Screw Size Chart: Find the Dimensions You Need (2024)

The first time you read the label on a box of screws, you’ll probably come across a lot of confusing numbers that seemingly don’t make any sense.

But here’s the kicker:

I’m here to tell you that sizing the right screw doesn’t have to be guesswork anymore!

By the end of this guide, you’ll be able to confidently read a screw size chart and decipher all the measurements on a screw callout by yourself.

From metric and imperial size standards to all the other related intricacies, I’ve got it all covered below.

Let’s get started!

How to Read a Screw Chart

If you live in the United States, you’re among the three countries in the world that use the imperial system.

This means that you’ll have to follow the standard screw sizes of ASME/ANSI, which is called the Unified Thread Standard (UTS).

All measurements in the UTS system are in inches.

For the rest of the world, you’ll probably be using the metric system, which follows the ISO screw thread standard.

Whether you’re trying to read a screw chart or a screw label with the callout printed on it, you should familiarize yourself with both systems

How to Read a UTS Screw Chart (Imperial System)

A UTS screw chart will typically have three things: the screw gauge, the number of threads per inch (TPI), and the thread standard.

Our extensive screw chart also specifies the screw pitch in inches, which is the distance between each thread.

This could come in handy if you’re converting from the metric system to the imperial system, but it’s not something that UTS screw charts generally have.

Finally, the callout on the screws box may have a tolerance class number, the LH symbol if the screws are left-handed, and the screw length.

So, to sum up, here are six terms you should familiarize yourself with within the UTS system:

1. Thread standard
2. Gauge
3. Threads per inch (TPI)
4. Screw length
5. Tolerance class
6. LH or RH

1. Thread Standard

There are two thread standards under the UTS chart: course (UNC) and fine (UNF).

There’s also the extra-fine screw thread standard (UNEF), but it’s not as popular as the other two.

2. Gauge (Major Diameter)

The screw gauge is the diameter of the outside thread in inches, better referred to as the “major diameter.”

Anything less than ¼” will be labeled from #0 to #12. #0 has an outside diameter of 0.06 inches, while #12 has an outside diameter of 0.216 inches.

After #12, the gauge size is labeled according to the outside diameter. If the screw has an outside diameter of 0.25 inches, the gauge size will be ¼”, and so on.

Knowing this will help you know what drill bit sizes to use when drilling into wood, metal, plastics and any other material.

3. Threads Per Inch (TPI)

TPI stands for the number of threads per inch. It’s specified right after the screw gauge.

Let’s put this into perspective:

If you have a screw that has a #8 gauge. This screw will have 32 threads in a one-inch section of a coarse screw (#8-32 UNC).

If the screw is from the fine series, it’ll have 36 threads per inch (#8-36 UNF).

5. Screw Length

Just like with nail sizing, a screw length is the length of the shaft. The shaft is the part of the screw that goes into the surface.

Some screws have heads that can be driven completely into the surface.

This means that the head of the screw is part of the screw length.

Screws that can be driven completely into a surface are known as countersinking screws.

Some screws, like truss-, round-, hex-, button-, and pan-head screws don’t go all the way into the surface.

These screws are known as non-countersinking. The screw length is measured from beneath the screw head to the tip.

When reading the callout on a box of screws, you’ll see the screw length specified at the very end of the callout.

So, if you see a .5 at the end of the callout, this means that the screw length is ½ inch long.

6. Tolerance Class

The tolerance class indicates which type of holes or nuts the screw can fit into.

You’ll find it specified after the screw length. There are five tolerance classes, from class 1 to class 5.

Size 1 screws fit more loosely than size 2 screws. This is because they have more space between their threads. Size 3 fits the tightest.

There may also be the letter A or B next to the number, which indicates a slightly different size.

7. Right-Handed or Left-Handed

Finally, left-handed screws are labeled with the symbol LH, right after the tolerance class.

Left-handed screws are threaded in reverse, which may confuse you if you’re not aware of the LH symbol.

Left-handed screws loosen when you turn them clockwise and tighten when turned counter-clockwise.

How to Read a Metric Screw Chart (ISO)

A metric screw size chart is fundamentally different from imperial screw charts since they don’t contain information on the number of threads per inch.

Instead, metric measurements specify the thread pitch only, which is the distance between the threads on the screw.

So, what’s the bottom line?

These charts will typically have three things:

• Screw diameter
• Screw pitch
• Screw length

We’ve also included the drill tap size for each screw size, to make it more convenient if you want to tap threads using a drill.

See our full tap drill size chart for more info!

Fortunately, I’ve already covered everything you need to know about the diameter, pitch, and length of the screws.

So, if you have a screw labeled with metric measurements, all you need to do is grab a calculator and convert them to inches.

Reading American Screws Callouts

Pop quiz! You find a box of screws with a callout printed on it.

It goes #3-48 UNC-2B-LH x .5

By now, you should be able to decipher all the information on the callout.

If you couldn’t get it all right, don’t fret!

Let’s break it down together:

1. Gauge size is #3 (0.099 inches in the screw diameter chart)
2. 48 is the number of threads per inch
3. UNC means the screw is coarse
4. 2B specifies the tolerance class
5. LH means you’ll have to begrudgingly turn it in the opposite direction
6. .5 means the screw length is ½ inch

Reading International Screws Callouts

Here’s a callout number on a box of screws. The callout starts with the letter M, so this should be a good indication that you’re dealing with international screws.

M10 x 1.5 x 75

Let’s break it down:

1. The first number of the letter M indicates the screw’s diameter, which is 10 mm.
2. The second number is the screw’s pitch, which is 1.5 mm.
3. The third number is the screw’s length, which is 75 mm.

Detailed Screw Size Charts for US and International Screws

Our extensive screw size charts include:

• The Unified Thread Standard (UTS) used mainly in the United States and Canada
• The ISO thread size chart used in the rest of the world

Imperial Screw Size Chart (UTS)

Metric Screw Size Chart (ISO)

FAQs

Here are some of the most frequent questions we’ve received.

What is the head diameter of a screw?

The head diameter of a screw is roughly double the shank diameter. The shank is the long section of the screw beneath the screw head.

Whether the shank is threaded, partially-threaded, or not threaded at all, the concept remains the same.

For example, a screw with an outside diameter of 1?4 inches should have a head diameter of about 1?2 inches.

However, you can never precisely calculate the screw head size from doubling the outside diameter alone.

The best way to determine the exact screw head size is by measuring it yourself:

How many types of screws are there?

There are over 60 types of screws available. The most common type of screw is the wood screw.

Other types of screws include:

• Lag bolts/screws (see our lag screw size chart here)
• Wood Screws (see our wood screw size chart here)
• Sheet metal screws
• Deck screws (see our deck screw chart here)
• Machine screws (see our chart for these here)
• Masonry screws
• Hex bolts
• Set-screws (see our set screw chart)
• Self-drilling screws (see our chart here)

Wrapping Up

Our extensive guide is meant to cover all the intricacies and specs that you’ll typically find in a screw size chart.

Whether you’re trying to read the label on your screws box or looking for the perfect screw size for your project, you’ll find all the info you need in this guide.