A metric drill bit is a tool that is used in a power drill motor to make holes in wood, metal and other materials. Used in most countries around the world, the metric drill bit is produced in sizes that are measured in units of the metric measuring system. This is contrary to the drill bits commonly used in the United States, which are measured in fractions of an inch as defined by the Society of Automotive Engineers (SAE) or the American National Standards Institute (ANSI). The metric drill bit is often used in the repair of machines and components manufactured outside of the US, and in some cases, products that are made in the US also use metric-type fasteners and drill bits.
When a component is designed to use a metric fastener, a metric drill bit must be used to provide the proper-sized hole for the fastener to fit into. If a drill bit other than a metric drill bit is used, the fit could be too large or too snug for the fastener. This could create a fastener that has improper shear strength or even a poorly fitting component. This is often a problem in repairing some small machines that may be manufactured using a metric fastener, however, when repaired, is subject to the use of an SAE bolt of an improper size to fit into the metric-sized fastener hole.
One advantage that the metric drill bit has over the typical American, fractionally measured drill bit is in the sizing of the bit. The metric drill bit is able to be sized with a common metric measurement, however, the American system of measuring a drill bit by a fraction of an inch does not allow for the sizing of particularly small drill bits. The system used to size the ANSI-sized, extremely small drill bits is by gauge. This can commonly be confusing to the user when a very small drill bit is required. First, the user must convert the size of the fastener from a fraction of an inch into a decimal equivalent in the thousandths, add 10 thousandths to the size of the hole for room to insert the bolt, then check a chart to match that number to the gauge size.
A metric drill bit is much easier to size, so the majority of machine shops in the US also use the metric system when machining materials. Several drill bit manufacturers offer kits marked with both the metric drill bit size and the ANSI size on each bit. This removes much of the confusion for the average home repair person.