Surface Area and Volume of a Torus
"Torus" is the technical mathematical name for the three-dimensional shape of a doughnut. A torus is generated by rotating a ring about an axis outside of the ring. To find the surface area and volume of a doughnut, you need to know the outer radius R and the inner radius r, i.e., the radius of the doughnut hole. You can apply the formulas described below, or you can use the convenient torus calculator.
Volume of a TorusIf the outer radius of the doughnut is R and the inner radius is r, then the volume of the solid is given by the equation
Volume = (1/4)(pi2)(R2 - r2)(R - r).
Recall that pi is the constant 3.14159265358...
Surface Area of a TorusThe surface area formula for a torus is
Surface Area = (pi)2(R2 - r2).
Suppose a bagel is 5 inches wide and the hole is 2 inches across. Then R = 2.5 and r = 1. The volume is thus
(1/4)(pi2)(2.52 - 12)(2.5 - 1)
= 19.43 cubic inches.
The surface area is
(pi)2(2.52 - 12)
= 51.82 square inches.
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