Fertile window on a 28-day cycle
My last period started on
28-day cycles ·
Pick the first day of your last period to see the chance of each day.
General information. Not a substitute for your doctor.
If you are not sure, leave it at 28 — the estimate already allows for that.
On a 28-day cycle, ovulation is estimated on day 15, 14 days before the next period. The fertile days are the six that end on it, day 10 to day 15, and the chance of pregnancy is highest on days 14 and 15 (Wilcox, Weinberg and Baird, 1995).
The days of a 28-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 15.
- 2
Fertile days: day 10 to day 15.
- 3
Highest chance: days 14 and 15, at 31% and 33% from sex on that day.
- 4
It is the cycle most counts assume: ovulation on day 15, 14 days after the period started.
| Cycle day | Chance |
|---|---|
| Day 10 | 10% |
| Day 11 | 16% |
| Day 12 | 14% |
| Day 13 | 27% |
| Day 14 | 31% |
| Day 15, 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 25 to 30 days were 65% of the total. In them, ovulation came on average 15.2 days after the period started, and the next period 12.4 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 28-day cycle
When do I ovulate on a 28-day cycle?
The estimate is cycle day 15, counting the first day of your period as day 1: 14 days before the next period. In real cycles of 25 to 30 days, ovulation came on average 15.2 days after the period started (Bull et al., 2019).
What are the fertile days on a 28-day cycle?
Day 10 to day 15: the five days before ovulation and the day itself. The chance from sex is highest on days 14 and 15 (Wilcox, Weinberg and Baird, 1995).
Does a 28-day cycle change the due date?
No. Naegele's rule, which adds 280 days to the first day of the last period, already assumes a 28-day cycle (ACOG, 2017).
Are 28-day cycles common?
In 612,613 real cycles, those of 25 to 30 days were 65% 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. Cycle apps got 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.