Ovulation Window Predictor

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

Inputs

Result

Loading calculator…
—

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.

Related calculators

Gestation Age Calculator
Compute gestational age (weeks + days) from last menstrual period. Not medical advice.
Pregnancy Week & Trimester Calculator
Pregnancy week, trimester, and weeks-until-due from LMP. Not medical advice.
Menstrual Cycle Predictor
Predict the next 6 expected period start dates from a recent period and cycle length. Not medical advice.
Due Date from Conception Calculator
Estimated due date = conception + 266 days. Not medical advice.
Conception Date from Due Date
Estimated conception window = due date − 266 days, ± a few days for fertile window. Not medical advice.
Days Between Dates Calculator
Count the exact number of days between any two dates — all calendar days, weekdays only, or business days (skipping US federal holidays). Pick whether to include both endpoints or just the difference.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Menstrual Cycle Predictor
Calc

Predict the next 6 expected period start dates from a recent period and cycle length. Not medical advice.

Pregnancy Week & Trimester Calculator
Calc

Pregnancy week, trimester, and weeks-until-due from LMP. Not medical advice.

Due Date from Conception Calculator
Calc

Estimated due date = conception + 266 days. Not medical advice.

Conception Date from Due Date
Calc

Estimated conception window = due date − 266 days, ± a few days for fertile window. Not medical advice.

Gestation Age Calculator
Calc

Compute gestational age (weeks + days) from last menstrual period. Not medical advice.

Pregnancy Due Date Calculator
Health

Estimated due date from last menstrual period (Naegele's rule), first-trimester ultrasound (gestational age + remaining weeks), conception date, or IVF transfer. Shows trimester + ACOG term-week milestones.