How do you decide which size clock is most appropriate for your needs? The best starting point is working out the viewing distance you require.

The viewing distance is the distance where the time can still be read accurately to the minute. These distances have been established in practical tests for clock faces of up to 2000mm with black chapters on white background. Viewing distances for face diameters of over 2000mm have been calculated from the empirically established legibilities.

In unfavourable lighting conditions – and designs with coloured clock faces – the viewing distance could be substantially reduced.

Once you have determined the viewing distance required, use the chart below to find the minimum size of clock.

For example, if the clock is going to be used in your office and the distance from the clock to the furthest workstation is 15m, a clock diameter of 300mm would be suitable, or you may wish to move up to 400mm just to be sure.