screw sizes

Screws: How To Pick The Right One For The Job

In this short, but comprehensive guide, we will explore the different types of screws, including their size and head type. You will also learn the difference between metric and imperial screw sizes.

By the end of this guide, you'll be able to make an informed decision on which screw you need for the job you're tackling.

Types of screws

screw type

Screws are categorised by the type of material they're suitable for fastening. 

Some screws are sold as ‘multi-purpose’, so double-check these screws are ideal for the material you're fastening.

The most common types of screws and their characteristics are listed below.

  • Wood: Has a pronounced thread (the raised line that spirals the length of the screw) and a sharp point to easily pierce wood
  • MDF: As MDF is prone to splitting, MDF screws are a particular version of a wood screw that is self-tapping (see Abbreviations, below)
  • Drywall: These screws are self-tapping and have a counter-sink head, so they sit just under the surface of a drywall panel, helping to maintain a smooth surface
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    Masonry: Do not have pointed tips. Therefore a hole needs to be pre-drilled in the concrete or brickwork before inserting masonry screws

For a comprehensive visual guide to screw types, check out this PDF from Bolt Depot.

Screw heads

screw heads

Screw heads have an indentation at the top for inserting a screwdriver. 

A slotted head is where the indentation goes straight across. Slotted heads are rarer these days, as the indentation is more likely to be a cross shape.

If the cross shape has another, shallower cross running through it, then it is a pozidrive head screw. Otherwise, the screw head is a phillips.

Phillips screwdrivers can be used for both pozidrive and phillips heads, but it’s better to use the corresponding screwdriver for the type of screw being used.

Screw heads can be flat, domed, or slightly raised.

Flat screws are also known as countersunk because they sink into the material and sit flush with the surface.

Domed screws, sometimes called rounded, have a pronounced dome on top and are used for decorative surfaces such as mirrors. 

Raised screws are, confusingly, flat on top but the head is thicker than flat screws, so they do not sit flush. Raised screws are also used for decorative surfaces, such as light switch covers.

Metric screw sizes vs. Imperial screw sizes

Metric screws are measured by the diameter and the length, both in millimetres.

Imperial screws are measured by gauge and length, the latter of which is measured in inches. Imperial gauge refers to the diameter of the screw, but is an assigned number, not a measurement. The higher the gauge number, the larger the screw.

You can convert between the two using this handy conversion chart.

Acronyms and abbreviations

  • ST: Self Tapping. You don't need to pre-drill a hole to fit these screws as they cut their own thread as they're screwed in
  • TT: Twin Thread. So-called as they have two threads running the length of the screw. Often stronger than single thread screws
  • TFT: Twin Fine Thread. As TT but the thread is wound tighter, so it screws in slower and can be adjusted. TFTs are better for use with thin materials like metal
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    ZP: Zinc Plated. Coated with zinc to help make them rust-proof and so are suitable for outdoors
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    ZYP: Zinc and Yellow Passivated. As ZP but with an extra layer of passivation which improves protection from corrosion.

How to pick the right screw for the job

length of screw

Your DIY store should have labelled screws according to what material they can be used for (wood, masonry etc.).

You then need to consider the screw length. For example, if you are screwing together two pieces of wood to make a box, at right angles to each other, and the wood is 24mm thick, then your screw length needs to be at least 48mm long. This 48mm figure includes 24mm to pass all the way through the first piece of wood plus another 24mm to firmly anchor into the second piece.

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Pro Tip: The Spruce recommends applying soap to the length of the screw to reduce friction as it enters the material

The type of screw head you choose depends on the finish you want. E.g. for a smooth, flat finish, use flat head screws rather than raised.

I hope this guide has been helpful in choosing the right screw for your job. By now you should understand the following: 

  • How different materials require different screws
  • When to choose a raised screw over a countersunk screw
  • How metric sized screws compare with imperial

You now have all the information you need to make an informed choice when at your local DIY store.

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how to choose the right screw

About the Author Vicky

Blogger who spent childhood suffering through many house renovations, but at least now is old enough to design rooms to her taste