Ovulation Calculator
Estimate your most fertile days and ovulation date from your cycle.
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How Ovulation Timing Works
Ovulation typically occurs about 14 days before your next period starts, because the luteal phase (the time from ovulation to the next period) is relatively constant at around 14 days. The follicular phase before ovulation is what varies between people and cycles.
Ovulation day = cycle length - 14For a 28-day cycle, ovulation falls around day 14. For a 32-day cycle, around day 18. The calculator counts from the first day of your last period.
The fertile window spans roughly the five days before ovulation plus the day of ovulation, because sperm can survive up to five days while the egg is viable for about a day.
Worked Example
With a 30-day cycle and a last period starting 1 March, ovulation is estimated around day 16 (15 March), with the fertile window spanning roughly 10–16 March.
The Menstrual Cycle Phases
| Phase | Timing (28-day cycle) | What happens |
|---|---|---|
| Menstrual | Days 1 – 5 | Period |
| Follicular | Days 1 – 13 | Egg matures |
| Ovulation | ~Day 14 | Egg released |
| Luteal | Days 15 – 28 | Womb lining thickens |
Frequently Asked Questions
How accurate is ovulation prediction?
It is an estimate based on cycle averages. Ovulation can shift from cycle to cycle. Tracking basal body temperature or using ovulation predictor kits gives more precise, real-time signals.
Why is the luteal phase fixed at 14 days?
The luteal phase is governed by the lifespan of the corpus luteum, which is fairly consistent at 12–14 days. Cycle-length variation comes mostly from the follicular phase.
What is the fertile window?
The roughly six days ending on ovulation day. Intercourse during this window gives the highest chance of conception because sperm can already be present when the egg is released.
Can I use this for contraception?
No. Cycle-based estimates are not a reliable contraceptive method on their own. Consult a healthcare provider about effective options.
Note: Estimates are for informational purposes only and are not medical advice or a contraceptive method. Consult a qualified healthcare provider.