Ovulation Calculator
Estimate your fertile window, ovulation date and next period for a regular cycle. Private browser calculations with clear assumptions and limitations.
A clearer view of your dates
See an estimated fertile window, ovulation date and next period.
Enter your details and select Calculate. No signup needed.
For general information only. This tool does not confirm pregnancy, diagnose fertility conditions or replace clinical care. Do not use it to prevent pregnancy.
How to estimate your fertile window
Enter the first day of your last period and your average cycle length. This simple model supports regular cycles of 21–35 days. Select Estimate fertile window to see the dates for that entered cycle.
We show one cycle, rather than projecting several months of increasingly uncertain dates. If you change either input, calculate again to generate a new result.
What the calculation assumes
The estimated next period is the entered start date plus the cycle length. The estimated ovulation date is 14 days before that next period. The displayed fertile window covers the five days before estimated ovulation and the estimated ovulation day.
Example: for a period starting September 1, 2026 and a 28-day cycle, the next period is estimated for September 29, ovulation for September 15, and the fertile window for September 10–15.
This model cannot observe ovulation. Timing varies between people and between cycles, including cycles that seem regular. The real fertile window may fall outside the dates shown.
Important limits
Do not use this tool for contraception. It cannot identify guaranteed infertile or “safe” days, and it does not guarantee pregnancy. A clinician can help you choose appropriate fertility-tracking or contraception methods.
If cycles are irregular, periods have stopped, or you are unsure of your cycle length, a calendar prediction may be misleading. Selecting the irregular-cycle checkbox stops this tool from presenting precise-looking dates.
Common questions
Is ovulation always on day 14?
No. This calculator estimates it relative to the next period, using a fixed 14-day assumption. Actual timing varies and cannot be established by calendar arithmetic alone.
Why are there several fertile days?
The fertile window includes days before ovulation because sperm can survive for several days. The window shown here is still only an estimate, not a measurement of your fertility.
Do you store my period date?
No. The calculator keeps entries only in the current page’s memory. It does not add them to URLs, recent-calculation history or analytics. If you copy or print your result, that creates a separate personal record under your control.
Can this diagnose a fertility problem?
No. It cannot diagnose infertility, confirm ovulation or interpret symptoms. Discuss concerns with a qualified clinician.
Sources and editorial transparency
Prepared by DateTimeWise. Calculation documentation updated September 24, 2026. This page has not been independently medically reviewed.
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