πΈ Ovulation Calculator
Your fertile window and ovulation date from cycle length.
Estimated Ovulation Day
Estimates only, not medical advice. Do not use this calculator for contraception. Talk to a clinician about fertility planning.
How to Use This Calculator
Enter the first day of your most recent period and your average cycle length. The calculator estimates your ovulation day, your six-day fertile window, and your next expected period.
Enter the first day of your last period (LMP). Spotting before full flow starts does not count.
Enter your average cycle length in days, counted from one period's first day to the next. The default is 28; most adults fall between 24 and 35.
Read your ovulation estimate and fertile window. The two days before ovulation are the most fertile.
Note your next expected period date. If your period is more than a week late, take a pregnancy test.
How the Calculation Works
The method counts backward because the luteal phase is stable. If your cycle is 28 days and your last period started January 15, your next period is expected February 12, and ovulation falls 14 days earlier on January 29. The fertile window runs January 24 through January 29. For a 32-day cycle starting the same day: next period February 16, ovulation February 2, fertile window January 28 through February 2. Shorter cycles move ovulation earlier, longer cycles move it later, always anchored 14 days before the next expected period.
Worked Example
Last period: January 15, 2026. Average cycle: 28 days. Next expected period: January 15 + 28 days = February 12. Ovulation: February 12 - 14 days = January 29. Fertile window: January 24 through January 29 (six days). Peak fertility: January 27 and 28, the two days before ovulation. If this person instead had a 35-day cycle, ovulation would land on February 5 and the fertile window would run January 31 through February 5.
Understanding Your Cycle Phases
A cycle has two main phases. The follicular phase runs from your period to ovulation: follicles mature, estrogen rises, and cervical mucus becomes clear and stretchy near ovulation. This phase varies in length and is what makes cycles longer or shorter. Ovulation itself lasts about a day: the egg releases and survives 12 to 24 hours. Then the luteal phase runs a steady 12 to 14 days while progesterone rises; if no pregnancy occurs, hormone levels fall and your period starts. Signs of approaching ovulation include egg-white cervical mucus, a slight basal body temperature rise the morning after, mild one-sided pelvic twinges (mittelschmerz), and increased libido. Tracking any of these alongside the calendar sharpens the estimate considerably. These are estimates, not medical advice, and this tool must not be used for contraception.
Frequently Asked Questions
How accurate are ovulation calculators?
They are good estimates for regular cycles and rough guides for irregular ones. The math assumes ovulation happens 14 days before your next period, which holds for most regular cycles but shifts with stress, illness, travel, and perimenopause. Studies of calendar methods show the fertile window prediction is right about 60 to 70 percent of the time for regular cycles. For the best accuracy, combine the calendar estimate with ovulation predictor kits or basal body temperature tracking.
What is the fertile window?
The six days ending on ovulation day: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in fertile cervical mucus, while the egg survives only 12 to 24 hours after release. That is why the days before ovulation matter more than the days after. Intercourse on any of these six days can lead to pregnancy, with the two days before ovulation being the most fertile.
Can you get pregnant outside the fertile window?
It is very unlikely but not impossible. The main reason is cycle variability: if you ovulate earlier or later than the calendar predicts, intercourse you thought was "safe" can fall inside the real fertile window. Sperm survival also varies. Calendar methods alone are not reliable contraception: typical-use failure rates run around 15 to 25 percent per year. Do not use this calculator to avoid pregnancy.
What if my cycle is irregular?
Enter your average cycle length over the last 6 months for the best estimate, and treat the fertile window as wider than shown. With irregular cycles, ovulation timing is genuinely unpredictable, so the calendar method loses accuracy fast. Tracking cervical mucus, using ovulation predictor kits, or talking to a clinician will give you far better information than dates alone.
Does ovulation always happen 14 days before the next period?
The luteal phase (ovulation to next period) is the most stable part of the cycle, usually 12 to 14 days and rarely varying by more than a day or two for a given person. The follicular phase (period to ovulation) is the variable part: it stretches or shrinks, which is what makes cycle lengths differ. That stability is exactly why the calculator counts backward 14 days from your expected next period rather than forward from your last one.