Ovulation Calculator — Free, Private
Project an assumed ovulation date from a period start and cycle length. The form does not measure ovulation or interpret a test result.
- Runs in your browser
- Nothing is uploaded
- No signup; device limits apply
Numbers stay on your device. Nothing is sent to a server.
How to use this tool
Enter a start date and an integer cycle length from 21 through 45 days. The next period is start plus cycle length; the assumed ovulation date is 14 days before that.
Results and examples
With a start of 2026-08-01 and a 28-day cycle, estimated ovulation is August 15. The engine displays August 10–16 as its window, implementing five days before the assumed date through the following day.
Limitations
- The displayed interval is not confirmation of an individual’s fertile days.
- This is not contraception or fertility care. NHS describes about 10–16 days from ovulation to the next period, so a fixed fourteen-day assumption cannot capture individual variation.
Fields
| Field | Guidance and constraints |
|---|---|
| First day of last period | Check the unit and format displayed beside this field. |
| Cycle length (days) | Accepted range: 21–45 · Input step: 1 |
Data handling
Processing stays in the current browser tab; the site does not upload tool input for the calculation itself.
Sources
Frequently asked questions
When does the displayed window end?
The current code ends it on the day after assumed ovulation.
Will it learn changing cycles?
No. It uses only the cycle length entered for the current calculation.
Related tools
You do not need to start over. Continue with the tool that fits your next step.
