Skip to content

CALCULATOR · Health

Ovulation Calculator

Estimate ovulation day and your fertile window from your cycle length — limitations stated honestly.

Show formula
ovulation ≈ LMP + (cycle − 14) · fertile window −5…0 days

Method reviewed:Fixed luteal-phase model: ovulation ≈ cycle length − 14 days · reviewed August 2026

Inputs

Calendar

Every date field, picker and result uses this calendar.

—
—

Result

Your result appears here

Enter First day of last period and the result fills in as you type.

How it works

  1. Enter the first day of your last period.
  2. Set your average cycle and period lengths.
  3. Read the estimated ovulation day and fertile window on the cycle map.

Frequently asked questions

How is the fertile window defined?

Sperm can survive up to about five days and the egg about 12–24 hours, giving an approximately six-day window ending on ovulation day.

Can I use this to avoid pregnancy?

No. Cycle-based prediction is not reliable contraception. Days outside the window are labelled "lower fertility", never "safe".

Examples

Period started on 1 March 2026, 28-day cycle
Ovulation around 15 March. The fertile window runs from 10 to 15 March, and the next period is due on 29 March.
The same start date with a 32-day cycle
Ovulation around 19 March — cycle length minus 14 days — with the fertile window from 14 to 19 March and the next period due on 2 April.

Related calculators