Metric & Standard Bolt & Screw Size Chart: M2, M4, M5, M6, M8, M10

Bolts and screws are integral parts of fastening or putting together things. Several people assume that these two materials are the same, but they are different. Physically, they look similar. They are still unique fasteners in terms of application.

Screws are very versatile, making them a popular choice among people. The threads provide a sturdy grip, allowing them to meet the requirements of any project. They come in several types and sizes, which you can use in metal and wood. Other types are used for drywall and concrete. Screws are self-tapping, but you still have the option to drill a hole for it. Should you decide to go with the latter, it is less time-consuming to drill all holes first before installing the screws. 

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Standard & metric screw size chart

Here’s the standard & metric screw size chart you need to help you find the right products at the shop! It doesn’t matter if you’re working outside or on furniture. ASME is the organization that is in charge of a lot of the standardization work. These standards dictate things like thickness and screw thread.

STANDARD SCREW SIZES & THREAD DIAMETER CHART
SizeMillimeterInchNearest FractionMajor diameterPitch diameterMinor diameter
#01.50.061/16
#11.80.075/64
#220.083/32
#32.30.097/64
#42.80.117/64
#53.10.121/8
#63.30.139/64
#84.20.165/32
#94.50.17711/64
#104.80.193/16
#125.50.217/32
#146.30.24¼
8-320.1640.14370.1257
6-320.1380.11770.0997
4-400.11120.09580.0813
10-240.190.16290.1389
10-320.190.16970.1517
2-560.0860.07440.0662

How do you determine screw size?

screws on metal

When looking into and determining screw size dimensions, there are two things you need to understand – the length and the thread count. The latter is also commonly referred to as threads per inch (TPI) or thread pitch. We use the TPI to determine the thread’s fineness. The number will also tell if a screw can thread through a bolt. To measure screw size, we take its diameter. If the measurement says 1/4-20, it means it has a diameter of 1/4 inch and 20 threads per inch. A ruler is all you need to determine thread count and size.

lots of screws

The first step in measuring the length is finding a surface to lay it down on an even surface. Start measuring wherever the head would rest when fully embedded into something. In some cases, you may need to hang it off the edge of a surface to lay it flat. Ones with a pan, round, or truss head should be measured from the bottom of the head to the tip. Ones with a flat or oval head should be measured from the top to the tip. Hex head ones are an exception to this rule, which means you need to measure from the bottom of the head.

Now that you know how to take the measurement, the next step is to place the ruler down the axis to get the thread count. For accuracy, count the first thread as zero. Within one inch of the length, count the number of thread gaps. Once you have these values, you can now divide the length by the number of thread gaps. If there are four thread gaps in a one-inch length, the thread pitch would be 0.250 inches (one inch / 4 thread gaps = 0.250 inches). In other words, the gap in between each thread is 0.250 inches, and it has a Thread Per Inch (TPI) of four.

Lay it down on a flat surface again. Using a ruler or measuring tape, get the measurement from one side of a thread to the other to get the diameter. The diameter in the imperial system is represented in either fraction of an inch or a gauge number. If you are using the metric system, use the cm or mm side of the ruler to measure the diameter. Keep in mind that when purchasing ones with measurements that are listed in the metric system, the initial number is the diameter. 

How do I identify screw size?

Identifying sizes can be tricky if you don’t know how to read them. They are represented in an alphanumeric code, which is difficult to understand if you are not familiar with it. Codes such as “1/8-10 x 4/6”, “M5-0.4 x 15”, and “4-30 x 0.10” may seem overwhelming and meaningless at the same time, but it is the most vital information.

What is the significance of knowing these things if you can walk into the hardware store and ask the sales clerk for assistance? Well, there is not always someone available to assist you. Sometimes, they do not know much about it. You may get false or inaccurate information. When it comes to things as intricate as sizes, knowing the bare minimum helps. It can save you from buying the wrong type, leading to unnecessary expenses and wasted time and effort. It can even cause a failed project.

Step 1: Identifying the first number of the size

The first number is the largest diameter measurement of the thread. For example, a 4-30 x 0.10 has a diameter of 0.112 inches. It is because 0.060 + (4 x 0.013) = 0.112 inches.

If it is larger than ten, it is expressed as a fraction of an inch. For example, a 1/8-10 x 4/6 has a diameter of 1/8 of an inch. 

Should you see the letter M, it means the measurement is in millimeters. For example, M5-0.4 x 10 has a major diameter of 5 mm. 

Step 2: Reading the second number

The second number expresses the number of threads per unit of distance in between these threads. You can also refer to it as the thread pitch. If you have a 4-30 x 0.10, the threads per inch (TPI) is 30.

In the metric system, the threads are expressed in millimeters per thread. An M5-0.4 x 15 has a thread for every 0.4 mm.

Step 3: Reading the third number

The third number is typically the one after the x, and it prescribes the length of the screw. You measure them all from the bottom of the head down to the tip. The only exception from the rule is flathead screws, which sit flush on the surface. You should measure this type from the top of the head down to the tip. An M5-0.4 x 15 has a length of 15 mm.

drilling into wood

What do the numbers mean on screw sizes?

If you often do DIY, one of the popular items you are using is screws. It comes in several sizes, which you can find in the packaging. Finding the correct size can be a daunting task if you don’t know the imperial or metric system. A conversion table would be handy if you often convert between these two systems.

Several companies indicate the metric and imperial size on the packaging, which is helpful to most consumers. The challenge often comes when purchasing online. Most retailers do not indicate both system sizes on the product page. Why? It makes the product page too long.

Imperial Sizes

The gauge indicates the diameter. A larger number means a bigger diameter, whereas a smaller number means a smaller diameter. For example, a number 3 is smaller than a number 11. 

The gauge sizes do not have a direct connection with the actual head size. When measuring the length of a flathead screw, start on top of the head down to the stem. On the other hand, start from the bottom of the head down to the stem for other types. The gauge and length determine the size.

To calculate the gauge and head sizes, you can use the formula below.

Gauge = (Head diameter in sixteenths of an inch X 2) – 2

Metric Sizes

Generally, the metric system is easier to understand. It may take additional time to adjust if you are already comfortable with the imperial system. Instead of using gauges in expressing diameter, the metric system uses millimeters. On the other hand, it still uses millimeters to measure length.

The gauge (imperial) is approximately the head’s size in millimeters (metric). However, there is no correlation or scientific explanation here. 

Metric System

The metric system was based on the meter, a system of measurement whose origin was in France in the 1970s. When taking the length of a screw, we use meters. 

For the most accurate length, measure using millimeters from where the screw head sits to the tip. You don’t always measure from the top of the head. It depends on where the head would sit on the surface. Use a ruler or a measuring tape to get the most accurate measurement.

Here are some points to keep in mind when getting the length:

  • Screw heads rest differently on surfaces. If the head is flat, it will most likely sit flush. Otherwise, there will be a bump.
  • For flat-headed ones, measure from the tip to the top to get the length.
  • For round-headed ones, measure from the tip to the bottom of the head. The rounded head will stick out from the surface, which should not be included when measuring. 
  • For any other types that are not countersunk, measure from the flat underside to the tip.

Once you have the length, it is time to get the diameter. Use millimeters to measure from one side of a thread to the other. If the packaging has measurements indicated in the metric system, the first digit represent the diameter. For example, 4.0 means the screw has a diameter of 4 millimeters (mm).

Lastly, you need to measure the distance from one thread to another. It’s also referred to as the pitch. In the metric system, we use pitch instead of thread spacing. To get this final measurement, use a ruler or measuring tape. In most cases, the pitch would only measure less than 1 mm. It is recorded in a decimal point of an mm.

In summary, diameter is listed first before the length. For example, 4.0 x 50 means the screw has a diameter of 4 mm and a length of 50 mm.

Standard System

The standard system is otherwise known as the imperial system. It was introduced in the British Weights and Measures Act of 1824. It evolved through a series of amendments in the said act. For some time, the imperial system had been the standard measurement system. By the late 20th century, several countries have adopted the metric system. Presently, Canada and the United Kingdom are countries still using the imperial unit system.

The imperial system uses inches when getting the length from the tip to where the head rests and the diameter. The measurement should start wherever the head rests. 

When getting the length using inches, here are some points to keep in mind:

  • Take the length of a flat head from the tip to the top of the head.
  • For round-headed ones that are not countersunk, measure from the flat underside to the tip.
  • For round-headed ones that are countersunk, start measuring where the countersunk and the oval top meet in the middle. To put it simply, this is where the oval top would rest on the surface.
  • For flat-headed ones that are countersunk, measure from the tip to the top of the head.

Take the diameter by measuring the thread’s width in fractions of an inch using a measuring tape or ruler. A gauge number or fraction of an inch represents the diameter in the imperial system. 

  • cA gauge number represents the fraction of an inch in diameter. To know the diameter, you need to refer to a gauge guide to match the gauge with a fraction of an inch.
  • For example, a #0 gauge is 1/16 inch in diameter. A #1 gauge is 5/64 inch, and a #2 gauge is 3/32 inch.

Lastly, count the threads in one inch to get the thread spacing value. To get an accurate measurement, lay the screw next to a measuring tape or ruler and count the number of threads. The thread count in the imperial system ranges from 35 to 40 threads per inch. Thread pitch and thread spacing mean the same thing, so don’t get those two confused. 

M2 screw size

We use the M2 screw in small items, such as electrical equipment, cameras, and mobile phones. They’re also common in laptops and the like. The “M” corresponds to the outer diameter in the metric system, so an M2 has a diameter of 2mm. Do keep in mind that in reality, the actual diameter would vary between 1.9mm to 2mm because of manufacturing tolerances.

The thread pitch in an M2 is 0.4mm. Some manufacturers offer non-standard pitches, so it is best to reach out to them for any concerns in customization. If the measurement reads M2 x 8, the diameter is 2mm and has a length of 8mm.

Brass, nylon, stainless steel, steel, aluminum, and titanium are a few of the materials used in fabricating this type. 

M4 screw size

An M4 screw is slightly larger than an M2. The M4 comes in several lengths but shares the same diameter of 4mm. The M4 screw size in inches is the equivalent of 0.15748.

M5 screw size

carpenter installing a door

The M5 screw size has a diameter of 5mm. The M5’s dimensions are significantly thicker than an M4 but can be used in several applications. Some have an anti-corrosive finish to prevent rust from building up. 

The M4 screw size in inches is the equivalent of 0.19685 inches.

M6 screw size

The diameter of an M6 screw is 6mm. It comes in several thread pitches, such as 12-24 and 10-32. A sample application of an M6 is attaching the equipment to the vertical uprights of a rack. We typically use the threads mentioned above in rack applications. 

M8 screw size

An M8 screw has a diameter of 8mm. It has a pitch thread of 1.00mm for fine and 1.25mm for standard. 

M10

The M10 screw has the widest diameter among these six kinds. It has a diameter of 10mm and comes in several lengths. The M12 is even wider and can shoulder heavier loads.

Buying Guide

Now that we have discussed some of the basics, it is time to guide you through what you need to know. Finding the right type and specifications will hold materials together nicely. 

Types

metal screws

Depending on the application, they come in different forms and sizes. Here are some of the most common ones.

Cement Board

The cement board type is fully threaded, securing the backer board to subfloors or wall studs. The coating prevents corrosion from happening as a result of mortar and moisture. You can use it in metal and wood. 

cement board

Wood

A wood screw has a pointed tip that aids in wood-to-wood fastening. One part has coarse threads, while the other is an unthreaded shank. It helps in fastening wood pieces together. A common type is the pan head with a round head.

Deck

Deck screws are for fencing and decks, but they can also be used for similar outdoor projects. Composite deck ones have fine threads and a small head. On the other hand, wood deck ones have an unthreaded shank and a coarse-threaded portion. 

Drywall

Drywall screws are for interior projects, specifically for securing drywall. Fine-threaded drywall ones work best with metal studs, while coarse-threaded ones work with wood studs. Do keep note drywall ones cannot support tile and cement board. 

Lag

We use a lag screw to fasten components that handle a heavy load. It has a hex head, so you need a socket and ratchet or a wrench to secure it in place. 

Structural Wood

Structural wood screws have an unthreaded shaft and coarse threads. Structural wood ones may appear similar to wooden ones, but they are more durable. 

Sheet Metal

Sheet metal screws secure sheet metal. They are fully threaded and are available for use with self-drilling points and pre-drilled holes. 

Machine

Machine screws are fully threaded, which you can use in fastening metal pieces together. The two main types are socket cap screws and socket sets. The has a cylindrical, raised, hex-socket head, while the latter is headless. 

screws forming a T

Multi-Material

You can use a multi-material one in several materials, such as drywall or sheet metal. Wood and plastic ones are also good options. 

Concrete

A concrete screw is what you need for fastening concrete. Some require you to pre-drill, while others are self-tapping. It allows you to drill directly into the surface without pre-drilling.

Drive Types

Phillips and slotted drives are the two main types. Some are a combination that works for either type.

There is also the Allen drive, which has a six-sided socket. The square drive (Robertson drive) and star drive (Torx) have a head that minimizes camming out. 

Thread Types

Fine-threaded screws are best used with pre-tapped holes. The closer distance between these threads allows a tight hold. However, it would take a longer time to remove or install.

Coarse-threaded ones have a wider space between threads. We often use it with softer materials, such as drywall or wood. It is easy to install and remove.

Here’s a good resource on masonry & brick. Make sure you get the right screw length, thread diameter and screw threads for your project. We also have a resource on Phillips head options.

FAQ

What is the screw size number?

This is an indication of how big it is, as well as the threads per inch. From this number, you can gather a lot of information.

How big is a #8 screw?

4.2mm in diameter.

What size is an M3 screw?

3.0mm in diameter.

What size is an M2 screw?

2.0mm in diameter.

screws on metal background

Time needed: 5 minutes.

How do I identify a screw size?

  1. Put the screw on a flat surface.

    You will want the entire thing to lay flat, which means the head will need to lie off of the edge of a table or other surface.

  2. On the axis, you will want to place the ruler.

    Start at the first thread, which gets the number zero.

  3. Within 1 inch of the screw, you’ll start counting the thread gaps.

  4. Find the TPI by dividing the thread gaps by the 1-inch length.

    If there are 6 thread gaps, divide 1 inch by 6, resulting in thread gaps of .1667 inches.

  5. As it is lying flat, measure across the diameter.

    The outermost part is where you will want to measure from. If you are doing measurements on a worn one, you’ll get inaccurate results.