Menstrual Cycle Calendar Arithmetic
Use an assumed constant cycle to calculate a next-period date and an ovulation window. This is not ovulation measurement or a pregnancy-probability prediction and must not be used for contraception.
- 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 the first day of the last period and usual cycle length. The implementation accepts 21–45 days and assumes ovulation 14 days before the next start, making irregular cycles especially uncertain.
Results and examples
The window extends five days before and one day after estimated ovulation. Being outside it does not rule out pregnancy, and delayed menstruation or symptoms require context beyond calendar arithmetic.
Worked example
Starting on 1 August 2026 with a 28-day cycle gives 29 August next start and 15 August assumed ovulation.
Limitations
- Tests, temperature, hormones and individual luteal-phase differences are excluded.
- The tool provides neither safe contraceptive days nor pregnancy probabilities.
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.
Frequently asked questions
What if my cycle varies?
The constant-cycle assumption can make dates inaccurate.
Are days outside the window safe?
No. Do not use it to exclude pregnancy or plan contraception.
Related tools
You do not need to start over. Continue with the tool that fits your next step.
