Ovulation calculator — how to find your fertile window accurately

📅 April 2026⏱ 5 min read🏷 Women's Health

This article is for informational purposes only and is not medical advice. Consult a healthcare provider for fertility questions.

Ovulation is when one of your ovaries releases an egg. It happens once per cycle, and the egg is viable for only 12–24 hours. However, because sperm can survive inside the body for up to 5 days, your fertile window is actually 5–6 days long — the 5 days before ovulation and the day of ovulation itself.

How to calculate your ovulation date

For a textbook 28-day cycle, ovulation occurs on day 14 — counting from the first day of your last period. The formula:

Ovulation date = First day of last period + (Cycle length − 14)

For a 30-day cycle: ovulation is on day 16. For a 26-day cycle: day 12. The "minus 14" is because the luteal phase (post-ovulation) is consistently 14 days in most women, regardless of cycle length. Only the follicular phase (pre-ovulation) varies.

Your fertile window

The fertile window spans 5 days before ovulation through 1 day after. For a 28-day cycle this is days 10–15. The highest probability days are the 2–3 days immediately before ovulation and ovulation day itself.

💡 Best days to conceive

Ovulation day and the two days before have the highest conception probability (20–30% per cycle for healthy couples under 35). Days 3–5 before ovulation have moderate probability (10–15%). Day after ovulation drops to near zero as the egg is no longer viable.

Physical signs of ovulation

Cervical mucus changes: Becomes clear, slippery and stretchy — like raw egg white. This is the most reliable physical sign.
Basal body temperature rise: BBT rises 0.2–0.5°C after ovulation and stays elevated until your next period. Tracking BBT over multiple cycles reveals your pattern.
Mild one-sided cramping (Mittelschmerz): About 20% of women feel a twinge on one side when the egg is released.
Increased sex drive: A hormonal effect that peaks at ovulation.

🌸 Free Ovulation Calculator

Enter your last period date and cycle length. See your ovulation date, fertile window and best days to conceive.

Calculate Now →

When calculations are less accurate

Calendar-based ovulation calculation assumes a regular cycle. It's less reliable if:

Your cycle length varies by more than 3 days month to month
You've recently stopped hormonal contraception (cycles may be irregular for 3–6 months)
You have PCOS (polycystic ovary syndrome), which affects ovulation timing
You're under significant stress, which can delay ovulation

In these cases, tracking physical signs (especially cervical mucus and BBT) alongside calculator predictions gives more reliable results.