Ovulation calculator
Ovulation usually happens about fourteen days before the next period starts, so the estimate works from the cycle length rather than its midpoint. The fertile window covers the days before ovulation, because sperm remain viable for several days.
- Fertile window
- 13/09/2026 – 19/09/2026
- Next period
- 02/10/2026
Rates and thresholds
- Estimated ovulation
- cycle length − 14 days
- Fertile window
- 5 days before ovulation to 1 day after
- Luteal phase
- About 14 days, fairly constant
- Cycle length
- Typically 21–35 days
- What it is not for
- A method of contraception
Why is ovulation counted back from the end of the cycle?
Because the second half of the cycle is far steadier than the first. Ovulation to period is usually about fourteen days whatever the total length; it is the part before ovulation that varies. Subtracting fourteen from the cycle length therefore estimates better than halving it — on a thirty-day cycle the two answers differ by two days.
Can this be relied on to avoid pregnancy?
No. This is a calendar estimate that assumes a regular cycle, and real ovulation shifts with illness, stress, travel and sleep — even for someone whose cycle is usually regular. The fertile window here is an estimate, not a boundary. For choosing a method of contraception the right place is a doctor or midwife, who can account for your health and circumstances.
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