How to Measure Bolt Size & Drill Bit Sizes


Bolts are fasteners that are used in areas where strength counts the most. A properly installed bolt will not come loose until you remove the retaining nut with your wrench to remove the object yourself. Drilling a hole through wood or metal to install a bolt requires you to know the correct bit to drill the right size hole. A hole that is too small means drilling a second time and too large means wasted materials. So, determining the proper size hole for your installation is essential. An adjustable wrench and a tape measure can tell you in moments what size drill bit you need for the bolts you intend to use on your project.

Things You'll Need

  • Bolt
  • Adjustable wrench
  • Tape measure
  • Selection of drill bits appropriate for the material being drilled
  • Hold the head of the bolt securely in one hand. Grasp the adjustable wrench in your other hand and open the jaws of your wrench completely.

  • Place the threaded shaft of the bolt between the jaws of your wrench resting on the non-moving side of the open tool. Slowly close the jaws of the wrench until they barely touch the threads on the bolt.

  • Remove the bolt from the wrench. Be careful not to touch the adjustment knob of the wrench and lose your measurement.

  • Place your tape measure across the top of the jaws of the wrench. Align the tape measure so that the 1 inch mark is resting on the inside of one jaw. Read and remember the mark that rests on the inside edge of the opposite jaw.

  • Subtract 1 inch from the measurement to account for the starting placement of the tape measure. Select the drill bit from your collection that matches your adjusted measurement. Slide the bit between the jaws of your wrench to double check the measurement.

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  • Photo Credit bolt image by Sergey Galushko from
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