Ovulation Calculator: Finding Your Fertile Window
Ovulation usually lands about two weeks before your next period, not two weeks after your last one. That single distinction makes the whole calculation work.
What is the Ovulation Calculator?
In a typical cycle, ovulation happens about 14 days before the next period, so this tool subtracts 14 from your cycle length and adds the result to the first day of your last period. The fertile window shown runs from five days before ovulation to one day after it, covering how long sperm and an egg can survive. The result is general information only, not medical advice, and it is unreliable for irregular cycles or as a contraceptive method.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length in days (often 28).
- Read your estimated ovulation day and fertile window.
Worked example
For a last period starting 2026-01-01 with a 28-day cycle, ovulation is estimated on 2026-01-15, with a fertile window from 2026-01-10 to 2026-01-16.
When do I ovulate in a 28-day cycle?
In a regular 28-day cycle, ovulation falls around day 14, counted from the first day of your last period. The fertile window runs roughly from day 9 to day 15, because sperm can survive several days while the egg lasts about a day. For a last period starting 1 January 2026, that means ovulation around 15 January and a fertile window of 10 to 16 January.
Frequently asked questions
Is ovulation always on day 14?
No — day 14 assumes a 28-day cycle. With a longer or shorter cycle, ovulation shifts, which is why the tool uses your own cycle length.
Can this be used to avoid pregnancy?
No — cycle timing alone is not reliable contraception, especially with irregular cycles, stress, or illness shifting ovulation.