Fertile Window EstimatorFree online date & time calculator — instant results, no sign-up
Estimate the fertile window from a cycle length and last period date.
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About the Fertile Window Estimator
The Fertile Window Estimator uses the first day of your last period and your cycle length to estimate ovulation, a surrounding fertile window, and your next period date. It assumes ovulation occurs 14 days before the next period and marks a window from five days before through one day after estimated ovulation. These dates are planning estimates, not confirmation of ovulation or a reliable way to prevent pregnancy.
- First day of last period
- The calendar date your most recent menstrual period began, entered as a date such as the default 2026-10-23.
- Cycle length (days)
- The number of days from the first day of one period to the first day of the next, with a default of 28 days.
Formula and method
N = D + C days; O = N − 14 days = D + (C − 14) days; S = O − 5 days; E = O + 1 day.
- DFirst day of the last period, used as the starting calendar date.
- CCycle length in days.
- NApproximate next period date.
- OEstimated ovulation date.
- SEstimated fertile window start date, five days before O.
- EEstimated fertile window end date, one day after O.
Add the cycle length as elapsed calendar days to the first day of the last period to estimate the next period. Subtract 14 days to estimate ovulation, then subtract five days and add one day to obtain the inclusive fertile window boundaries. With D = 2026-10-23 and C = 28, the next period is Fri, November 20, 2026, estimated ovulation is Fri, November 6, 2026, and the fertile window runs from Sun, November 1, 2026 through Sat, November 7, 2026.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- First day of last period1970-01-01
- Cycle length28 days
Results
- Estimated ovulationThu, January 15, 1970
- Fertile window startsSat, January 10, 1970
- Fertile window endsFri, January 16, 1970
- Next period (approx.)Thu, January 29, 1970
- NoteEstimate only — cycles vary
How to use the Fertile Window Estimator
- 1Enter the first day of your last period; for the worked example, use 2026-10-23.
- 2Enter your cycle length in days; use 28 for the worked example.
- 3Run the calculation to obtain the estimated ovulation, fertile window, and next period dates.
- 4Read the fertile window as including both its start and end dates, rather than as a single predicted day.
- 5Treat the results as estimates and update the inputs when your next period begins or your typical cycle length changes.
Reading your result
Estimated ovulation = Fri, November 6, 2026 is the model's predicted ovulation date, not a measured event. Fertile window starts = Sun, November 1, 2026 marks five days before that estimate. Fertile window ends = Sat, November 7, 2026 marks one day afterward; both boundary dates are included. Next period (approx.) = Fri, November 20, 2026 is the last period date plus 28 days. Note = Estimate only — cycles vary means the actual dates may differ, and pregnancy is not guaranteed within the window or ruled out outside it.
Limitations
This calendar model assumes the entered cycle length applies to the current cycle and that ovulation occurs 14 days before the next period. Neither assumption is certain: ovulation timing and the interval between ovulation and menstruation vary. Irregular cycles, postpartum changes, breastfeeding, perimenopause, illness, and hormonal medications can make the estimate less useful. The tool does not detect whether ovulation occurs, assess fertility, or provide safe days for unprotected sex. Do not use it alone for contraception; ovulation tests, cervical mucus observations, or clinical advice may help clarify timing when trying to conceive.
Common questions
Why does the default example estimate ovulation on November 6 rather than count the period start as day one of an offset?+
The calculator uses elapsed date arithmetic: October 23, 2026 plus 28 days gives November 20, then subtracting 14 days gives November 6. Its offset is 14 elapsed days after the starting date, not a conversion to cycle day 14.
Why does the fertile window extend from five days before ovulation to one day after?+
Sperm can survive for up to about five days in favorable conditions, while an egg generally remains capable of fertilization for about 12–24 hours after ovulation. The calculator represents this with an inclusive seven-date planning window, but the predicted ovulation date may be wrong.
What cycle length should I enter if my periods are irregular?+
You can enter a typical length based on recent cycles, measured from one period's first day to the next period's first day. However, a single length cannot capture irregular timing, so the resulting window may miss your actual fertile days.
Can I assume sex outside the estimated fertile window cannot cause pregnancy?+
No. Ovulation can occur earlier or later than predicted, so dates outside this estimated window are not reliably infertile. Use an appropriate contraceptive method if you want to avoid pregnancy.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.