This calculator requires the use of Javascript enabled and capable browsers. This script determines the annulus facial surface area within a given set of conditions. What is an annulus you asked (embarrassed and looking to make sure no one sees you ask...). One definition is "A space contained between the circumferences of two circles, one within the other." Another is "The solid formed by a circle revolving around a line which is the plane of the circle but does not cut it." OK, all of that said, what is it really? It looks like a washer that goes with a nut and bolt. Picture a donut... Don't eat it; just picture it. Better than that, for this calculator, picture it without depth. The outer edge of the donut is one circle and the inner edge is another. The space in between is the annulus and it is an area, not a volume, and it has no calories. (For volume and other additional annulus calculations, see our Annulus Volume Calculator. In simple terms, it is the difference, in area (square units), between two areas of two concentric circles. Other definitions do not mention concentric; only contained within. Our calculator works for both. The formula is, assuming the inner radius is IR and the outer radius is OR, and AA is the annulus area of either the top or bottom (but not both and excluding the sides), AA = Square root(OR squared  IR squared). While an annulus CAN have volume (as you can see in the graphic below), this calculator only deals with facial surface area of the top or bottom.
Enter two of the initial three values of outer radius, inner radius, and area; it does not matter which two. Click on Calculate. The third value, along with both diameters, is calculated. Calculations are rounded to three decimal places.
If you are really into this sort of math and you are experimenting with this at midnight or after, you need to get out more. This will make you old in a hurry!
