Ovulation Calculator for a 21-Day Cycle
Estimate your ovulation date, fertile days, and next period from your last period and cycle length.
Processed entirely in your browser — nothing you enter here is ever uploaded.
Result
About this tool
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.
Example. A last period on 1 January 2024 and a 28 day cycle returns ovulation on 14 January, a fertile window from 9 January through 14 January, and the next period on 29 January.
Limit. Cycle length is accepted only from 21 to 35 days, and ovulation is always placed by subtracting a fixed 14 day luteal phase.