One-tenth of an inch is equal to three millimetres in width. The width of a penny is little over 1.5 millimetres, so three millimetres is roughly equivalent to two pennies.
A millimetre is a unit of measurement that is one thousandth of a metre in the metric system. The smallest measurement visible on a metre stick is the millimetre. An inch measures 25.4 millimetres in metric units, which is the same as standard measurement.
Coin widths are measured in millimetres by the United States Mint. The width of a dollar coin is 2 millimetres, that of a dime is around 1 1/3 millimetres, and that of a nickel is almost 2 millimetres.