Non-SI units are still used in many areas due to a combination of convenience and historical reasons. When using non-SI units the risk of error is high, especially when using the values in quantity equations. A number of non-SI units are accepted for use with the SI. These include units of time (minute, hour, day), volume (litre) and logarithmic ratio quantities (neper, bel, decibel). The majority of non-SI units are not accepted for use with the SI, despite their common use (Box 1).