What is diecast scale? This is the first question most beginning diecast collectors encounter when they start to buy diecast cars.
Diecast scale is really just a measure of diecast car size. It may seem confusing at first, but it's really not so complicated.

1:18 scale
Length (approx.) 23 cm, 238 mm, 9.1 inches

1:24 scale
Length (approx.) 16.5 cm, 179 mm, 6.5 inches

1:32 scale
Length (approx.) 12 cm, 134 mm, 4.7 inches

1:43 scale
Length (approx.) 10 cm, 100 mm, 4 inches
Thus, a 1/18 scale diecast car is usually about 24 - 30cm (depending on the size of the actual car). A 1/24 scale diecast car is generally about 16 - 20cm. Many of the cars made in the smaller scales (1/32-1/38) are actually made so that the car will be exactly 5 inches long, regardless of the scale.
Finally, it's important to note that larger vehicles, such as trucks or construction vehicles, are often made on a smaller scale (1/50 or 1/64), but are in fact about the size of 1/24 scale cars, since the actual vehicle is much larger.