Ovulation Window Predictor

Predicted ovulation date and fertile window from LMP and cycle length. Not medical advice.

Inputs

Result

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General calculation reads

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How Not to Be Wrong
Jordan Ellenberg - math thinking
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The Joy of X
Steven Strogatz - accessible math
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Scientific calculator (Casio fx-991EX)
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How to use this calculator

  • Enter first day of last period.
  • Enter typical cycle length.
  • Read ovulation date and fertile window.

About this calculator

Ovulation is roughly 14 days BEFORE the start of the next period (regardless of cycle length). The fertile window — the range during which conception is possible — extends 5 days before ovulation (sperm survives ~5 days) through ~1 day after (egg survives ~24h). This is a formula calculator; not medical advice.

Frequently asked

Why 14 days before next period?+
The luteal phase (post-ovulation) is more consistent at 14 days; the follicular phase varies. So ovulation is typically 14 days before the next period regardless of total cycle length.
How accurate is this prediction?+
Useful for irregular cycles only as a rough guide. Most accurate way to confirm ovulation: ovulation predictor kits (OPKs), basal body temperature, or ultrasound monitoring.
Why is the fertile window 6 days?+
Sperm survives 3-5 days in the female reproductive tract; egg survives ~24 hours after ovulation. Conception is possible across this 5-day-pre to 1-day-post-ovulation window.
Is this medical advice?+
No. For fertility planning, especially after trying without success, consult a reproductive endocrinologist.
What if my cycles are irregular?+
This formula assumes a consistent cycle length. For irregular cycles, OPKs or fertility-tracking apps that use multiple signals (BBT, cervical fluid) are more reliable.

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