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Last edited 05 Aug 2022

Scale rule

Scale rules are used to measure distances on scale drawings and physical models.

The term scale describes the relationship between a depiction of a building, object, area of land etc compared to its actual size. Scale us usually expressed as a ratio of one unit of the represented scale compared to the actual full size dimension. So full scale is 1:1, whereas a scale on fifth of the actual size is 1:5. For more information see: Scale.

Scale drawings are used to illustrate items that it is not useful or convenient to draw at their actual size. This may be because drawing the item at full size would be unmanageable, or would not easily fit on a single sheet of paper (such as a building). For more information see: Scale drawing.

Whilst a conventional ruler allows measurement of full scale (1:1) dimensions, a scale rule also allows measurement of drawings that are not at full scale without the need for any conversion.

Scale rules typically measure full scale dimensions up to 300mm. They are generally either flat (actually a flat oval shape), similar to a conventional ruler, with 8 different scales along their four edges (2 on each edge), or a three pronged star shape (generally referred to as triangular) with 12 different scales along their 6 edges.

The scales on a four edged flat scale rule are generally: 1:1, 1:100, 1:20, 1:200, 1:5, 1:50, 1:1250 and 1:2500

The scales on a 6 edged triangular scale rule are generally: 1:1, 1:10, 1:2, 1:20, 1:5, 1:50, 1:100, 1:200, 1:500, 1:1000, 1:1250 and 1:2500. However, they may only have 6 scales, one on each edge: 1:20, 1:25, 1:50, 1:75, 1:100 and 1:125.

Scale rules can be made of plastic, wood or metal.

Scale rules should only be used for measuring. They should not be used for drawing or cutting straight lines as this can damage their edges, making accurate measurement difficult.