Ovulation & Fertile Window Calculator

Privacy-first: We do not save your data! Use the comprehensive mode to add more signals (LH, BBT, cervical mucus) for a personalized estimate.

Mode:

First day of your last period.

Typical range is 21 to 45 days.

How this works

Fertile window spans the five days before ovulation and the day of ovulation. Sperm can survive up to ~5 days; an egg is typically viable for less than 24 hours.

Calendar estimate uses LMP + (average cycle length - luteal phase). The luteal phase is often assumed to be ~14 days on average, but can vary by person and cycle.

LH tests (ovulation predictor kits) anticipate ovulation ~24 to 36 hours after the first positive. BBT confirms ovulation retrospectively (we set ovulation about one day before a sustained temperature rise). Cervical mucus peak (egg-white/slippery) often occurs near ovulation and can nudge the estimate.

Due date shows two methods: LMP + 280 days, and (if conception/ovulation known) conception + 266 days.