Guide for Choosing the Right Size Screw

Do you want to know how to choose the right size screw for your project? It helps to learn about measuring systems and techniques for getting accurate measurements. Here are DIY tips on how to choose the right screw size.

Table of Contents

What Are the Main Types of Screws?

The type of screw you use depends on the application. The main types of screws include:

  • Tek screws: Tek screws, also called self-drilling screws, drill their pilot holes as you insert them, making them ideal for materials without pre-drilling. They are available in various diameters and lengths, so you can choose the size you need without compromising the product design. 
  • Wood screws: These are screws used for wood and have large spaces between the threads compared to metal screws. They are smaller in diameter and are usually made of stainless steel or zinc. Wood screws typically have a partial thread and a slotted button head. 
  • Metal screws: Metal screws, also known as machine screws, are available in varying materials, including steel, stainless steel and zinc. Metal screws also have different lengths, drive types and head shapes. They are robust and resistant to high temperatures, tearing and shearing.
  • Sheet metal screws: Some sheet metal screws have self-driving tips, while others have self-tapping tips. Head styles, length, thread count and diameter categorize sheet metal screws. 
  • Drywall screws: You can identify drywall screws by a curve on the junction connecting the head to the shaft to prevent tearing when screwing them into drywalls. They are available in course and fine threads. The fine threads are suitable for connecting drywall to metal shafts, while course threads are suited for attaching drywall to wood shafts.
  • Lag screws: These are available in various lengths with hexagonal heads and partial threads. They are mostly made of stainless steel or zinc. Lag screws have pointed tips, making it easier to sink them into material. 

What Are the Main Types of Screws?

5 Things You Should Look at When Determining the Size of a Screw

When learning to pick the right screw size, there are five things to know:

1. The Measurement Systems

There are two systems for determining screw sizes — the metric and the imperial systems. The difference lies in the unit of measurement. For example, while the imperial system measures thread size, length and diameter in inches, the metric system does it in millimeters. Each method has unique characteristics and considerations for determining a screw size. 

2. The Diameter

The diameter of the screw you choose depends on the materials you work with and the hold or anchor required. Using the correct diameter for safe and accurate application is also vital. The screw’s thickness should match the material’s width to avoid tears.

The screw gauge, also referred to as the major diameter, is the diameter of the external thread in inches. Anything below a quarter inch is labeled from #0 to #12. The outside diameter of a #0 gauge is 0.06 inches, while that of a #12 gauge is 0.216 inches. Beyond #12, gauge size will reflect the screw's diameter. A quarter-inch diameter screw will be marked that way. 

3. The Thread

The thread is the spiral shape that runs around the screw’s cylinder. The thread size may differ depending on the diameter of the screw. The threads can be tapered or parallel, or full or partial. 

Thread per inch (TPI) is the number of threads in one inch of the screw. This typically follows the screw gauge. For example, a screw labeled #10x36 has a #10 gauge and 36 threads per inch. 

4. The Length

The screw’s length is essential when choosing the right screw size. While thicker materials may require longer fasteners for a firm hold, short screws may be suitable for thinner materials. The general rule is that the screw should enter at least half of the attached material’s thickness. 

The screw length is determined by the shaft size, which is the part of the screw inserted into the material. This rule generally applies to non-countersinking screws such as round, hex, button, truss, round and pan-head screws. 

Screws such as flat-head screws sink entirely into the material. You can measure those from the top of the head to the tip of the threads.

5. The Material 

Know the material you will work on with the screw, whether wood, metal, PVC, concrete or plaster. Different screw types are suitable for varying materials. Each screw type has a unique build and characteristics to optimize its application. 

How to Measure Screw Size With the Imperial System

Measuring the size of a screw can be easier than you think. All you need is a ruler or a measuring tape, a gauge guide and the screws you want to measure. 

The Length

To determine the length of the screw:

  1. Take the inches side of the ruler and measure the screw from the tip to where the head rests. 
  2. If the screw is a countersunk, measure from the top of the head to the tip.
  3. If the screw is a countersunk with a round head, measure from where the oval top rests on the material’s surface. 
  4. For non-countersunk round-headed screws, measure from the flat side underneath the head to the tip. 

The Diameter 

To determine the diameter of the screw:

  1. For the imperial system, you want to find the diameter by the gauge number or in the fractions of an inch. 
  2. Measure one side of a thread to the other side to determine the diameter. Then, match your measurement with the gauge guide to get the gauge number or major diameter. 

The Threads Per Inch 

To measure the TPI:

  1. Place the ruler on the screw and count the number of threads in 1 inch to get the thread pitch or spacing value.
  2. The general TPI range for the imperial system is from 35 to 40.

How to Determine Screw or Bolt Size From a Hole

There are three ways to determine a bolt or screw size from a hole. 

The Alginate Method

You can determine the screw size from a hole with alginate with these steps:

  1. Pour a mixture of alginate into the hole and place a stick or screw in the middle while it's liquid. The stick helps you to pull the shape out when it solidifies.
  2. Use a bolt gauge or measuring tape to measure the size. 

The Vernier Caliper Method

You can measure the screw size from a hole by following these steps: 

  1. Place the Vernier caliper in the hole and adjust the scale on both sides to read the hole’s diameter.
  2. Place a stick or straw in the hole and mark it at the point where it reaches the end. 
  3. Use the caliper to measure the length you measured on the straw or stick.

The Size Measurement Scale Method

Screw size measurement scales help you determine the fastener size for holes. To take the measurements:

  1. Place the bolt scale on top of the hole.
  2. Move the scale until you find a matching hole size.
  3. Read the number on the scale to determine the size.

Get Quality Screws From Construction Fasteners and Tools

Get Quality Screws From Construction Fasteners and Tools

Knowing how to pick the right screw size is essential to the success of your project. From there, you need the right screws. Construction Fasteners and Tools has a wide range of products to meet your everyday needs. We offer only the best services and help customers make the optimum choices. 

Find the right screw for your project from Construction Fasteners and Tools.