Ovulation Calculator
Find your fertile window based on your cycle. Know when ovulation is most likely — whether you're trying to conceive or planning around it.
How ovulation timing works
Your menstrual cycle has two phases. The follicular phase (period to ovulation) varies in length — it can be 10 days or 25. The luteal phase (ovulation to next period) is remarkably consistent at around 14 days. So to estimate ovulation, we count backward from your next expected period, not forward from your last one.
Why a 6-day fertile window?
Sperm can survive in the female reproductive tract for up to 5 days under good conditions. An egg, once released, lives only about 12-24 hours. So the window of possible conception is the 5 days before ovulation plus ovulation day itself.
Within that window, the two days before ovulation are statistically the most likely to result in conception.
Signs of ovulation you can watch for
- Cervical mucus changes — it becomes clear, slippery, and stretchy (egg-white consistency) right around ovulation.
- Basal body temperature rise — about 0.5°F (0.3°C) higher after ovulation. Confirms ovulation, doesn't predict it.
- Mid-cycle twinge — some people feel a mild ache on one side ("mittelschmerz").
- Heightened libido — a hormonal nudge from biology.
- Ovulation predictor kits (OPKs) — detect the LH surge that precedes ovulation by ~24-36 hours.
What if my cycles are irregular?
Calendar-based calculators are best when your cycles are within a few days of each other month-to-month. If yours vary widely, consider tracking BBT, using OPKs, or using an app that learns your individual rhythm. For persistent irregularity, talk to your provider — it can have a treatable cause.
Frequently asked questions
When is my fertile window?
For most people, the fertile window is the 5 days before ovulation plus ovulation day itself — a 6-day stretch. Sperm can survive up to 5 days; the egg lives only ~24 hours after release.
Why 14 days before next period?
The luteal phase (ovulation to next period) is remarkably consistent across people — about 14 days regardless of overall cycle length. So we count back 14 days from the start of your next expected period.
Is this calculator accurate for irregular cycles?
Less so. If your cycles vary by more than a few days, calendar-based calculators are rough estimates. Consider basal body temperature tracking, ovulation predictor kits (OPKs), or fertility apps that learn your individual cycle.
Can I use this to avoid pregnancy?
Calendar-based "rhythm method" alone has a high failure rate (~24% per year) for typical use. If you're trying to avoid pregnancy, talk to your provider about more reliable options.
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