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How to use a scale rule

By Marty McGuirk | October 17, 2022

This tool comes in handy for measuring scale plans, changing scales, and more

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hand with pencil and scale rule

You’ve found that perfect structure in an article or prototype drawing in Model Railroader magazine. The only problem is the plan is S scale and you’re an N scaler. You could use a photocopier to quickly enlarge or reduce a published plan to your scale. Or you can use the drawing and convert dimensions from one scale to another using a scale rule. Add an architect’s scale and you can transfer dimensions from published layout plans to your own layout, or see if some neat feature on a layout will fit your space.

Model railroad scale rule

You can get scale rules marked for only one modeling scale, but I suggest you purchase one marked with all the standard indoor modeling scales (N, HO, S, and O scale are commonly found on most rules). Most rules show the zero mark set in from the edge. Be sure to start your measurements from “0” – not the end.

Most published plans include prototype dimensions in feet and inches. Use the markings on the rule to transfer these dimensions directly to your model. Use the prototype dimensions and the scale of the drawing doesn’t matter. I used the O scale rule to transfer measurements from an HO plan directly to a sheet of styrene.

If you know the scale of the drawing, measure the plan directly with the appropriate scale markings, note the dimensions, and then mark that dimension in your scale with the rule.

Architect’s scale
An architect’s scale is another type of rule that’s useful for model railroading. Architect’s scales are available at most stationery or art supply stores. The scale is marked in a variety of standard drawing scales, including those we often use for track plans in MR.

When you’re building a layout from a published plan, an architect’s scale offers a simple way to transfer dimensions directly from the plan to the layout. Match the scale of the drawing to the scale on the rule. Then transfer the measurement to the layout. This technique works equally well when used with scale track plan drawings you’ve made yourself.

hands with pencil using scale rule

Transferring dimensions. After reading the prototype dimensions from an HO scale plan, use an O scale rule to draw the outline of a door onto styrene.

hand holding scale rule over drawing

Measuring scale plans. Measure a window opening on an HO scale plan with the HO scale calibrations on the rule. The opening is 30″ (or 2-1⁄2 feet) wide.

hand with pencil and scale rule

Changing scales. To enlarge the window opening on the HO plan to O scale, flip the rule to O scale and measure the 30″. Be sure to measure from zero, not from the end of the rule.

hand with drawing and scale rule

Using an architect’s scale. An architect’s ruler is handy for measuring published track plans. In this case the scale of the drawing is 3/4″ to the foot.

Learn more about where to buy large-scale rules.

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