Ovulation Calculator (Fertile Window)
Enter the first day of your last period and your cycle length to see your fertile days.
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Ovulation Calculator (Fertile Window) by TheCalcTools
How the Ovulation Calculator (Fertile Window) works
Ovulation usually happens about 14 days before the next period (the luteal phase), so in a 28-day cycle it falls around day 14. The fertile window covers the 5 days before ovulation and the day of ovulation, because sperm can survive up to about 5 days.
Cycles vary from month to month, so this is an estimate. Ovulation tests, basal body temperature and cervical mucus tracking give more precise timing. This is not a contraceptive method.
The formula
Worked example
Using the default values shown in the calculator:
| First day of your last period | 2026-10-03 |
|---|---|
| Average cycle length (days) | 28 |
| Luteal phase length (days) | 14 |
- Period started 3 October 2026, 28-day cycle: ovulation around Saturday, 17 October 2026.
- Fertile window 12 October 2026 to 18 October 2026; next period 31 October 2026.
Result:
| Estimated ovulation day | Saturday, 17 October 2026 |
|---|---|
| Fertile window | 12 October 2026 to 18 October 2026 |
| Next period expected | 31 October 2026 |
| Pregnancy test from (if your period is late) | 1 November 2026 |
| Ovulation on cycle day | 15 |
Frequently asked questions
What if my cycles are irregular?
Use your shortest and longest recent cycles to bracket the window, or use ovulation test kits.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation timing varies.
Sources and further reading
- Wilcox AJ et al. (1995) Timing of sexual intercourse in relation to ovulation. NEJM 333:1517
- Wikipedia: Ovulation
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This calculator gives general estimates for information and education. It is not medical advice and does not diagnose anything. Talk to a qualified health professional about your own situation. See our disclaimer.