Ovulation Calculator 🔒 Runs locally in your browser — nothing is uploaded.

Fertile window & ovulation day estimator

⚠ Please note

For informational purposes only — not medical advice or a method of contraception. Ovulation timing varies between cycles and individuals; this is a statistical estimate based on average cycle patterns, not a precise prediction. Consult a healthcare professional for fertility planning or contraception guidance.

The day your last period started.

Count from the first day of one period to the first day of the next. Most cycles range from about 21 to 35 days — use your own average if you know it, not just the 28-day textbook figure.

Your estimate

Estimated ovulation day
Fertile window
Next estimated period

Fertile window on the calendar

Fertile window Ovulation day Next period (est.)

About this tool

Ovulation — the release of an egg from the ovary — is the single most fertile moment of the menstrual cycle, and reproductive-health guidance points to a consistent pattern behind it: the luteal phase (the stretch from ovulation to the start of the next period) stays close to 14 days for most people, cycle after cycle. It's the follicular phase (from the first day of your period until ovulation) that stretches or shrinks to account for the rest of your total cycle length. That's why this calculator asks for your typical cycle length rather than assuming everyone is exactly 28 days — a 24-day cycle and a 34-day cycle both ovulate about 14 days before their next period, just on very different calendar days.

The fertile window shown here — the 5 days before ovulation plus ovulation day itself — reflects two biological realities: sperm can survive in the reproductive tract for several days waiting for an egg, while the egg itself is viable for only about 24 hours after release. Together they create a roughly 6-day stretch each cycle when conception is possible. Cycle length can vary from month to month even for the same person, stress, illness, and travel can shift ovulation earlier or later, and no calendar-based method is as reliable as clinical tools like ovulation predictor kits or basal body temperature tracking — so treat this as a starting estimate, not a guarantee.

Frequently asked questions

How accurate is this compared to ovulation predictor kits?

This calendar-based estimate is meaningfully less precise than actual physiological tracking methods. Ovulation predictor kits (which detect the LH hormone surge in urine), basal body temperature charting, or cervical mucus tracking all respond to what your body is ACTUALLY doing in real time, while this calculator only uses average cycle-length math applied retroactively — it can't detect cycle-to-cycle variation, stress-related delays, or anything else that shifts your actual ovulation day. Use this as a general planning reference, not a substitute for those direct-observation methods if precision genuinely matters for your situation.

What if my cycle length varies month to month?

This is genuinely common — many people don't have a perfectly consistent cycle length, and that's within the range of normal. If your cycles vary, try tracking a few months and entering your MOST TYPICAL or AVERAGE recent cycle length rather than any single unusual month, or run the calculation with both your shortest and longest recent cycle lengths to see the range of possible ovulation days rather than relying on one single estimate. If your cycles vary a lot (more than a week or so between your shortest and longest), that variability itself is worth mentioning to a healthcare provider, since this tool's single-cycle-length math becomes less reliable the more irregular your actual pattern is.