Inside dimensions of a 55 gallon drum?

A metal fifty-five gallon lube oil drum is thirty five inches tall.

Given: Area of a circle = 1/4piDiameter2 and: 231in3 / gallon

Take the square root of: 55 gal x 231in3 / gallon x 4 / 35in / pi

I come up with about 21.5 inches for the diameter, which sounds about right.

But of course individual drums vary a bit, so grab a tape measure if you have a drum handy.