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Ovulation Calculator

Estimate your ovulation day and fertile window from your last period start date and average cycle length.

Results update instantly.Change any value and the estimate recalculates on the spot.

Health

Ovulation

Estimate your ovulation day and fertile window from your last period and cycle length.

days

Estimated ovulation day

-

  • Fertile window-
  • Next period (estimate)-

Estimate only, not medical advice. Ovulation timing varies; talk to your doctor for personal guidance.

How to use this calculator

1. Enter the first day of your last period. 2. Enter your average cycle length in days. 3. Read your estimated ovulation day, fertile window, and next period date.

Worked example

With a last period starting January 1, 2026 and a 28-day cycle, ovulation falls around January 15, 2026, the fertile window runs January 10 through January 15, and the next period is estimated around January 29.

How the math works

Ovulation usually happens about 14 days before the next period, so ovulation day = last period start + (cycle length - 14) days. The fertile window covers the 5 days before ovulation plus ovulation day itself, since sperm can survive up to 5 days.

When does ovulation happen?
Ovulation usually occurs about 14 days before your next period starts. In a 28-day cycle that lands around day 14, counting from the first day of your last period.
What is the fertile window?
The 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse in this window can lead to pregnancy.
How accurate is this calculator?
It is an estimate based on averages. Stress, illness, and natural variation can shift ovulation by several days, so treat the dates as a guide, not a guarantee.
What if my cycles are irregular?
The estimate is less reliable with irregular cycles. Tracking ovulation signs like basal body temperature or LH test strips gives a more personal picture.
Can I get pregnant outside the fertile window?
Pregnancy outside the fertile window is unlikely but not impossible, because ovulation can occasionally happen earlier or later than expected.

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How it works

How the Ovulation Calculator works

Quick answer: Ovulation usually happens about 14 days before the next period. With a 28-day cycle starting January 1, 2026, estimated ovulation is Jan 15, 2026 and the fertile window is Jan 10 - Jan 15, 2026.

Formula

Ovulation ≈ period start + (cycle length − 14 days)
Fertile window ≈ the 5 days before ovulation plus ovulation day

The next period is estimated at period start + cycle length.

Worked example

Period started January 1, 2026, 28-day cycle: ovulation Jan 15, 2026, fertile window Jan 10 - Jan 15, 2026, next period Jan 29, 2026.

Assumptions and limits

  • Cycles vary month to month; ovulation tests or temperature tracking are more precise.
  • Not a method of contraception.

Frequently asked questions

When does ovulation happen?

Ovulation usually occurs about 14 days before your next period starts. In a 28-day cycle that lands around day 14, counting from the first day of your last period.

What is the fertile window?

The 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse in this window can lead to pregnancy.

How accurate is this calculator?

It is an estimate based on averages. Stress, illness, and natural variation can shift ovulation by several days, so treat the dates as a guide, not a guarantee.

What if my cycles are irregular?

The estimate is less reliable with irregular cycles. Tracking ovulation signs like basal body temperature or LH test strips gives a more personal picture.

Can I get pregnant outside the fertile window?

Pregnancy outside the fertile window is unlikely but not impossible, because ovulation can occasionally happen earlier or later than expected.

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