The numerical scale indicates how many times the length measured on the models is reduced. It is given by the ratio 1: M, where M is the scale number.

Example from practice: 
The McLaren Mercedes MP4/13 race car from Minichamps is 1:18 scale. According to the technical parameters of a real car, it´s length is 455 cm. Divide the length of the monopost by a scale number,  455/18 = 25.27 cm. The result means that the model from Minichamps is 25.27 cm long.

Of course, this example also works in the opposite direction, that means we multiply the measured length of the model by the scale number and the result we get is the real length of the race car. 

Comparison of McLaren Mercedes MP4/13 model from Minichamps in 1:18 and 1:43 scale:

Comparison of 1:12, 1:18 and 1:43 scale F1 models:

Comparison of 1:18, 1:24 and 1:43 scale road car models:

Comparison of 1:32 and 1:43 scale truck models:

Comparison of helmet models in scale 1:2 and 1:5:

Comparison of road car and motorcycle model, in both cases scale 1:18: