Inch Bolt Wrench Size Chart

Quick answer: a wrench fits the width across the flats of the bolt head, not the bolt diameter. A 1/4-inch standard hex bolt commonly takes a 7/16-inch wrench; a 3/8-inch bolt takes 9/16 inch; and a 1/2-inch bolt takes 3/4 inch. Heavy-hex heads are larger than standard hex heads.

Standard and Heavy-Hex Bolt Wrench Sizes

Use the table for standard hex and heavy-hex bolt heads. The heavy-hex column begins at 1/2-inch bolt diameter. Square-head bolts, lag screws, nuts, and other fastener styles can use different dimensions and are not represented here. When the head style or standard is uncertain, measure across the flats or check the fastener specification.

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Why Heavy-Hex Bolts Need a Larger Wrench

A heavy-hex bolt has a wider head across the flats than a standard hex bolt of the same nominal diameter. For example, a 1/2-inch bolt uses a 3/4-inch wrench with a standard hex head and a 7/8-inch wrench with a heavy-hex head. Structural fasteners must be identified by their markings and specification, not by appearance alone.

Get a Secure Fit

Measure across two opposite flat sides of the head.
Use a fully seated six-point socket or box wrench when breaking loose a tight fastener.
Check whether the fastener is inch or metric before applying torque.
For structural connections, follow the project specification and fastener manufacturer guidance.

Related references: socket sizes in order, American wrench sizes, and Allen wrench sizes.