The Period and Ovulation Calculator estimates the next period, ovulation date, fertile window, and a suggested pregnancy-test date from the last period date, average cycle length, and period length.
Calculation method
The model estimates the next period by adding cycle length days to the last period date. Ovulation is estimated 14 days before that next period. The fertile window starts five days before estimated ovulation and ends one day after it. The pregnancy-test date is one day after the estimated next period.
The calculator accepts cycle lengths from 21 to 45 days and period lengths from 1 to 10 days.
Worked example
For a last period date of 1 January, a 28-day cycle, and a 5-day period:
- Next period estimate: 29 January.
- Ovulation estimate: 15 January.
- Fertile window estimate: 10 through 16 January.
- Pregnancy-test date used by the model: 30 January.
The period length validates the input but does not shift the ovulation formula.
Questions
Is ovulation always 14 days before the next period?
No. Fourteen days is a calendar assumption used by this tool. Ovulation timing varies, especially with irregular cycles.
Can this be used as contraception?
No. Calendar estimates alone are not reliable contraception. Use evidence-based medical guidance.
What is the fertile window?
It is the estimated five days before ovulation through the day after ovulation. It is a planning estimate, not a guarantee of conception.
Health notice and limitations
This calculator is educational and does not replace pregnancy testing or care from a qualified clinician. It does not account for hormonal contraception, postpartum cycles, illness, irregular bleeding, ovulation tests, or individual medical history.