Needed: I do not have a micrometer. I used a machinist's clamp and a feeler gage set. You also need a sample piece of steel or wire you want to size.
Step 1: Place the Clamp
Step 2: Measure the Gap
Search the Internet for <wire gage size>, like this one: http://www.efunda.com/DesignStandards/gages/wire_forward.cfm Enter the thickness of the piece as you measured it. The web site will tell you the gage or the range of measurements for each gage.