Scale factors and proportions
A scale of 1:n means each unit on the drawing stands for n real units, so multiply the drawing measurement by n.
A worked example
On a 1:200 plan, a wall drawn 6 cm long is really 6 x 200 = 1 200 cm, which is 12 m.
The mistake people actually make
Convert both measurements to the same unit before dividing, or the scale factor comes out a hundred times too small.