How ovulation is actually calculated

Ovulation is counted backwards, not "in the middle"

The most common mistake is to assume ovulation falls in the middle of the cycle. That works only for a 28-day cycle, which is why "day 14" has lodged itself so firmly in everyone's memory.

The actual calculation runs the other way: backwards from the expected start of the next period. The reason is that a cycle is made of two parts of unequal stability — and you have to work from the stable one.

Two parts of the cycle, and only one of them is stable

• The follicular phase — from the start of the period to ovulation. This is the part that floats: its length varies most between women and between one woman's own cycles. It is why the overall length of the cycle moves.
• The luteal phase — from ovulation to the next period. It is considerably steadier: around 14 days on average, usually within 11–17.

Hence the method. If the stable part is the second one, then the thing to work from is its end — that is, the next period.

The reason for this asymmetry is physiological. The luteal phase is the lifespan of the corpus luteum, a temporary structure that forms where the follicle was after the egg is released. Its term is fairly tightly set. How long a follicle needs to mature, on the other hand, depends on a great many things — and that is where a cycle stretches or shortens.

What that looks like in numbers

Taking the luteal phase as roughly 14 days:

• a 28-day cycle — ovulation around day 14;
• a 32-day cycle — around day 18;
• a 24-day cycle — around day 10;
• a 30-day cycle — around day 16;
• a 35-day cycle — around day 21.

So "day 14" is not a rule but one case among many. On a 24-day cycle, expecting ovulation "in the middle", on day 12, puts you two days out — in the direction that matters.

The error is asymmetric. Because the fertile window sits mostly before ovulation, expecting it later than it happens misses the beginning of the window, which is where the most informative days are. On a cycle shorter than 28 days, "day 14" misses in exactly that way: it points at a day when ovulation has already happened. On a longer cycle the same mistake runs the other way and costs less — you simply start watching early.

The fertile window is not a single day

Ovulation is the shortest phase of the cycle, but the window is wider than it, because sperm can remain capable of fertilisation for several days. So the fertile window is usually taken as the few days before ovulation, the day of ovulation itself, and roughly a day after.

The asymmetry is significant: most of the window falls on the days before ovulation, not after. An egg lives considerably less time than sperm do.

What can be measured rather than calculated

A calendar calculation is an estimate drawn from the past. Some things concern the current cycle, and they give a different kind of information.

• Basal body temperature. Measured immediately on waking, before getting up. After ovulation it rises slightly under the influence of progesterone and stays higher until the period. The main limitation: the rise is visible after ovulation has already happened. For planning the current cycle that is too late, but for learning your own luteal phase length it is exactly right.
• Cervical mucus. Before ovulation it becomes clearer, thinner and more stretchy. This is a sign that appears before the event rather than after, which is why it is considered practically useful. But it is an observation rather than a measurement, with a lot of subjectivity in it.
• LH tests. These respond to the surge of luteinising hormone that precedes ovulation by roughly a day. They give the narrowest window of the three, but they cost money and require some idea of when to start testing — which means they still rest on the calendar.

None of these methods makes a forecast accurate. They make it yours instead of average.

How Kvit calculates it

Kvit works the same way — backwards from the expected start of the next period, not by halving the cycle.

By default it takes the same roughly 14 days of luteal phase. But if you measure basal body temperature, Kvit determines your own luteal phase — accepting values from 9 to 18 days — and calculates from that instead. Your day of ovulation may then differ from the examples above, and that is fine: the examples describe an average, not you.

And one more thing worth knowing in advance: when there are few entries, Kvit labels where the number came from — the forecast rests on what you entered during setup. And when a reliable calculation is not possible from the entries available, it does not draw a date at all.

Why your own records beat the formula

The formula above rests on two quantities: the length of your cycle and the length of your luteal phase. The first is knowable only from your records. The second is an average, and in any given cycle it may differ.

So any ovulation calculation is an estimate, not a fact. The more cycles recorded, the better founded it is.

In practice that means the app does not become useful immediately. One cycle is data entered. Three or four cycles is a range. Six months is a picture you can do something with, including show to a clinician.

Common mistakes worth clearing away

• "Ovulation is always on day 14." Only on a 28-day cycle.
• "A regular cycle means ovulation on the same day." Even in regular cycles the day of ovulation shifts.
• "No symptoms means no ovulation." Many people notice nothing at all, and that is not an indicator.
• "The app knows the exact day." No app does. It calculates an estimate from your data, and it ought to say so.
• "Outside the window is safe." No. The window is a planning guide, not a method of contraception.

When to see a clinician

• a cycle shorter than 21 days or longer than 35, and not as a one-off;
• a cycle that has suddenly become very irregular;
• pregnancy has not happened after a year of regular trying (or six months if you are 35 or older);
• severe pain around ovulation or during your period that gets in the way of ordinary life;
• bleeding between periods.

On how the phases relate to one another and what happens in each, see Phases of the menstrual cycle.