While it identifies high-risk days, stress, illness, or hormonal shifts can delay ovulation unexpectedly, making "safe" days unsafe.
Short cycles often indicate anovulation or a short luteal phase (Luteal Phase Defect), which can impact fertility. Consult a reproductive endocrinologist if cycles are consistently under 21 days.