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How often have you started to do a small job in the house only to find you don't have a suitable screw!  They are either too fat, too long, not fat enough or not long enough.  If by some miracle the screw fits, you don't have a rawl plug!  

A trip to the DIY shop beckons, but by the time you have found the screws, sorted out which size and which head shape you want, queued at the checkout and got home, there is no time left to do the job.  

Sounds familiar - well here is a little bit of basic information which may help in future.

Size    A wood screw is described by two measurements. The length and diameter (of the shank).

The length is obvious.*

The shank is given a gauge number with 6, 8 and 10 being the most commonly used.  

The thing to remember is the higher the number the thicker the screw.  e.g.  No.10 is thicker than No. 6.

* Gary Scarr has kindly pointed out that this is not obvious!

A one inch countersunk screw is shorter than a one inch raised head, which is shorter than a one inch round head. Why? Because the length is the length that should be buried in the wood, not the physical length of the screw.


Head shapes  

There are three head shapes to choose from, all intended for different uses.         

Countersunk -  this is intended to screw right down until the head is flush with the wood surface, leaving nothing protruding.

In order to accomplish this, the hole has to be made big enough on the surface of the wood only, to allow the screw head (which of course, is larger than the shank) to drop down past the surface. Used in most woodwork, door hinges and is essential when making children's toys in order to prevent accidents.

Raised - these have a flat tops and are usually used on such things as door furniture that have partly countersunk holes.

Round - these are used where the holes are not countersunk at all.



The most common type of Slots slots in screw heads are the traditional single slot on which you use the standard flat screwdriver, and the cross slot.  

This cross slot was originally designed for the Phillips screwdriver and some of the foreign screws are still of this design, where only a Phillips type screwdriver should be used.  However, the majority of cross head screws are now Supadriv or Pozidriv which require a screwdriver with a special tip.  If you try to use a Phillips screwdriver on Supadriv screws you will find they won't grip.

N.B.   If you use a power screwdriver choose screws with the Supadriv slot.  The screwdriver locks into this making the job much easier,.  



Twin-start screws have a sharp thread and hardened body, this enables them to be driven in much faster and makes them ideal for use with power screwdrivers.

Standard threads are O.K. for general jobs using a traditional screwdriver.


I must thank Mr. George Gouraud for the following information regarding a screw which I have omitted.

"It is a Canadian invention called the Robertson Head.  This screw has a square, flat bottomed "slot" and is available in all the sizes found in other head styles, as well as having 4" slot sizes.  The advantages of this type of head are:-

1:  It will not "cam out" as you tighten or loosen it, as happens with the Phillips head.

2:  The screw can be placed on the driver bit and will stay there even if you are driving it into either an overhead or downward facing work piece."

Thank you Mr. Gouraud.


  • Here's a formula sent in by Geoff Sharpe, for working out the exact diameter, in inches, of the shank of a screw. 

    Multiply the screw Number by 0.013 and add 0.060.

    Examples -  

    (No.) 8 x 0.013 + 0.060 = 0.164

    (No.)10 x 0.013 + 0.060 = 0.190


    My thanks to Jason Lee Olson for correcting the original formula shown

    Jason has also suggested you could convert the screw number to to decimal 
    e.g. No. 8 = .008 and use the following formula.

    Screw No. 8 - .008 x 13 + .060 = .164 (No. 8 diameter in inches)

    Screw No. 6 - .006 x 13 + .060 = .138 (No. 6 diameter in inches) 


    Should you require the diameter in mm then multiply the answer by 25.4.

    Screw No. 8 - .008 x 13 + .060 = .164 (No. 8 diameter in inches) x 25.4 = 4.166 diameter in mm

    Screw No. 6 - .006 x 13 + .060 = .138 (No. 6 diameter in inches) x 25.4 = 3.505 diameter in mm


  • Be sure to get the right screw for the job, particularly with regard to length.  Just a fraction too long can result in damage to a decorative surface or the screw coming right through to the other side. Too short and they may not be up to the job in hand.

  • To make screws easier to remove at a later date, rub soap on the thread before screwing in.

    Thanks to Geoff Smith of this one

    We have, however, been advised that soap can attract moisture which results in the screw rusting, (see next tip).

  • If you apply a little grease or Vaseline to the thread when inserting new screws they will be easier to remove should the need arise.   Do not use soap as this attracts moisture which can result in the screw rusting.

  • To avoid splitting the wood make start holes before attempting to insert the screw.  This can be done with a bradawl for small screws in soft wood or, alternatively, with a very small drill bit.

  • Avoid several screws following a wood grain as this can lead to the wood splitting down the grain.





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