Fertility Window Calculator Formula
Understand the math behind the fertility window calculator. Each variable explained with a worked example.
Formulas Used
Fertile Window Opens (Day)
fertile_start_day = ovulation_day - 5Peak Fertility (Day)
peak_day = ovulation_day - 1Ovulation Day
ovulation_day_display = ovulation_dayFertile Window Length
fertile_window_length = 6Days Until Fertile Window
days_until_fertile = max(ovulation_day - 5 - days_since_period, 0)Variables
| Variable | Description | Default |
|---|---|---|
cycle_length | Average Cycle Length(days) | 28 |
days_since_period | Days Since Last Period Started(days) | 8 |
ovulation_day | Derived value= cycle_length - 14 | calculated |
How It Works
Fertility Window
The fertile window is the 6-day period ending on the day of ovulation.
Ovulation Day = Cycle Length - 14 Fertile Window = Ovulation Day - 5 to Ovulation Day
Conception Probability by Day
Worked Example
A 28-day cycle, currently on day 8.
- 01Ovulation = Day 14
- 02Fertile window = Day 9 to Day 14
- 03Peak fertility = Day 13
- 04Days until fertile window = 9 - 8 = 1 day
Frequently Asked Questions
When am I most fertile?
The day before ovulation and the day of ovulation have the highest conception probability. The 2-3 days before ovulation are also highly fertile.
How long does the fertile window last?
The fertile window is approximately 6 days: the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract.
Can I get pregnant outside the fertile window?
It is very unlikely. Conception requires a viable egg (12-24 hours after ovulation) and sperm (survives up to 5 days). Outside this window, the probability is near zero.
Ready to run the numbers?
Open Fertility Window Calculator