Metric & SAE Bolt Size Chart: Head Sizes, Thread Sizes & Wrench Sizes
Look up the wrench or socket size, thread pitch, and strength grade for any metric or SAE bolt below.
What Is a Bolt Size Chart?
A bolt size chart lists a bolt's nominal diameter, its head size across the flats, its thread pitch, and the wrench or socket that fits it. Bolt "size" refers to the diameter of the threaded shaft, not the head. An M10 bolt has a 10mm shaft diameter, but its hex head is typically 17mm across the flats. Knowing the head size, not the shaft diameter, is what actually determines which tool to grab.
This distinction trips up a lot of people new to fasteners: asking for "a 10mm wrench" for an M10 bolt is a common mistake, since the correct tool is almost always the larger 17mm size. This page covers metric bolts M6 through M24, SAE bolts from 1/4 inch to 1 inch, thread pitch and grade markings for both systems, and how to identify the right tool without trial and error.
Metric Bolt Size Table (M6 to M24)
Head size is the number that matters for tool selection. The SAE equivalent column shows the closest standard wrench if only an SAE set is on hand.
| Bolt | Head (mm) | Thread Pitch (mm) | Socket / Wrench | SAE Equivalent |
|---|---|---|---|---|
| M6 | 10mm | 1.0 | 10mm | 3/8" |
| M8 | 13mm | 1.25 | 13mm | 1/2" |
| M10 | 17mm (or 16mm) | 1.5 | 17mm | 11/16" |
| M12 | 19mm (or 18mm) | 1.75 | 19mm | 3/4" |
| M14 | 22mm (or 21mm) | 2.0 | 22mm | 7/8" |
| M16 | 24mm | 2.0 | 24mm | 15/16" |
| M18 | 27mm | 2.5 | 27mm | 1-1/16" |
| M20 | 30mm | 2.5 | 30mm | 1-3/16" |
| M22 | 32mm | 2.5 | 32mm | 1-1/4" |
| M24 | 36mm | 3.0 | 36mm | 1-7/16" |
SAE Bolt Size Table
SAE head size runs 1.5 times the nominal bolt diameter for most common sizes, which is a fast way to estimate an unfamiliar size in the field.
| Bolt | Head Size | TPI (UNC / UNF) | Wrench |
|---|---|---|---|
| 1/4" | 7/16" | 20 (UNC) / 28 (UNF) | 7/16" |
| 5/16" | 1/2" | 18 / 24 | 1/2" |
| 3/8" | 9/16" | 16 / 24 | 9/16" |
| 7/16" | 5/8" | 14 / 20 | 5/8" |
| 1/2" | 3/4" | 13 / 20 | 3/4" |
| 9/16" | 13/16" | 12 / 18 | 13/16" |
| 5/8" | 15/16" | 11 / 18 | 15/16" |
| 3/4" | 1-1/8" | 10 / 16 | 1-1/8" |
| 7/8" | 1-5/16" | 9 / 14 | 1-5/16" |
| 1" | 1-1/2" | 8 / 12 | 1-1/2" |
Metric Bolt Grades and Property Classes
Metric bolt strength is stamped on the head as a two-number property class. The first number is roughly one-tenth of the minimum tensile strength in MPa. The second number, after the decimal point, is the yield-to-tensile ratio, or how close the bolt gets to permanent deformation before it snaps.
| Grade | Marking | Approx. Strength | Typical Use |
|---|---|---|---|
| Class 8.8 | 3 radial lines (SAE Grade 5 equivalent) | ≈800 MPa tensile | General structural and chassis bolts |
| Class 10.9 | 6 radial lines (SAE Grade 8 equivalent) | ≈1040 MPa tensile | Suspension, engine mounts, driveline |
| Class 12.9 | 12.9 stamped, no lines | ≈1220 MPa tensile | Cylinder head bolts, connecting rods, torque-to-yield fasteners |
Always replace a torque-to-yield or stretch bolt with the same grade specified by the manufacturer. Fitting a lower-grade bolt in a high-stress location, such as a cylinder head or a connecting rod, risks the bolt stretching permanently or shearing under normal operating load.
US Bolt Head and Wrench Sizes Reference
SAE bolt grades are marked with radial lines cast or stamped into the head: no lines indicates Grade 2, a low-strength general hardware bolt. Three lines indicate Grade 5, the most common automotive grade. Six lines indicate Grade 8, a high-strength bolt used in suspension and towing hardware. The head size runs approximately 1.5 times the nominal bolt diameter, so a 1/2 inch bolt uses a 3/4 inch wrench, and a 3/8 inch bolt uses a 9/16 inch wrench.
Hex Bolt Dimensions
Hex bolt dimensions follow ANSI/ASME B18.2.1 for SAE fasteners and DIN 933/931 or ISO 4014/4017 for metric fasteners. A standard hex bolt has a chamfered hex head for easier socket engagement, a washer face machined flat under the head to seat evenly, and a shaft that is either fully or partially threaded depending on the application. Flange-head bolts add an integrated washer for load spreading, button-head and socket-head cap screws use a hex recess instead of an external hex, and each variant still follows the same thread pitch standards covered on the thread size chart.
Unified Thread Standards: UNC and UNF
UNC (Unified National Coarse) and UNF (Unified National Fine) are the two main SAE thread families. Coarse threads install faster and tolerate dirt, minor damage, and repeated use in soft materials better than fine threads. Fine threads pack more threads into the same length, which increases tensile stress area and improves resistance to loosening under vibration. A 1/2 inch-13 bolt has 13 threads per inch and is UNC. A 1/2 inch-20 bolt has 20 threads per inch and is UNF. Before cutting new threads for either standard, check the tap drill chart for the correct pilot hole size.
Bolt Grade Markings: How to Identify Them
On a metric bolt, look for a single stamped number on the head, such as 8.8, 10.9, or 12.9. On an SAE bolt, count the radial lines: three lines is Grade 5, six lines is Grade 8, and no markings usually means a low-strength Grade 2 bolt. Stainless steel bolts often carry an A2 or A4 marking instead of a numeric grade, indicating the alloy family rather than a tensile class.
What Socket Fits Which Bolt
The socket has to match the head size across the flats, not the bolt's thread diameter. M10 bolts most commonly use a 17mm head, though some compact fasteners on European vehicles use a 16mm or even 15mm head on the same M10 thread, so it pays to check before assuming. A 1/2 inch SAE bolt always uses a 3/4 inch head. When only the opposite system's socket set is on hand, the socket conversion chart shows the closest safe substitute and its tolerance rating.
Bolt Size Chart FAQ
What size wrench fits an M10 bolt?
A 17mm wrench fits most M10 bolt heads. Some compact or European fasteners use a 16mm or 15mm head on an M10 shaft instead, so check the actual head before assuming 17mm on unfamiliar hardware.
Is bolt size the shaft diameter or the head size?
Bolt size refers to the shaft diameter, not the head. An M10 bolt has a 10mm-diameter threaded shaft, but its hex head is typically 17mm across the flats, which is the number that determines wrench or socket size.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) threads have fewer, deeper threads per inch and install faster with more tolerance for dirt or minor damage. UNF (Unified National Fine) threads have more threads per inch, giving higher tensile strength and better resistance to loosening from vibration.
Can I use a Grade 5 bolt where a Grade 8 bolt is specified?
No. A Grade 8 (or metric 10.9/12.9) specification means the application carries loads a Grade 5 bolt cannot handle safely. Substituting a lower grade risks the bolt stretching or shearing under normal operating stress.
How do I read metric bolt grade markings?
Metric bolts stamp the property class directly on the head as two numbers separated by a period, such as 8.8, 10.9, or 12.9. The first number is roughly one-tenth of the minimum tensile strength in MPa, and the second is the yield-to-tensile ratio.
What socket fits a 1/2 inch SAE bolt?
A 3/4 inch socket fits a standard 1/2 inch SAE bolt head, since SAE hex heads are sized at 1.5 times the nominal bolt diameter across most common sizes.