Models come in dozens of different scales. Typical scales are 1:12, where one inch equals one foot full scale (common for doll houses), and 1:6 for larger dolls like Barbies where two inches equals one foot. Other scales popular with modelers include HO scale and N scale used for model railroading.
Find out what scale your model was made in. The scale is usually listed on the box and sometimes on the model itself. You need the scale to find out how large the model would be if life-sized.
Measure the model. This can be difficult for odd shaped items. One way to do this accurately is to place the model on a sheet of paper. Push two square objects against the ends of the model you want to measure. Take the model away without moving the blocks. Make a pencil mark along the bottom edges of the blocks to mark the length of the model. Now use a ruler to accurately measure the length of the model by the marks.
Convert the scale to a ratio (if it is not given as a ratio). Common gauges are: G (1:20); Half Scale (1:24); I scale (1:32); O scale (1:48); S scale (1:64); HO Scale (1:87.1); TT scale (1:120); N scale (1:160); and the tiny Z scale (1:220).
Convert the model size to life size. Use the ratio of the scale for your model, and multiply the length of the model by the denominator of the scale (second number in the scale). This will give you the length of your model in life size. For instance, if your model is N scale and measures 2 inches long, multiply 2 inches by 160 to get 320 inches for life size (convert inches to feet by dividing the number of inches by 12 - 320 inches = 320/12 = 26.6 feet). You can make measurements in feet and inches or metric--just make sure the units are the same for the scale model and life-size object.
Find the scale from the life-sized object. If you have a model of a real craft like an aeroplane and you want to know the scale, find out the measurements of the real object. Convert the measurement into inches or centimetres, then measure your model. Divide the length of the real object by the length of the model and you have the scaling factor.
You can find the scale of a model railroad by measuring the width of the track. Standard width of railroad track in the U.S. and U.K. is 4 feet 8 1/2 inches or 56.5 inches. Divide 56.5 inches by the width of the model track to determine the scaling factor.