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Free online ovulation calculator

Ovulation is when an egg is released from the ovary, typically 14 days before the next period. This calculator shows your ovulation date, the 6-day fertile window, and days until.

About this calculator

How to use

  1. Enter the first day of your most recent period.
  2. Set your average cycle length (default 28 days).
  3. Adjust the luteal phase if you know it (default 14 days).
  4. The calculator shows your ovulation date, the fertile window, and next expected period.

The formula

follicularPhase  = cycleLength − lutealPhase
ovulationDay     = lastPeriodDate + follicularPhase
fertileStart     = ovulationDay − 5 days
fertileEnd       = ovulationDay + 1 day
nextPeriod       = lastPeriodDate + cycleLength

Source: Wilcox AJ et al. “Timing of sexual intercourse in relation to ovulation.” NEJM 1995;333:1517–1521.

Worked example

Last period: April 5, 2026. Cycle: 28 days. Luteal phase: 14 days.

  • Follicular phase = 28 − 14 = 14 days
  • Ovulation = April 5 + 14 = April 19, 2026
  • Fertile window = April 14 – April 20
  • Next period = April 5 + 28 = May 3, 2026

Notes

  • The luteal phase is the most stable part of the cycle, typically 12–16 days.
  • If your cycles are irregular, this calculator provides an average estimate.
  • This calculator is for family planning awareness, not medical advice.

References

Frequently asked

How is the ovulation date calculated?
Ovulation day = last period date + (cycle length − luteal phase). With a 28-day cycle and 14-day luteal phase, ovulation occurs on day 14. The luteal phase is the time from ovulation to the next period, which is fairly constant at about 14 days for most people.
What is the fertile window?
The fertile window spans 6 days — the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse during this window can result in conception. This calculator shows the window as ovulation minus 5 days through ovulation plus 1 day.
My cycle length varies — how does that affect the result?
Only the follicular phase (the time before ovulation) varies between cycles. The luteal phase stays roughly constant. If your cycles are irregular, the calculated dates are an estimate. Tracking basal body temperature or using LH predictor tests gives a more precise result.
Can I share my calculation?
Yes. The "Share with my numbers" button copies a URL that restores your exact inputs when opened.
How accurate is this calculator?
The calculation assumes a consistent cycle length and a 14-day luteal phase. It is a planning estimate, not a clinical test. Actual ovulation can vary by several days even in regular cycles.

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