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What Size Screw?
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Screw Size Chart

So, just what do all of those screw numbers mean? It’s a good thing to know – plans, instructions, specs and other woodworkers refer to specific kinds and sizes of screws all the time. Below, Lee Grindinger clears up the mystery in his response to a Woodworker’s Journal eZine reader’s question.

Q. Screw numbers - He's sure it's something really simple but still wants to know why some screws are listed by diameter and threadcount (3/8-16) and others are just #6 or whatever? Or, perhaps more to the point, what does a #6 or #8 screw translate into?

A. Lee Grindinger: "Well, it's not as simple as you may suspect. All screws are listed by diameter but the terms used for different types of screws vary. A 3/8"-16 screw is a machine screw, the "3/8" is the diameter and the "16" is the number of threads per inch. This screw will thread into a nut or threaded hole.

Wood screws are tapered and thread count is not part of the description. A gage number like "6" is the diameter and the length is the length of the screw that goes into the wood not inclusive of the head unless it's a flat head. The smaller the gage number the smaller the screw diameter (opposite of nails and wire diameter, sheesh).

Heads come in a plethora of configurations as well, from driver type like slotted, phillips, square and Robinson, to shape of the head like round, flat and oval. Drywall screws are sized by gage (diameter) and length and come in either coarse or fine thread. Get a full scale chart to hang on your wall in the shop. Once you get the hang of the system identifying the screw you need is not too difficult, but it is confusing at first.

From the Woodworker's Journal eZine archives

Make sense? If you need screws, including some of the more exotic varieties, like square-X drive lube finished screws or double locking assembly screws, you'll find them - along just about every woodworking fastener you'll ever need - at Rockler.

posted on May 23, 2007 by Rockler
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7 thoughts on “What Size Screw?”

  • Danny

    what type and size screw would you recommend for 22" full extension drawer slides rated for 100 lbs

  • Blog Editor

    Most often, the appropriate screws come in a package with the drawer slides, but since you're asking, that must not always be the case.<br /><br />Full extension ball bearing drawer slides frequently come with #6 x 1/2" screws that have a low profile head to avoid interference with the slide mechanism. A 5/8" screw will offer superior hold - if material thickness allows. You'll find both <a href="http://www.rockler.com/product.cfm?Offerings_ID=10899" rel="nofollow">here</a>.<br />

  • Dave Turnes

    I need a round head slotted wood screw, 3 inches long,
    The shaft in 1/4 inches.
    I have a 2 1/2 inch screw and need more depth
    It is for a rocker that belonged to my Grandfather.
    The structure of the rocker is held together by hidden bolts and screws
    Can you help me find this screw and what is the proper description?

  • Tony is Swaq


  • trish jack

    If I'm putting holes in three quarter thick wood. what size screw do I use

  • Sam

    Is this blog entry still being monitored by the Rockler editorial staff? I see that the first comment/question received a response, but subsequent ones have not. If, as it seems, this post is closed for comments and questions, please say so and remove the "Leave A Comment" option.

    (Potential) Customers (like me!) often have questions that need to be answered in order to make purchases, and it's very frustrating when those questions go unanswered.

    I look forward to a reply.


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