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Ovulation Calculator for a 21-Day Cycle

Estimate your ovulation date, fertile days, and next period from your last period and cycle length.

Estimated ovulation date

Forecast for the next 3 cycles

This is an approximate calculation for general orientation, not medical advice. Cycles naturally vary — please consult your doctor for contraception or fertility guidance.

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Result

This page opens the ovulation calculator preset for a short 21-day cycle — enter the first day of your last period to see the estimate. With only 21 days from period to period, ovulation is expected around day 7, and the fertile window covers days 2 through 7 — remarkably early, sometimes overlapping the last days of menstruation.

Short cycles compress the follicular phase: the egg matures faster, while the luteal phase keeps its usual ~14 days. Practically this means the fertile days can begin almost as soon as a period ends, which surprises many people who assume mid-cycle timing. A consistently short cycle is usually a personal norm, but if your cycles have suddenly shortened, it is worth mentioning to a gynecologist.

The forecast table shows the next three cycles so early fertile windows do not catch you off guard. All calculations stay in your browser. This is an approximate calendar estimate, not medical advice — short cycles especially benefit from confirmation with ovulation tests.

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Accepted Formats & Rules

  • Ovulation estimate: cycle length minus 14 days.
  • Fertile window: 5 days before ovulation plus the ovulation day.
  • Forecast table for the next 3 cycles.

Key Benefits

✓ Shows the full picture at once: ovulation date, six fertile days, next period, and a forecast for the three following cycles.
✓ Adapts to your body: any cycle length from 21 to 35 days, using the standard 14-day luteal phase.
✓ Completely private: dates are calculated in your browser and never leave your device.

Use Cases

  • • Find your estimated ovulation date and 6-day fertile window from the last period and cycle length.
  • • See when the next period is expected and plan ahead with a forecast for the next 3 cycles.
  • • Compare short (21-day) and long (35-day) cycles to understand how the fertile days shift.