How this calculator works
Cycle length is the interval from one period start to the next. Period end dates count the start as bleeding day one. The table repeats the same cycle and bleeding duration six times; it cannot model irregularity. For each upcoming cycle it estimates ovulation 14 days before the following period and shows the preceding five days through that ovulation date. This simplified calendar is unsuitable for contraception and does not confirm ovulation. The result is an estimate for information only and is not medical advice.
Formula
Upcoming start(k) = last start + k × cycle days, k = 1 to 6. End = start + bleeding days - 1. Estimated ovulation = start + cycle - 14 days.
Worked example
With a January 1, 2026 start, 28-day cycles and five bleeding days, the next predicted period is January 29 to February 2. Ovulation in that upcoming cycle is estimated February 12.
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Frequently asked questions
Why is the end date four days after a five-day start?
The first date counts as day one, so five bleeding dates span four elapsed days.
Does this work with irregular cycles?
A fixed repetition is often inaccurate when cycle length varies. The table is only a planning illustration.
Can I use the fertile window as birth control?
No. The model assumes a fixed luteal interval and cannot account for actual ovulation variation.