Ovulation Calculator

Enter the first day of your last period and your average cycle length to estimate your ovulation day and the fertile window — the days conception is most likely.

Enter your last period and cycle length, then press Find fertile window.

The fertile-window method

Ovulation typically happens about 14 days before the next period starts, because the luteal phase (after ovulation) is fairly constant. So:

Ovulation day ≈ LMP + ( cycle length − 14 )

The fertile window is roughly the five days before ovulation plus ovulation day itself — about six days — because sperm can survive several days and the egg lives about a day. Conception is most likely in the two days before ovulation.

Worked example

Last period January 1, 28-day cycle:

Ovulation: Jan 1 + (28 − 14) = day 14 → January 15.
Fertile window: about Jan 10 – Jan 15.
30-day cycle: ovulation shifts to about January 17.

Accuracy and limits

This calendar method assumes regular cycles and average luteal length, which isn't true for everyone. Stress, illness and natural variation move ovulation around. Methods like tracking basal body temperature or ovulation predictor kits (which detect the LH surge) are more precise. Use this as a starting estimate, not contraception.

Note: the calendar method is not reliable as birth control. For family-planning or fertility concerns, speak with a clinician.

Frequently asked questions

When do I ovulate?

Usually about 14 days before your next period. For a 28-day cycle starting January 1, that's around January 15 (day 14). Longer cycles push ovulation later; shorter cycles earlier.

What is the fertile window?

The roughly six days ending on ovulation day — the five days before plus ovulation itself — because sperm survive several days and the egg about one. Conception is most likely in the two days before ovulation.

How accurate is calendar ovulation tracking?

It's a rough estimate that assumes regular cycles. Stress, illness and natural variation shift ovulation. Basal body temperature charts and ovulation predictor kits (LH tests) are more precise.

Can I use this as birth control?

No. The calendar method is not reliable contraception because ovulation timing varies. For preventing or planning pregnancy, talk to a clinician about appropriate methods.

MB
Mustafa Bilgic · Editor, Calcool
The fertile-window estimate follows standard cycle-tracking guidance. For reproductive-health information see the American College of Obstetricians and Gynecologists (ACOG). This is an estimate, not contraception or medical advice.

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