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Unravel the mystery of screws, screw sizes, different types of screw and uses


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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.

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

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



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


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

Thank you Mr. Gouraud.


  • Here’s a formula sent in by Geoff
    , 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


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

    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.