Ovulation Calculator
Estimate ovulation around 14 days before the next period, from cycle length and the last period start.
days
About this calculator
Ovulation is estimated as 14 days before the next period. The fertile window is treated as the five days before ovulation through ovulation day. These dates are averages, not a guarantee of fertility.
Limits
- Assumes a regular cycle and ovulation about 14 days before the next period.
- Not a contraceptive method and not a fertility diagnosis.
How to calculate
Enter the first day of the last period and typical cycle length in days. Most cycles fall between 21 and 35 days; 28 is the default example.
Next period = LMP + cycle length
Ovulation = next period − 14 days
Fertile window = ovulation − 5 days through ovulation
When to use it
- A calendar sketch of a typical cycle
Examples
28-day cycle, LMP on the 1st
Next period is the 29th. Ovulation is estimated on the 15th. The fertile window is about the 10th through the 15th.
Ovulation day in a regular cycle (count from day 1 = LMP)
| Cycle length | Estimated ovulation |
|---|---|
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
FAQ
Why subtract 14 days?
The luteal phase after ovulation is often close to 14 days. Cycle-length differences usually come from the days before ovulation.
Can I use this as contraception?
No. Cycle timing is not reliable enough on its own to prevent pregnancy.