The different steel types are identified as a specific type by a four-digit number system, 10XX where 10 is the indication of the steel being plain carbon, and XX indicating the percentage of carbon (measured in hundreths of percentage points – 0.00).

For example 1020 steel contains 0.20% C (carbon). This system is designed by the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE).

The AISI-SAE grades of tool steel is the most common scale used to identify various grades of tool steel.