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Scale converter

Converts real dimensions to model dimensions in any common scale, and shows the gauge discrepancy that British OO carries and almost nothing explains.

The question this answers: “How big is a real door in OO — and why does my track gauge not match?”

A door is about 2000mm; a terraced house frontage about 5000mm.

Result

Model size
78.9mm
OO (1:76). 7.89cm.
Scale
1:76
Track gauge used
16.5mm
A correctly scaled standard gauge would be 18.88mm.

Worth knowing about this answer

  • OO (1:76) runs on 16.5mm track, but a correctly scaled standard gauge at this ratio would be 18.88mm — the track is narrow by about 13%. That represents a real-world gauge of roughly 1254mm rather than 1435mm. It is a historical compromise, everybody lives with it, and it is why scaling a real track gauge with this divisor gives an answer that does not match the track you can buy.
Show the working
  1. Divide by the scale6000mm ÷ 7678.9mm
  2. Scaled standard gauge1435mm ÷ 7618.88mm
  3. Gauge discrepancy(16.5 − 18.88) ÷ 18.88-12.6%

Model size: 78.9mm. Scale: 1:76. Track gauge used: 16.5mm

How it works

Division by the scale denominator. The second half compares the gauge actually used against a correctly scaled 1435mm standard gauge, and reports the percentage error and the prototype gauge it implies.

What it assumes

  • Standard gauge is taken as 1435mm for the discrepancy figure.
  • British N is treated as 1:148 rather than the continental 1:160.

Where it stops being reliable

  • Scenes are not scaled like objects. Selective compression — shortening a scene while keeping its key features — is standard practice above about thirty prototype metres.
  • It converts dimensions and says nothing about whether a model will run. Wheel standards, flange depth and minimum radius are separate compatibility questions that a correctly scaled measurement does not answer.
  • Scale names are not used consistently between countries or between manufacturers, and a box marked with a scale is not a guarantee of the ratio inside it. Where the two disagree, the ratio is the thing to work from.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Model railways.