When I started machining a few years ago, I immediately purchased a set of dial calipers- they're probably the single most useful tool for use in machining, aside from the lathe or mill themselves. I quickly realised that calipers were just as useful for scale modeling. Since then, my workbench hasn't been without a set.
Simply put, it's a precision measuring tool. Used with a simple calculator, it's the best scale conversion tool I've ever found.
Digital and dial models. The digital is pretty obvious to read. The dial is easy also, it just takes a moment to learn to use it:
This caliper reads 0.750 inches, or 3/4". The scale that runs the length of the beam shows that it's open just over 7/10 of an inch, and the dial shows how much more. 0.700 + 0.050 = 0.750. Quick and easy when you've learned how.
Dial calipers are availible in either decimal inch or metric- or you can just multipy/ivide by 25.4 to convert between. Digital calipers, you just push a button to convert.
Either set can be used for very small measurements.
This enamel wire is 0.011" in diameter- just over one one hundreth of an inch. Multiply this by 24, and in 1/24 scale, it would be 0.264" - perfect for 7mm spark plug wire.
The wheels in the Dragon BMW M3 kit measure 0.780" in dia. Times 24 = 18.72". A bit large for the 17" they're supposed to be- but that's kit accuracy for you.
Pressing the inch/mm button quickly tells you that they're 19.82mm- for those of you who enjoy the metric scale.
Inside measurements can also easily be done-
The rims are 0.683" wide on the inside, or 16.4" in 1/24. Minus a little bit for the brake calipers, and that's the largest size Brembo disk I can fit into this wheel.
It also works just as well in reverse:
The seat belts on a full size M3 are 1.860" wide. Divide by 24, and in 1/24, the seat belts should be 0.077".
A very nice set of calipers (like Mitutoyo) can run over $100 USD- however, there are many less expensive ones availible. I've used both, top of the line and 'generic import', and honestly haven't been able to tell a difference. The two that I have each cost about $20-25 USD, and they can be found on Amazon for less than $16 USD.
I wouldn't model without a set!