Ovulation Calculator (Cycle-Adjusted) Formula
Understand the math behind the ovulation calculator (cycle-adjusted). Each variable explained with a worked example.
Formulas Used
Ovulation Expected on Cycle Day
ovulation_on_day = ovulation_dayFertile Window Starts Day
fertile_start = ovulation_day - 5Fertile Window Ends Day
fertile_end = ovulation_day + 1Days Until Ovulation
days_to_ovulation = max(ovulation_day - days_since_period, 0)Variables
| Variable | Description | Default |
|---|---|---|
cycle_length | Average Cycle Length(days) | 28 |
days_since_period | Days Since Period Started(days) | 10 |
ovulation_day | Derived value= cycle_length - 14 | calculated |
How It Works
Ovulation Timing
Ovulation typically occurs 14 days before the next period, not 14 days after the last one.
Ovulation Day = Cycle Length - 14
The fertile window spans from 5 days before ovulation through 1 day after, because sperm can survive up to 5 days and the egg lives about 24 hours.
Example Cycle Lengths
| Cycle Length | Ovulation Day | |---|---| | 25 days | Day 11 | | 28 days | Day 14 | | 32 days | Day 18 | | 35 days | Day 21 |
Worked Example
A 28-day cycle, currently on day 10.
cycle_length = 28days_since_period = 10
- 01Ovulation day = 28 - 14 = Day 14
- 02Fertile window = Day 9 to Day 15
- 03Days until ovulation = 14 - 10 = 4 days
Ready to run the numbers?
Open Ovulation Calculator (Cycle-Adjusted)