Taco Bout Fertility Tuesday · Patient Tool

Fertile Window Calculator

Estimate your fertile window from your last period, your cycle length and, optionally, your ovulation test.

Last reviewed: November 2026 · v1.0

Read this first. This is a calendar estimate, not a measurement. Ovulation varies from cycle to cycle even in regular cycles. It is not birth control and cannot tell you whether you are or are not fertile on a given day.
1 Your cycle
days
days

The luteal phase is the time from ovulation to your next period. It is usually 12 to 14 days and stays fairly steady for the same person. If you don’t know yours, leave 14.

2 Optional: first positive ovulation (LH) test

If you enter this, the estimate uses it instead of the calendar, because ovulation usually follows the LH surge by about 24 to 36 hours.

How the math works and what the research shows

Ovulation ≈ next period − luteal phase  ·  Fertile window = 5 days before ovulation + ovulation day

In a study of 221 healthy women trying to conceive, nearly all pregnancies came from intercourse in the six days ending on ovulation day. The chance of conception was about 10% for intercourse five days before ovulation and about 33% on ovulation day itself (Wilcox, Weinberg & Baird, NEJM 1995).

The window ends on ovulation day, so waiting for a positive LH test can miss most of it. A calendar estimate is only a guess: even women with regular cycles vary in when they ovulate (Wilcox et al., BMJ 2000).

Sources and change log
  • Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-21.
  • Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ 2000;321:1259-62.
  • Luteal-phase length and the LH-surge-to-ovulation interval are summarized from patient-education sources.

Change log. v1.0, November 2026: split into its own tool.

Educational tool from Taco Bout Fertility. It is not medical, legal or financial advice and does not create a doctor–patient relationship. Nothing you type leaves your device.

Talk with your own fertility doctor about your dates, your protocol and your costs.