For analogue clocks the size of face and hands decide the distance from which the clock can be read. Colour combinations of the face, hands and marks have as well impact on readability. Black hands and marks on a white face are regarded as the best combination from readability aspects. Built-in illumination provides good readability in dark environments.

In some countries there are national standards for the time scale and hands, e.g. DIN-standard in Germany.

This table presents some general guidelines for reading distances of analogue clocks. (Black hands and marks against white face)

For digital clocks it is difficult to set simple guidelines for readability since the spread of readability among various designs is large. LED is the most frequently used technology for digital clocks. The readability of this technology is very much depending on light intensity and reading angle of diodes and segments used.

Large out-door clocks are often built by electro-mechanical digits for which the size and colour are critical factors for readability.

In general the size of digits sets the limit for the reading distance of a digital clock, even if other factors also have major impact. In the table below we give some general guidelines for reading distances of LED digital clocks.