Understanding screw sizes

Discussion in 'Carpenters' Talk' started by MajorDisaster, May 12, 2014.

  1. MajorDisaster

    MajorDisaster New Member

    Hi,
    believe it or not, whenever I've needed to use screws, I've just selected them from a box containing all different types and sizes, by sight only. When it comes to actually selecting a screw of a specific size from the screwfix catalogue it becomes tricky because I can't hold the thing in my hand.

    So my question is "how do I interpret the sizes?"

    For instance I'm looking at this particular wood screw:
    http://www.screwfix.com/p/turbogold-countersunk-screws-3-x-16mm-pack-of-200/16957

    It's describes as 3 x 16mm. Fine, I imagine that 16mm is the length in mm, but is that the length from tip to the top of the screw or just the length of the thread? And what is the 3? Is that 3mm? or some other measurement? And what is it referring to, the width of the top of the screw, the head, or the width of the shaft?

    So confusing. Please help.
     
  2. Hurley

    Hurley Member

    Length is over all with countersunk and from under head to tip with other heads. A no 8 or no 10 is general common thickness, have a look on your wall plugs they say screw size on there. Alternatively get a box with various sizes and replenish whichever ones you use most with correct size.
     
  3. MajorDisaster

    MajorDisaster New Member

    Thanks for the length info. I was hoping for some guide as to what the '3' means in the particular screw I mentioned. I'm pretty sure it doesn't mean 3mm, but then what? is there a chart somewhere that I can relate the number quoted with a size in mm?
     
  4. Hurley

    Hurley Member

  5. Hurley

    Hurley Member

    I have had a look and yes, it is metric sized, 3mm diameter shaft. A very small screw.
     
  6. Hurley

    Hurley Member

    Here is a 3.5 x 16 to give you an idea of size
     

    Attached Files:

  7. MajorDisaster

    MajorDisaster New Member

    Thanks Hurley. That's very useful. If the screwfix website gave some sort of size gauge, it would be much easier to use. Instead, taking the wood screws for example, it's obvious that all the photos are identical.

    In the screw you photographed, could you possible measure the width of the head at its widest point for me?
     
  8. Phil the Paver

    Phil the Paver Screwfix Select

    The sizes are mm, the head size will be fairly similar on smaller screws due to the type of screwdriver required, ie: Philips or pozi 2 etc.
     
  9. Hurley

    Hurley Member

    7mm
     
    MajorDisaster likes this.
  10. MajorDisaster

    MajorDisaster New Member

    Great. Thanks Hurley.
     
  11. malkie129

    malkie129 Screwfix Select

    Just had a look out of interest, a 3mm Turbo Gold screw is actually Pozi No 1 size.
     
  12. MajorDisaster

    MajorDisaster New Member

    Thanks for that Malkie.
    Thanks to everyone for your input.
    I reckon I now know 100 x more than I did before.
    Learnin' all the time...

    Cheers!
     
    Hurley likes this.
  13. chippie244

    chippie244 Super Member

    With screws there is a difference between and imperial.
    With imperial the first no is the length ie 1 1/4" and the second is the diameter but expressed as a gauge ie 8 but it roughly works out at double the dia in mm so a no 8 is about 4mm dia
    Metric is easier, the first no is the length and the second is the dia in mm.
    The screwdriver tip size is related to the dia, an imperial 4 or metric 3 or will take a no 1 pozi, an imp 6-10 or met 3.5-5 will take a pozi 2 and higher will be a 3 or larger.
    Screws designed to be fitted by a machine ie electrical or plasterboard are likely to be Phillips and are graded like pozi's.
    Other screwdriver tips are available.
     
  14. Sigman

    Sigman New Member

    For what its worth I was told by an old joiner that the screw sizes were gauged by length and head diameter, therefore a 1" x 8 screw was 1 inch in length and the head diameter was 5/16" . the way the head diameter was worked out was to half the number, in this case 8=4 and add 1=5 and then call the result as sixteenths of an inch,so a 8 screw head was 5/16 in diameter and a number 10 screw head was 3/8" diameter ( 10 divided by 2 = 5 and add 1 =6 and call it sixteenths which = 3/8". for odd number screw sizes eg. 5s 7s etc the procedure was the same but you then went into 32nds therefore a No7 screw had a head diameter of 9/32".
     
  15. malkie129

    malkie129 Screwfix Select

    TTF for Metric screws. :confused:
     
  16. chippie244

    chippie244 Super Member

    I've never bothered to find out what the gauge meant and if I've been taught it I can't remember, I have been taught what the gauge meant on sheet metal thicknesses but I am just as clueless on that.
     
  17. Hurley

    Hurley Member

    You just know that when you need a 10x1" and when you need a 12x2" could be apples and bananas for all you care
     

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