No calculator knows the date of your next period. What can be calculated is the range it falls inside, and that is what the one below does: it gives you a window built from what you told it, then explains why the window is that wide.
Find your next period window
Two things: the first day your last period started, and your usual cycle length in days — the count from the first day of one period to the first day of the next.
A window, not a day20 October – 1 November
Your next period
13days
20 October – 1 November
The one after
17days
15 November – 1 December
And the third
21days
11 – 31 December
Each window is wider than the last, because every month adds its own uncertainty to the one before.
Estimated fertile days
7 – 13 October
These days are an estimate, and not a method of preventing pregnancy. Ovulation moves with stress, travel and illness, and no calculation can pin it down.
Why a window and not a date
Because the event that sets the date — ovulation — does not happen on a fixed day. The NHS puts it at 10 to 16 days before the next period in most women. Saudi Arabia's Ministry of Health gives almost the same range, 12 to 16 days, and adds the line that settles the whole question: it is "not necessarily in the middle of the cycle".
Follow real women day by day and the range gets wider still. In a study that tracked ovulation by hormone measurement rather than by calendar, the fertile window fell entirely between days 10 and 17 in only about 30% of women — and on every day from day 6 to day 21, a woman had at least a one-in-ten chance of being in her fertile window.
The familiar numbers, and the measured ones
Most of what we remember about the cycle came from a textbook rather than from a measurement. Here are four familiar figures, next to what showed up when someone counted.
| The figure | Commonly said | What the measurements show |
|---|---|---|
| Cycle length | 28 days | 29.3 days on average; 81% of cycles fall between 21 and 35 |
| Day of ovulation | Day 14 | 12 to 16 days before the next period, not necessarily mid-cycle |
| A regular cycle | Arrives on the same day | Moves by about two and a half days month to month, in the same woman |
| Fertile days | Days 10 to 17 | Fall entirely in that range for only about a third of women |
How the window is worked out
The arithmetic is simple; being honest about it is the harder part. We take the first day of your last period, add your usual cycle length, and that gives the centre of the window. Then we open the window on either side of that centre by as much as we expect your cycle to move.
That amount is not a guess. Across more than 600,000 measured cycles, an individual woman's cycle moved by about two and a half days from month to month on average, and by more than that for women who describe their cycle as variable. Since this page knows nothing about you beyond two fields, it assumes the wider figure rather than the narrower one — and that width is honest, not something to apologise for.
The window then widens for each cycle further out. The second is built on an estimate of the first, and the third on an estimate of the second, so the not-knowing accumulates. Drawing all three the same width would be the most flattering version and the least true one.
What moves your dates
The causes health services list are more ordinary than we assume: the first years after periods begin, the approach to menopause, weight gained or lost, heavy physical training, stress, some hormonal contraceptives, and conditions such as PMOS — the condition still widely called PCOS — or a thyroid disorder.
So a period that shifted a few days during a hard month is not necessarily a sign of anything. What deserves attention is the pattern, not the single month.
About the fertile days it shows
Sperm can survive inside the body for days — up to seven, according to the NHS — while the egg itself lives only about a day. That is why fertility is not counted as the day of ovulation alone, but as the five days before it plus the day itself.
And those days here rest on an estimate of when ovulation happens, which rests in turn on your cycle length. An estimate built on an estimate: that is why it is wide.
How the window gets narrower
By one thing: measuring instead of assuming. This page assumes how much your cycle moves, because it has no way of knowing. Log several real cycles and that figure stops being an assumption and becomes a number measured on you — and the window narrows to whatever your own cycle actually allows.
That is what the Shaden app does: it learns from the cycles you log, narrows the window as it learns, and tells you how confident it is rather than hiding it. Your data stays on your phone.