A frequent question on train forums involves a particular diecast toy car, usually available for a limited time but at a good price, and asking if it’s suitable to use in a particular scale. It seems not everyone knows how to determine scale themselves.

I understand why. I’ve never seen anyone explain how to do the math to figure it out, but it’s really not hard. All you need is a search engine, a ruler, and a calculator.

The first thing to do is find out the dimensions of the real thing. Let’s say the item in question is a Toyota Camry. A quick Google search reveals that a 2016 Toyota Camry is 190.9 inches long.

Next, convert that length to millimeters. Yes, we’re going to do the work in metric–it’s easier that way. Google will do that too–type **190.9 inches in mm** into Google and now you know it’s 4828.54 millimeters.

What you do next depends on whether you have the item in your possession or not. To determine the length the model should be in your given scale, just divide 4828.54 by the denominator of your scale. If your target is 1:48 (O scale), divide by 48. For 1:64 (S scale), divide by 64. If it’s 1:87 (HO scale), divide by 87. And so on. You can use any scale you want of course.

Doing the math, a perfect O scale Honda Accord would be 100.59 mm long. An S scale Accord would be 75.45mm, and an HO scale Accord would be 55.5mm. That’s about 4 inches in O, 3 inches in S, and a little under 2.25 inches in HO.

If you can measure the actual toy or model, you can determine its precise scale. Measure the item in millimeters, then divide the length of the real thing–4828.54 in this case–by the length of the model. If the toy is 112 millimeters long, you’ll get a result of 43.11196, which tells you it was intended to be a 1:43 model and wasn’t far off the mark.

But what you’re likely to find is that toy cars are often all close to the same length. So a supposed 1:43 Honda Civic and Accord will both be nearly 4.5 inches long, even though a Civic is 10 inches shorter than an Accord in real life. So if you’re an O scaler who wants vehicles to be as close to 1:48 as possible, do the math yourself. Look for “1:43” models of large cars. Smaller cars probably really are close to 1:43.

In the case of Hot Wheels and Matchbox cars, S scalers are better off going with smaller cars like Civics. HO scalers will occasionally find usable material among the large cars and trucks. For example, an HO scale F-150 pickup would be about 66 mm long (2.6 inches), since the real thing is 5788 mm long.

The moral of the story is not to believe the scale on the package. If you want accurate scale, determine or calculate it yourself.