The **hartley** (symbol **Hart**), also called a **ban**, or a **dit** (short for **d**ecimal dig**it**), is a logarithmic unit which measures information or entropy, based on base 10 logarithms and powers of 10, rather than the powers of 2 and base 2 logarithms which define the bit, or shannon. One ban or hartley is the information content of an event if the probability of that event occurring is 1/10.^{[1]} It is therefore equal to the information contained in one decimal digit (or dit), assuming *a priori* equiprobability of each possible value.

As a bit corresponds to a binary digit, a ban corresponds to a decimal digit. A **deciban** is one tenth of a ban; the name is formed from *ban* by the SI prefix *deci-*.

**One hartley corresponds to log**_{2}**(10)**** ****bit**** ****= ln(10)**** ****nat****, or approximately 3.322**** ****Sh****,**^{[a]}** ****or 2.303**** ****nat****. A deciban is about 0.332 Sh.**

Though not an SI unit, the hartley is part of the International System of Quantities, defined by International Standard IEC 80000-13 of the International Electrotechnical Commission. It is named after Ralph Hartley.