How to Figure Out the Size of Screws, Bolts, and Nuts

If you have some stray screws and bolts (a.k.a. fasteners) lying around and aren’t quite sure what size they are, or if you need to replace a missing bolt on a piece of machinery and need to know what size to get, here are some methods to easily figure that out.

Check Out the Hardware Store

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Most (if not all) hardware stores have sizing plates that you can test your fasteners on, with both male and female options, as well as imperial and metric.

Lowe’s has one off to the side at the very end of the aisle (pictured above). It’s almost sort of hidden, so I wouldn’t be surprised if a lot of DIYers didn’t know it was there to begin with.

The only downside to using these in-store sizing plates is that you can’t just bring in your engine that needs a new bolts, hoist it up onto the sizing plate and start checking different sizes—that’s usually impractical, and sometimes impossible. So something like this only works with bolts and nuts that you can bring with you to the store, but not all hope is lost!

Buy Your Own Sizing Plates & Thread Checkers

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If you plan on using them a lot, it might be worth it to invest in some sizing plates and thread checkers for yourself, some of which you can use on that engine as discussed above.

Your best bet is to buy a complete set of nut and bolt thread checkers, which come in individual pieces for each size (as well as male and female options), making them ideal for figuring out the bolt size needed on big machinery with not a lot of room to work with.

Alternatively, you can buy sizing plates, which are much cheaper, but only include female terminals. So you wouldn’t be able to check the size of nuts, for instance—just screws and bolts.

Print Out Size Charts for Free

The poor man’s version of sizing plates and thread checkers are size charts that you can print out at home. The Bolt Depot has some excellent charts that you can print off for both imperial and metric fasteners, as well as nuts and washers.

The important thing here is that you need to print them out at actual size. So if your printer has an option like “Fit to Page”, you’ll need to uncheck that so there’s no downscaling. Once you’ve taken care of that, though, you’re off to the races, and can begin figuring out the sizes of your stray fasteners.

One downside to this method is that you have to eyeball it instead of actually screwing the fastener into a physical thread checker—so it’s much less accurate.

For example, I overlaid a bolt onto the print-out chart to see if I could figure out what size it was. It seemed to fit the best with the #10-32 model, but #8-32 was pretty darn close as well. At that point, it would probably be best to get the bolt in both sizes and try them out.

Title image from Basil Arteomov/Flickr

Craig Lloyd writes about smarthome for How-To Geek, and is an aspiring handyman who loves tinkering with anything and everything around the house. He's also a mediocre gamer, aviation geek, baseball fan, motorcyclist, and proud introvert.