Considering the flare to be a frustrum of a right perpendicular cone (bear with me...), there is always a narrow end with a circular cross-section at the knee (or equivalent if not trousers), and a wider end, with a larger circular cross-section, at the foot. We will call the radius of the smaller circle r, and the radius of the larger circle R. Since it is often easier to measure the width of these parts on trousers laid flat, it is more convenient to use equations using these values, which are logically half the circumference. We shall use the symbols W and w for the larger and smaller widths respectively. Thus W=πR and w=πr. The length, l, is simply the distance between these two. Click on the thumbnail at the beginning of this paragraph for a diagram of the measurements.

Perhaps the most obvious statistic is the angle of the flare, relative to the vertical axis of the leg. In this equation, θ_F is the flare angle, R, r, W, and w are as defined above, and l is the length of the leg, that is the distance between the first circle and the second. While this value makes intuitive sense, the values do not necessarily. It is difficult to imagine a 4° flare, except to say that 4° is a pretty small angle. Actually, 4° is quite big, and substantially bigger than most trousers sold as flares, but then some flares reach angles over 20°, so what does flare angle really mean?

Another statistic would be the extra volume enclosed by the flare(s), compared to a straight leg. This may be calculated by the following equation:
, V_F being the additional volume of a single leg, and 2V_F being the extra volume in the pair of trousers. If the value of cosθ_F is close to one, then we may approximate the volume by

If θ_F is less than 8°, then the estimate will be accurate to within 1% (and always overestimated).

Some people like to think of their flares as being big enough to cover their shoes. This of course depends on the shoe size in question, as well as on the size of the flares. No-one seems to know the exact circumference of a shoe, so again, I've approximated, this time using the Ramanujan Formula for the circumference of an ellipse. Yes, I know shoes aren't elliptic, but it was the best I could do. Anyway, the calculator below will now tell you what shoe sizes will be covered by your flares, assuming you have the standard width fitting. (It seems to be fairly accurate for me. If you have any comments, please use our message board.)