Fertile window and ovulation calculator
Estimate your fertile window and ovulation day from the last period and cycle length.
Estimate based on a regular cycle. Not a reliable contraceptive method.
How the calculation works
The estimate rests on a physiological fact: ovulation happens about 14 days before the next period — not 14 days after the last one. That's why the key input is your average cycle length.
From the last period (LMP) and the cycle length:
- Next period = LMP + cycle length
- Ovulation = next period − 14 days
- Fertile window = 3 days before to 1 day after ovulation
Why the window lasts ~5 days
Sperm can survive up to ~5 days in the body, and the egg about 24 hours. So the chance of pregnancy exists a few days before ovulation and up to just after it — hence the "fertile window".
Step-by-step example (28-day cycle)
LMP on the 1st → next period on the 29th → ovulation on the 15th (29 − 14) → fertile window from the 12th to the 16th.
Note: this method assumes regular cycles and is for estimation, not contraception. Cycles vary month to month.
Examples
- 30-day cycle, LMP on Mar 5: ovulation ≈ Mar 21; fertile window Mar 18–22.
- 26-day cycle: ovulation around day 12 of the cycle (26 − 14).
Frequently asked questions
Is it a contraceptive method?
Not reliably. Cycles vary and the calendar method often fails — it should not be the only form of birth control.
What if my cycle is irregular?
The estimate loses accuracy. In that case, ovulation (LH) tests and basal temperature tracking are more reliable than the calendar.
Best day to conceive?
The two days before ovulation and ovulation day itself hold the highest chance. Still, the whole window is fertile.
Can I use this to avoid pregnancy?
It is not advisable. Calendar-based methods are among the least reliable, with a high typical-use failure rate.
Does ovulation always happen on day 14?
No. The luteal phase is what stays stable (12–14 days), so ovulation happens about 14 days before the next period.
Why is the fertile window six days?
Sperm can survive up to about 5 days, while the egg is viable for 12 to 24 hours after ovulation.
How do I confirm ovulation?
Basal temperature rises 0.3–0.5 °C after ovulation, confirming it. To predict, cervical mucus and LH tests work better.
When should I see a specialist?
After 12 months of trying, or 6 months if over 35. Also for very irregular cycles or severe pelvic pain.
Sources and references
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Updated on June 17, 2026 · by Rafael Rossi · Methodology & sources