How Is an Open End Wrench Measured

An open-ended wrench grips on two sides of a bolt.

Open-end wrenches are hand-held tools used for loosening and tightening bolts. An open-end wrench is exactly what the name entails. The end of the wrench that fits over the head of the bolt is open on one end. Two sides of the wrench grip parallel sides of the bolt head. Although wrenches are labeled with their sizes, you might find that the sizes are missing on older, rusted or well-used wrenches. There are many ways that you can determine the correct size of an open-end wrench.

Things You'll Need

• Veneer calipers
• Precision ruler
• Drill bit set

Instructions

1. Extend the "inside diameter" jaws located on a pair of veneer calipers between the inside edges of the gripping surface on the wrench. The gripping surfaces are the parts of the wrench that grip the bolt head. The reading on the veneer caliper will tell you the size of bolt the wrench can loosen or tighten.

2. Place a precision ruler on the surface of a table, then place the open-end wrench over the precision ruler. Place the left side gripping surface against the left side or "zero side" of the ruler. Take the measurement at the right side gripping surface.

3. Place a drill bit between the gripping surfaces of the open-end wrench. Graduate sizes until you reach a side that is a firm fit but still slips between the gripping surfaces. Inspect the shank of the drill bit for the size of the bit. This tells you the size of the wrench.

Tips & Warnings

• You can purchase low-cost plastic veneer calipers at most home improvement stores. You can also purchase precision rulers, but they tend to cost more than plastic veneer calipers.