A model's scale describes its size in relation to its real-life subject and takes the ratio form 1:[number]. The first number of the ratio is always "1." The second number explains how many times larger the real subject is when compared to the model. For example, a scale ratio of 1:12 means that every inch of the scale represents 12 inches, or 1 foot, of the subject.
Measure the length of the model in inches. As an example, you might have a train engine that's 8 inches long.
Reference or measure the length of the real-world subject. This measurement will likely be in feet, so multiply feet by 12 to convert to inches so that you use the same units. For instance, the represented train engine might measure 19.3 feet in length; multiply this figure by 12 to convert it to 232 inches.
Divide the subject's length by the model's length to calculate the scale factor. In the example, divide 232 by 8 to calculate a scale factor of 29. If you have a remainder, round the number to the nearest integer.
Write a "1:" in front of the scale factor to present it as a ratio. In the example, the model's scale is 1:29, which means every inch of model represents 29 inches of the subject.