Fertile window on a 21-day cycle
On a 21-day cycle, ovulation is estimated on cycle day 8, and the fertile days run from day 3 to day 8. Enter the first day of your last period to see the dates.
Full moon· turn the moon to see another day
My period started on and my cycles usually last .
Each day shows the moon of that night.
If you are not sure, leave it at 28 — the estimate already allows for that.
The days of a 21-day cycle
Counting the first day of your period as day 1. Ovulation is estimated 14 days before the next period, and the fertile days are the six that end on it.
- 1
Estimated ovulation: day 8.
- 2
Fertile days: day 3 to day 8.
- 3
Highest chance: days 7 and 8, at 31% and 33% from sex on that day.
- 4
Compared with a 28-day cycle, ovulation comes 7 days earlier.
On a cycle this short, the first fertile days can fall while the period is still going.
| Cycle day | Chance |
|---|---|
| Day 3 | 10% |
| Day 4 | 16% |
| Day 5 | 14% |
| Day 6 | 27% |
| Day 7 | 31% |
| Day 8, ovulation | 33% |
What 612,613 real cycles show
A study followed 612,613 cycles with a detected ovulation, logged by 124,648 users of a fertility app.
Cycles of 21 to 24 days were 8% of the total. In them, ovulation came on average 12.4 days after the period started, and the next period 11 days after ovulation.
Only 13% of cycles were exactly 28 days long, and 91% were between 21 and 35 days.
Source: Bull et al., 2019.
Questions about the 21-day cycle
When do I ovulate on a 21-day cycle?
The estimate is cycle day 8, counting the first day of your period as day 1: 14 days before the next period. In real cycles of 21 to 24 days, ovulation came on average 12.4 days after the period started (Bull et al., 2019).
What are the fertile days on a 21-day cycle?
Day 3 to day 8: the five days before ovulation and the day itself. The chance from sex is highest on days 7 and 8 (Wilcox, Weinberg and Baird, 1995).
Does a 21-day cycle change the due date?
Yes. Naegele's rule adds 280 days to the first day of the last period and assumes a 28-day cycle (ACOG, 2017). On a 21-day cycle ovulation comes earlier, and the due date calculator moves the date with it: 7 days earlier.
Are 21-day cycles common?
In 612,613 real cycles, those of 21 to 24 days were 8% of the total, and 91% were between 21 and 35 days. Only 13% were exactly 28 days long (Bull et al., 2019).
Can I trust this estimate?
It is an average. A calendar gets the ovulation day right at most 21% of the time (Johnson, Marriott and Zinaman, 2018), and the phases of the cycle vary from person to person. With your own dates, the calculator above shows the chance of each day.
Can the calculator be used to avoid pregnancy?
No. It is for information only and is not a method of contraception.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995;333(23):1517–21.
- Johnson S, Marriott L, Zinaman M. Can apps and calendar methods predict ovulation with accuracy? Current Medical Research and Opinion, 2018;34(9):1587–94.
- Bull JR, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019;2:83.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017;129(5):e150–4.
General information. Not a substitute for your doctor. Updated September 27, 2026.