Fertile window calculator
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.
Cycle apps got the day of ovulation right at most 21% of the time. So we show the chance for each day rather than a single window.
Source: Johnson, Marriott & Zinaman, 2018.
How the calculator gets to each day
The calculator starts from two things you enter: the first day of your last period and how long your cycle is. From them it estimates ovulation and gives each of the six fertile days the chance of pregnancy measured in a study.
- 1
The next period: the first day of the last one, plus your cycle length.
- 2
Ovulation: 14 days before the next period.
- 3
The fertile days: the six ending on ovulation, each with its own chance of pregnancy, measured for sex on that day.
Worked out from what you entered. The days ahead are still a prediction.
| Day | Chance |
|---|---|
| 5 days before | 10% |
| 4 days before | 16% |
| 3 days before | 14% |
| 2 days before | 27% |
| 1 day before | 31% |
| Ovulation day | 33% |
Questions about the fertile window
When is the fertile window in a 28-day cycle?
In a 28-day cycle, the calculator estimates ovulation on day 15, counting the first day of your period as day 1. The fertile window runs from day 10 to day 15, and the chance is highest on the last three.
What if my cycle is shorter or longer?
Ovulation moves with your cycle length, because it is counted back from the next period: in a 25-day cycle the estimate is day 12; in a 35-day cycle, day 22.
How many fertile days are there in each cycle?
Six: the five days before ovulation and the day itself. Outside them, the chance of pregnancy is close to zero (Wilcox, Weinberg & Baird, 1995).
Why does the calculator show a chance for each day instead of a window?
Because cycle apps got the day of ovulation right at most 21% of the time (Johnson, Marriott & Zinaman, 2018). A chance for each day shows what is known, and how much is still a prediction.
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.
General information. Not a substitute for your doctor. Updated September 27, 2026.