Roymech engineering encyclopedia

BS 46 Square Key and Keyway Dimensions


This page provides imperial dimensions for square keys and keyways to the legacy British Standard BS 46 Part 1:1958. It includes shaft diameter ranges, square key sizes, shaft and hub keyway dimensions, tolerances, corner radii and preferred key lengths.

Need help understanding the different types of keys?
See the Key and Spline Selection Guide for an explanation of parallel, square, taper and gib-head keys, where they are generally used, and the main considerations when selecting a keyed shaft-to-hub connection.
Legacy standard: BS 46 Part 1:1958 has been withdrawn. The dimensional information below is retained by RoyMech as a reference for existing machinery, legacy drawings, replacement components and older imperial designs.

For new metric designs, see BS 4235-1 Parallel Keys and Keyways and confirm requirements against the applicable current standard.

BS 46 Part 1:1958 Square Keys

Reference standard: BS 46 Part 1:1958 – Keys and keyways and taper pins. Specification. Keys and keyways.

Square keys have equal nominal width and height. The dimensions on this page are retained specifically for work involving equipment and drawings based on the former BS 46 imperial system.

All dimensions in the following tables are in inches.

BS 46 Part 1 square key and keyway dimensions

Square Key and Keyway Dimensions

Use the shaft diameter range to identify the corresponding nominal square key size W × T. For these keys the nominal width W and height T are equal. The table then gives the dimensional limits for the shaft and hub keyways together with the specified corner radius.

The numerical values below reproduce the legacy BS 46 dimensional data previously published by RoyMech. Where a component is being manufactured, inspected or replaced against an existing drawing, the original drawing and applicable standard should take precedence.

Shaft Diameter Width / Height   Keyway – Shaft   Keyway – Hub Radius r
W H   W h
Over To / Inc. W & T   Min Max Max Min   Min Max Min Max  
1/4 1/2 1/8   0.124 0.125 0.072 0.078   0.125 0.126 0.060 0.066 0.010
1/2 3/4 3/16   0.187 0.188 0.107 0.113   0.188 0.189 0.088 0.094 0.010
3/4 1 1/4   0.249 0.250 0.142 0.148   0.250 0.251 0.115 0.121 0.010
1 1 1/4 5/16   0.311 0.312 0.177 0.183   0.312 0.313 0.142 0.148 0.010
1 1/4 1 1/2 3/8   0.374 0.375 0.213 0.219   0.375 0.376 0.169 0.175 0.010
1 1/2 1 3/4 7/16   0.437 0.438 0.248 0.254   0.438 0.439 0.197 0.203 0.020
1 3/4 2 1/2   0.499 0.500 0.283 0.289   0.500 0.501 0.224 0.230 0.020
2 2 1/2 5/8   0.624 0.625 0.354 0.360   0.625 0.626 0.278 0.284 0.020
2 1/2 3 3/4   0.749 0.750 0.424 0.430   0.750 0.751 0.333 0.339 0.020
3 3 1/2 7/8   0.874 0.875 0.495 0.501   0.875 0.876 0.387 0.393 0.062
3 1/2 4 1   0.999 1.000 0.566 0.572   1.000 1.001 0.442 0.448 0.062
4 5 1 1/4   1.248 1.250 0.707 0.713   1.250 1.252 0.551 0.557 0.062
5 6 1 1/2   1.498 1.500 0.848 0.854   1.500 1.502 0.661 0.667 0.062


Preferred Square Key Dimensions and Lengths

The following table gives the nominal square key sizes, dimensional limits and preferred lengths listed on the original RoyMech BS 46 Part 1 reference.

All dimensions are in inches.

BS 46 Part 1 square key dimensions

Width / Height W & T   Preferred Length
W & T Max Min   From To
1/8 0.127 0.125   3/4 1
3/16 0.190 0.188   3/4 2
1/4 0.252 0.250   3/4 3
5/16 0.314 0.312   3/4 3
3/8 0.377 0.375   1 4
7/16 0.440 0.438   1 1/2 4
1/2 0.502 0.500   2 1/2 5
5/8 0.627 0.625   2 1/2 6
3/4 0.752 0.750   3 1/2 6
7/8 0.877 0.875   3 1/2 6
1 1.003 1.000      
1 1/4 1.253 1.250      
1 1/2 1.504 1.500      

Using Legacy BS 46 Square Key Dimensions

These dimensions remain useful when maintaining older machinery, interpreting historic engineering drawings or manufacturing replacement components for equipment originally designed using BS 46 imperial keys.

A square key has equal nominal width and height, but this does not mean the shaft and hub keyway depths are simply half the nominal key height. The specified keyway dimensions and tolerances in the table should be used when reproducing an existing BS 46 connection.

Do not substitute the nearest metric key solely because its nominal dimensions appear similar. The shaft and hub keyways, engagement, tolerances and required torque capacity should be checked before changing an existing connection.

For a new shaft-to-hub connection, current standards and current engineering requirements should normally be considered rather than specifying the withdrawn BS 46 standard simply because the imperial dimensions remain available.

Related Key and Spline Information



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