A bolt size chart enables people to immediately determine which bolt is best used in a specific situation without having to make guesses.
Measuring bolt sizes goes beyond just measuring their diameter; there are other factors that should be considered when doing so.
This article explains the two types of systems used in measuring bolts and the common areas where sizing errors are likely to occur.
How Bolt Sizes Are Defined
Three measurements define a bolt: nominal diameter, thread pitch, and length. Understanding all three is necessary before referencing any bolt dimensions chart.
Nominal Diameter
Nominal diameter refers to the diameter of the threaded section in millimeters in the case of metric bolts and in inches in the case of imperial bolts. The nominal diameter of an M10 bolt is 10 mm, whereas that of a 3/8″ bolt is 0.375″.
Thread Pitch
Thread pitch is defined as the distance between threads. Metric bolts are expressed in millimeters for thread pitch.
For example, the thread pitch of M10 x 1.5 shows a 10 mm diameter bolt and 1.5 mm distance between threads.
In the case of imperial bolts, there are threads per inch (TPI). For example, in the case of a 3/8-16 bolt, there are 16 threads per inch.
A coarse thread is a standard type, while a fine thread is resistant to vibration looseness.
Length
Bolt length is measured from below the head of the bolt to its tip. Countersunk bolts are measured from the head top. Getting length wrong is one of the most common ordering mistakes in fabrication and assembly.
Metric Bolt Size Chart (M3 to M36)
Metric bolt sizes follow ISO standards. The table below covers the most commonly used metric bolt sizes with standard coarse thread pitch and typical wrench sizes.
| Bolt Size | Diameter (mm) | Coarse Pitch (mm) | Wrench Size (mm) |
| M3 | 3.0 | 0.5 | 5.5 |
| M4 | 4.0 | 0.7 | 7 |
| M5 | 5.0 | 0.8 | 8 |
| M6 | 6.0 | 1.0 | 10 |
| M8 | 8.0 | 1.25 | 13 |
| M10 | 10.0 | 1.5 | 17 |
| M12 | 12.0 | 1.75 | 19 |
| M16 | 16.0 | 2.0 | 24 |
| M20 | 20.0 | 2.5 | 30 |
| M24 | 24.0 | 3.0 | 36 |
| M30 | 30.0 | 3.5 | 46 |
| M36 | 36.0 | 4.0 | 55 |
Fine pitch variants exist for all sizes above. For example, M10 x 1.25 is a fine thread alternative to the standard M10 x 1.5. Fine thread bolts are specified where vibration resistance or precise torque control is required.
Imperial and SAE Bolt Size Chart
Imperial bolt sizes use a combination of diameter in fractions of an inch and thread count in TPI. The Unified National Coarse (UNC) thread series is the standard for most general applications. The Unified National Fine (UNF) series is used where a fine thread is required.
| Bolt Size | Diameter (inches) | UNC (TPI) | UNF (TPI) | Wrench Size (inches) |
| #10 | 0.190 | 24 | 32 | 3/8 |
| 1/4″ | 0.250 | 20 | 28 | 7/16 |
| 5/16″ | 0.3125 | 18 | 24 | 1/2 |
| 3/8″ | 0.375 | 16 | 24 | 9/16 |
| 7/16″ | 0.4375 | 14 | 20 | 5/8 |
| 1/2″ | 0.500 | 13 | 20 | 3/4 |
| 5/8″ | 0.625 | 11 | 18 | 15/16 |
| 3/4″ | 0.750 | 10 | 16 | 1-1/8 |
| 7/8″ | 0.875 | 9 | 14 | 1-5/16 |
| 1″ | 1.000 | 8 | 12 | 1-1/2 |
SAE grade markings on the bolt head indicate tensile strength. Grade 2 is standard low-carbon steel. Grade 5 is medium-strength. Grade 8 is high-strength, used in structural and high-load applications.
How to Measure a Bolt: Step-by-Step Guide
Knowing how to measure bolt size correctly will help avoid any mistakes in ordering and ensure that the fitting will not cause you trouble at the location.
Step 1: Diameter Size Measuring
Use calipers to measure the diameter of the threaded shaft in millimeters. In the case of a metric bolt, the figure you get will be the nominal size. For an imperial bolt, you should convert the figure from decimal to fraction form.
Step 2: Length Measuring
Lay the bolt down flat and measure it from under the head all the way up to the end of the bolt. If the head is flat or countersunk, measure it from the top of the head.
Step 3: Measure the Thread Pitch
In case of metric bolts, use a thread pitch gauge and match the pitch in millimeters. In case of imperial bolts, measure the number of threads per inch. That count gives the TPI value.
Step 4: Check the Head Type and Drive
Hex heads are the most common. Other heads include socket cap, button, flat, and flange. Head type determines what kind of tool should be used for installation and if the fastener can be countersunk into the material.
Metric vs Imperial Bolts: Key Differences
Both systems are not compatible. Trying to fit a metric bolt into an imperial hole will damage the threads of both components.
| Feature | Metric | Imperial |
| Diameter notation | M followed by mm | Fraction or decimal inches |
| Thread measurement | Pitch in mm | Threads per inch (TPI) |
| Standard | ISO 898 | SAE / ASTM |
| Common regions | Europe, Asia, global OEM | USA, Canada, legacy equipment |
Most modern manufacturing uses metric. Imperial fasteners remain common in North American construction, agriculture, and maintenance of older equipment.
Stud bolts used in flanged pipe connections often follow ASME B16.5 or ASME B7 standards, which specify both diameter and thread pitch for each flange pressure class.
Common Bolt Size Mistakes to Avoid
Sizing mistakes happen to even experienced purchasers and may lead to delays and rework. Below, there are the most typical errors one should be aware of beforehand.
The confusion between the nominal size and the thread root diameter often leads to misfits. The nominal size is not the actual thread size.
Combining imperial and metric threads in a single assembly leads to misfits, which sometimes can go unnoticed until the assembly stage.
Ordering by bolt head size rather than shank diameter is another recurring error, particularly with hex bolts where the wrench size is often mistaken for the bolt size.
For non-standard applications, custom bolt sizing removes the guesswork. CNXD supplies custom bolt dimensions across metric and imperial specifications.
They offer material options covering carbon steel, alloy steel, and stainless steel to match structural, process, and high-temperature requirements. Contact CNXD for custom bolt sizing.
