# Torus

Go to Surface Area or Volume.

Torus Facts

Notice these interesting things: • It can be made by revolving a
small circle (radius r) along a line made
by a bigger circle (radius R).
• It has no edges or vertices
• It is not a polyhedron Torus in the Sky
.
The Torus is such a beautiful solid,
this one would be fun at the beach !

## Surface Area Surface Area = 4 × π2 × R × r

### Example: r = 3 and R = 7

Surface Area = 4 × π2 × R × r
= 4 × π2 × 7 × 3
= 4 × π2 × 21
= 84 × π2
≈ 829

The formula is often written in this shorter form:

Surface Area = 4π2 Rr

## Volume

Volume = 2 × π2 × R × r2

### Example: r = 3 and R = 7

Volume = 2 × π2 × R × r2
= 2 × π2 × 7 × 32
= 2 × π2 × 7 × 9
= 126 π2
≈ 1244

The formula is often written in this shorter form:

Volume = 2π2 Rr2

Note: Area and volume formulas only work when the torus has a hole!

## Like a Cylinder

The volume is the same as if we "unfolded" a torus into a cylinder (of length 2πR): As we unfold it, what gets lost from the outer part of the torus is perfectly balanced by what gets gained in the inner part.

And the same is true for the surface area (not including the cylinder's bases). And did you know that Torus was the Latin word for a cushion?

(This is not a real roman cushion, just an illustration I made)

When we have more than one torus they are called tori

## More Torus Images

As the small radius (r) gets larger and larger, the torus goes from looking like a Tire to a Donut:  