How to Measure Bolt Length

The Rule

Bolt length is measured from the surface the head sits on to the end of the bolt. This varies by head type.

Hex Bolts & Hex Cap Screws

Measure from under the head to the end. The head is not included in the length.

• A "2 inch" hex bolt has 2 inches of shank and threads
• Total length including head is longer than stated size

Carriage Bolts

Measure from under the head (below the domed top) to the end. Same as hex bolts.

Socket Head Cap Screws

Measure from under the head to the end. The cylindrical head is not included.

Flat Head / Countersunk Screws

Measure total length including the head. The head is designed to sit flush with the surface.

Lag Bolts / Lag Screws

Measure from under the head to the tip. Same as hex bolts.

Determining the Right Length

1. Measure the total thickness of materials being joined
2. Add thread engagement (at least 1.5x bolt diameter for steel)
3. Add washer thickness if using washers
4. Round up to the nearest standard length

Common Standard Lengths (SAE)

Bolts come in standard increments: 1/2", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/2", 3", 3-1/2", 4", 5", 6"...