This SAE and metric wrench size guide helps you understand how standard fractional-inch sizes relate to millimeters. If you learned one system first, the comparisons and exact conversions below make the other easier to recognize. I put this reference together from my years as a mechanic and industrial technician, when knowing the sizes by memory was part of the job.
SAE Standard and Metric Wrench Sizes
Compare common fractional-inch sizes with their exact millimeter conversions, see how the two sizing systems progress, and use the printable chart and calculator as quick references. The goal is to understand the measurements, not to treat nearby sizes as interchangeable.
I built this reference to make the relationship between SAE and metric measurements easier to learn. SAE wrench sizes are commonly written as fractions of an inch, while metric sizes are written in millimeters. Once you know that 1 inch equals exactly 25.4 mm, you can convert any SAE measurement and understand how it compares with familiar metric sizes.
For example, 7/16 inch converts to 11.1125 mm, while 9/16 inch converts to 14.2875 mm. These decimal values show the actual relationship between the systems. Remember that the size stamped on a wrench describes the opening across the flats of a nut or bolt head, not the diameter of the threaded shaft.
Wrench Sizes - SAE to Metric Conversion
SAE to Metric Conversion Chart
Standard Wrench Sizes Chart with SAE to Metric Conversions: The following chart lists the most common standard wrench and socket sizes, including their corresponding sizes one step smaller and larger, as well as the metric equivalents. For easy reference, you can also download the Wrench Sizes in Order - SAE Size and Metric Equivalent Conversion Chart as a PDF file.
📄 Print this wrench size chart to keep in your toolbox, garage, or glovebox.
SAE and Metric Sizes Compared in Ascending Order
The following table shows SAE sizes in ascending order, with the previous and next SAE sizes for context. Its final column is a whole-millimeter clearance reference, not the exact conversion; use the conversion chart above for the precise millimeter value.
One inch equals exactly 25.4 millimeters. Multiply an inch measurement by 25.4 to convert it to millimeters. Because many fractional-inch sizes fall between whole millimeter values, the exact conversion may be a decimal. A nearby metric size helps illustrate scale, but it is not an exact equivalent and should not be assumed interchangeable.
SAE to Metric: Exact Size Conversions
These are exact mathematical conversions, followed by the nearest whole-millimeter value for quick comparison. The nearest whole-mm value is not necessarily the size of a suitable substitute wrench or socket. Tool availability and fastener fit are separate questions.
| SAE Size (inches) | Exact Conversion (mm) | Nearest Whole mm |
|---|---|---|
| 1/8" | 3.175 mm | 3 mm |
| 3/16" | 4.7625 mm | 5 mm |
| 1/4" | 6.35 mm | 6 mm |
| 5/16" | 7.9375 mm | 8 mm |
| 3/8" | 9.525 mm | 10 mm |
| 7/16" | 11.1125 mm | 11 mm |
| 1/2" | 12.7 mm | 13 mm |
| 9/16" | 14.2875 mm | 14 mm |
| 5/8" | 15.875 mm | 16 mm |
| 11/16" | 17.4625 mm | 17 mm |
| 3/4" | 19.05 mm | 19 mm |
| 13/16" | 20.6375 mm | 21 mm |
| 7/8" | 22.225 mm | 22 mm |
| 15/16" | 23.8125 mm | 24 mm |
| 1" | 25.4 mm | 25 mm |
| 1 1/16" | 26.9875 mm | 27 mm |
| 1 1/8" | 28.575 mm | 29 mm |
| 1 1/4" | 31.75 mm | 32 mm |
| 1 5/16" | 33.3375 mm | 33 mm |
| 1 3/8" | 34.925 mm | 35 mm |
| 1 7/16" | 36.5125 mm | 37 mm |
| 1 1/2" | 38.1 mm | 38 mm |
Conversion rule: multiply the inch value by 25.4 to get millimeters. For example, 1/2 inch is exactly 12.7 mm. A 13 mm tool is a commonly labeled whole-millimeter size, but that does not make it an exact 12.7 mm equivalent or guarantee interchangeability.
| SAE Size (Inches) | Previous Size | Next Size | Metric Size Listed (not exact) |
|---|---|---|---|
| 1/4" | - | 5/16" | 7 mm |
| 5/16" | 1/4" | 3/8" | 8 mm |
| 3/8" | 5/16" | 7/16" | 10 mm |
| 7/16" | 3/8" | 1/2" | 12 mm (11 too snug) |
| 1/2" | 7/16" | 9/16" | 13 mm |
| 9/16" | 1/2" | 5/8" | 15 mm (14 too snug) |
| 5/8" | 9/16" | 11/16" | 16 mm |
| 11/16" | 5/8" | 3/4" | 18 mm |
| 3/4" | 11/16" | 13/16" | 20 mm |
| 13/16" | 3/4" | 7/8" | 21 mm |
| 7/8" | 13/16" | 15/16" | 23 mm |
| 15/16" | 7/8" | 1" | 24 mm |
| 1" | 15/16" | 1 1/16" | 26 mm |
| 1 1/16" | 1" | 1 1/8" | 27 mm |
| 1 1/8" | 1 1/16" | 1 1/4" | 29 mm |
| 1 1/4" | 1 1/8" | 1 5/16" | 32 mm |
| 1 5/16" | 1 1/4" | 1 3/8" | 34 mm |
| 1 3/8" | 1 5/16" | 1 7/16" | 35 mm |
| 1 7/16" | 1 3/8" | 1 1/2" | 37 mm |
| 1 1/2" | 1 7/16" | 1 9/16" | 39 mm |
| 1 9/16" | 1 1/2" | 1 5/8" | 40 mm |
| 1 5/8" | 1 9/16" | 1 11/16" | 42 mm |
| 1 11/16" | 1 5/8" | 1 3/4" | 43 mm |
| 1 3/4" | 1 11/16" | 1 13/16" | 45 mm |
| 1 13/16" | 1 3/4" | 1 7/8" | 47 mm |
| 1 7/8" | 1 13/16" | 1 15/16" | 48 mm |
| 1 15/16" | 1 7/8" | 2" | 50 mm |
| 2" | 1 15/16" | - | 51 mm |
SAE to Metric Wrench Size Conversion Calculator
Use this interactive SAE to metric wrench size conversion calculator to instantly convert standard Imperial fractional inch sizes into exact millimeters and understand the exact metric measurement. Enter or select an SAE size to see its decimal millimeter equivalent. This conversion explains the measurement only; it does not determine whether another wrench or socket is suitable for a particular fastener.
This tool converts measurements, not tool compatibility. A nearby labeled wrench size is not necessarily interchangeable.
Quick Size Relationships Summary
Each SAE wrench size typically increases by about 1/16". Here's a quick-reference list showing how the SAE sizes progress. Each entry below gives the exact millimeter conversion and the nearest whole-millimeter value rounded to the nearest integer. These rounded values are for measurement comparison only; they do not guarantee that a wrench or socket of that labeled size will fit properly.
- 1/4" = 6.35 mm exactly; nearest whole mm: 6 mm.
- 5/16" = 7.9375 mm exactly; nearest whole mm: 8 mm.
- 3/8" = 9.525 mm exactly; nearest whole mm: 10 mm.
- 7/16" = 11.1125 mm exactly; nearest whole mm: 11 mm.
- 1/2" = 12.7 mm exactly; nearest whole mm: 13 mm.
- 9/16" = 14.2875 mm exactly; nearest whole mm: 14 mm.
- 5/8" = 15.875 mm exactly; nearest whole mm: 16 mm.
- 11/16" = 17.4625 mm exactly; nearest whole mm: 17 mm.
- 3/4" = 19.05 mm exactly; nearest whole mm: 19 mm.
- 13/16" = 20.6375 mm exactly; nearest whole mm: 21 mm.
- 7/8" = 22.225 mm exactly; nearest whole mm: 22 mm.
- 15/16" = 23.8125 mm exactly; nearest whole mm: 24 mm.
- 1" = 25.4 mm exactly; nearest whole mm: 25 mm.
- 1-1/16" = 26.9875 mm exactly; nearest whole mm: 27 mm.
- 1-1/8" = 28.575 mm exactly; nearest whole mm: 29 mm.
- 1-1/4" = 31.75 mm exactly; nearest whole mm: 32 mm.
- 1-5/16" = 33.3375 mm exactly; nearest whole mm: 33 mm.
- 1-3/8" = 34.925 mm exactly; nearest whole mm: 35 mm.
- 1-7/16" = 36.5125 mm exactly; nearest whole mm: 37 mm.
- 1-1/2" = 38.1 mm exactly; nearest whole mm: 38 mm.
- 1-9/16" = 39.6875 mm exactly; nearest whole mm: 40 mm.
- 1-5/8" = 41.275 mm exactly; nearest whole mm: 41 mm.
- 1-11/16" = 42.8625 mm exactly; nearest whole mm: 43 mm.
- 1-3/4" = 44.45 mm exactly; nearest whole mm: 44 mm.
- 1-13/16" = 46.0375 mm exactly; nearest whole mm: 46 mm.
- 1-7/8" = 47.625 mm exactly; nearest whole mm: 48 mm.
- 1-15/16" = 49.2125 mm exactly; nearest whole mm: 49 mm.
- 2" = 50.8 mm exactly; nearest whole mm: 51 mm.
Tip: While many SAE and metric sizes are close, always use the exact match when possible to avoid rounding fasteners, especially under torque.
Bolt Size to Wrench Size (Common Matches)
Knowing which wrench fits which bolt can save you time and frustration. Below is a guide to common SAE and metric bolt sizes along with the correct wrench sizes to use.
🔩 Wrench sizes refer to the bolt head size, not the bolt diameter. For example, a 9/16" wrench fits the head of a 3/8"-16 bolt. The metric column shows a whole-millimeter size larger than the SAE measurement. It is not the exact conversion and should not be read as a tool-substitution recommendation.
| Bolt Size | Wrench Size (SAE) | Wrench Size (Metric) |
|---|---|---|
| 1/4"-20 | 7/16" | 12 mm |
| 5/16"-18 | 1/2" | 13 mm |
| 3/8"-16 | 9/16" | 15 mm |
| 7/16"-14 | 5/8" | 16 mm |
| 1/2"-13 | 3/4" | 20 mm |
| 5/8"-11 | 15/16" | 24 mm |
| 3/4"-10 | 1-1/8" | 29 mm |
| 7/8"-9 | 1-5/16" | 34 mm |
| 1"-8 | 1-1/2" | 39 mm |
As you can see, many metric and SAE sizes overlap. If you're in a pinch, a close metric equivalent can sometimes substitute for an SAE wrench - but always prefer the exact match to protect the fastener.
Understanding the 7/16" and 9/16" Metric Equivalents
These two familiar SAE sizes are useful examples of how fractional inches convert to millimeters.
7/16" equals 11.113 mm. The exact conversion is 11.1125 mm. A 12 mm wrench has a larger opening, but it is not an exact metric equivalent. An 11 mm wrench - despite being commonly listed as "close" - has an opening of only 11.0 mm, which is smaller than the bolt head and will not go on. The 12 mm wrench fits with about 0.89 mm of play, which is workable in a pinch.
9/16" equals 14.288 mm. The exact conversion is 14.2875 mm. A 15 mm wrench has a larger opening, but it is not an exact metric equivalent. A 14 mm wrench opens to only 14.0 mm and will not clear the bolt head. The 15 mm wrench fits with about 0.71 mm of play.
This is a good reminder that "close in millimeters" does not always mean "fits." When precision matters - especially with torque or rusted fasteners - always use the exact SAE size rather than a metric substitute.
Socket Drive Size Recommendations
When selecting the appropriate ratchet or impact wrench, it's helpful to know which drive size matches the typical range of SAE socket or wrench sizes. Below is a general guide to help you match socket drive sizes with standard SAE ranges:
| SAE Size Range | Recommended Drive Size |
|---|---|
| 1/4" to 3/8" | 1/4" Drive |
| 3/8" to 5/8" | 3/8" Drive |
| 5/8" to 7/8" | 1/2" Drive |
| 7/8" and Larger | 3/4" Drive or Larger |
Tip: Using the correct drive size improves tool longevity and safety, especially when applying torque to larger fasteners.
Torque Wrench Drive Size Recommendations
Torque wrenches follow the same basic drive size relationships as regular ratchets, but they're precision tools designed for specific torque ranges. Here's a general guide to help you choose the correct torque wrench drive size based on SAE socket sizes:
| SAE Socket Size Range | Torque Wrench Drive Size | Common Torque Range |
|---|---|---|
| 1/4" to 3/8" | 1/4" Drive | 20-200 in-lb (inch-pounds) |
| 3/8" to 5/8" | 3/8" Drive | 10-80 ft-lb |
| 5/8" to 7/8" | 1/2" Drive | 30-250 ft-lb |
| 7/8" and larger | 3/4" or 1" Drive | 100-600+ ft-lb |
Choosing the right torque wrench drive size not only ensures proper torque application but also helps prevent overtightening or damaging fasteners. Smaller drive sizes like 1/4" and 3/8" are ideal for light-duty applications such as engine covers or small suspension components, where precision at lower torque values is critical. For larger fasteners, like lug nuts, suspension bolts, or industrial hardware, 1/2" and 3/4" drive torque wrenches provide the strength and range needed to achieve accurate, high-torque output without overloading the tool. Always match the drive size to the task to maintain both tool accuracy and fastener integrity.
Tips for Choosing the Right Torque Wrench:
- Use a torque wrench where your target torque falls in the middle 60-80% of its range for best accuracy.
- Avoid using drive adapters when possible, as they may reduce torque accuracy.
- For most automotive tasks, a 3/8" or 1/2" drive torque wrench will cover everything from spark plugs to lug nuts.
Types of Wrenches and Sockets
In addition to knowing the size you need, it's important to understand the types of wrenches and sockets available. Common types include:
- Standard Wrenches: These are the traditional, open-ended wrenches you're probably used to seeing in any toolkit.
- Box Wrenches: These are closed-loop wrenches that fit securely around the nut or bolt, providing more torque and reducing the chance of stripping.
- Socket Wrenches: These are used with interchangeable sockets and allow for a greater range of motion, making them ideal for tight spaces.
- Torque Wrenches: A special tool that ensures fasteners are tightened to a specific torque value. This is particularly important for automotive work.
Socket and Wrench Sizes and Use Case
The socket and wrench sizes listed above are standard for most mechanical and automotive tasks. Here's a practical breakdown of typical use cases based on wrench size ranges:
- Small Sizes (1/4" to 3/8" / 7mm to 10mm): Best for light-duty tasks like working on electronics, bicycles, household furniture, and small appliances.
- Medium Sizes (7/16" to 3/4" / 12mm to 20mm): Frequently used in vehicle maintenance, motorcycles, general mechanical repairs, and lawn equipment.
- Large Sizes (13/16" and up / 21mm and up): Ideal for heavy-duty work such as industrial machinery, truck engines, farm equipment, and large plumbing connections. These often require more torque and may need a breaker bar or impact wrench.
In addition to size, the measurement system also affects your choice of tools:
- SAE (Society of Automotive Engineers): Used primarily in American-made products, older cars, and legacy equipment.
- Metric: Standard in European, Asian, and modern manufacturing. Increasingly common across all industries today.
Imperial to Metric Tool History & Fun Facts
Ever wondered where wrench sizes came from? Here are some interesting facts about SAE and Metric measurements:
- The SAE system was developed primarily for American-made vehicles and machinery.
- Metric tools became widely used in the U.S. after the 1980s, as foreign car manufacturers gained popularity.
- The adjustable wrench (also known as a "Crescent Wrench") was invented in 1892 by Johan Petter Johansson.
- The phrase "righty tighty, lefty loosey" is a mnemonic for remembering the standard threading direction.
- Some aircraft use a mix of SAE and metric fasteners, making proper tool selection critical for aviation mechanics.
Next time you pick up a wrench, remember that you're holding a piece of engineering history!
Frequently Asked Questions (FAQs)
Here are some commonly asked questions regarding metric vs standard SAE wrench and socket sizes:
Wrench and Ratchet FAQ
What's the difference between SAE & Metric wrench sizes?
SAE (Society of Automotive Engineers) wrench sizes are measured in inches and are mostly used in the United States, especially for older or American-made vehicles. Metric sizes, on the other hand, are measured in millimeters and are more commonly used worldwide.
Can I use a Metric wrench on an SAE bolt?
SAE and metric wrench sizes use different units. Convert inches to millimeters by multiplying by 25.4, and treat rounded values as approximations rather than proof that tools are interchangeable. For example, 9/16 inch converts to 14.2875 mm, so 14 mm is not its exact metric equivalent. This is a unit conversion only; it does not mean a 14 mm wrench will fit a 9/16-inch fastener.
How do I convert SAE sizes to Metric?
Convert an SAE measurement to millimeters by multiplying the inch value by 25.4. Keep the decimal result when you want the exact conversion. For example, 1/2 inch is 12.7 mm, and 9/16 inch is 14.2875 mm. Rounding to a whole millimeter gives an approximation, not an exact equivalent.
What are the most common wrench sizes?
Common SAE sizes: 7/16", 1/2", 9/16", 5/8", 3/4". Common Metric sizes: 10mm, 13mm, 15mm, 17mm, 19mm. These sizes are frequently used in automotive and home repair projects.
What type of wrench should I use for tight spaces?
A socket wrench with a swivel or a ratcheting box wrench works best for tight or hard-to-reach spaces. These tools provide flexibility and reduce knuckle-busting frustration.
What drive size ratchet should I use for a 1/2" socket?
A 3/8" drive ratchet is commonly used for 3/8" to 5/8" sockets, including 1/2".
What wrench size fits a 1/4"-20 bolt?
A 7/16" wrench fits the head of a 1/4"-20 bolt. The closest metric wrench that will fit is 12mm.
What are the two competing measurement systems for wrench sizes?
The two competing measurement systems for wrench sizes are SAE (Society of Automotive Engineers), which uses the Imperial system measured in inches, and the metric system, which is measured in millimeters.
Why does a toolbox often have both SAE and metric wrenches?
A toolbox often has both SAE and metric wrenches because American cars traditionally used SAE sizes, while the metric system became popular globally and in the US since the 1980s with the rise of European and Asian cars, so both systems are needed to work on different vehicles.
How do common SAE wrench sizes convert to metric?
Compare the exact dimensions rather than assuming two labeled sizes match. For example, 7/16 inch converts to 11.1125 mm, 9/16 inch converts to 14.2875 mm, and 3/4 inch converts to 19.05 mm. Their nearest whole-millimeter values are useful for learning the scale, but the small differences remain.
What are the exact metric equivalents of 7/16-inch and 9/16-inch?
7/16 inch equals 11.1125 mm, and 9/16 inch equals 14.2875 mm. These decimal values are the exact conversions. The nearest whole-millimeter sizes are 11 mm and 14 mm respectively, but rounding to whole numbers loses some precision and does not by itself establish tool compatibility.
What does the size stamped on a wrench refer to?
The size stamped on a wrench refers to the distance across the flat sides of the bolt's head (the wrench opening), not the diameter of the threaded shaft.
Can you give examples of bolt shaft sizes and their corresponding wrench sizes?
Yes: a 1/4"-20 bolt needs a 7/16" wrench, a 3/8"-16 bolt needs a 9/16" wrench, and a 1/2"-13 bolt needs a 3/4" wrench. The wrench size always refers to the bolt head width, which is larger than the threaded shaft diameter.
What are the common ratchet drive sizes and their typical uses?
Common ratchet drive sizes include 1/4" for small, delicate tasks; 3/8" as the all-rounder for most common tasks; 1/2" for bigger jobs like suspension bolts; and 3/4" for heavy-duty industrial work.
What are the five most important tips for using wrenches and ratchets effectively?
The five most important tips are:
- Always use the exact size match when possible to avoid stripping bolts.
- Use the exact conversion chart above to compare the measurement systems. Do not assume a nearby labeled size is interchangeable.
- 7/16 inch converts to exactly 11.1125 mm; the nearest whole-millimeter value is 11 mm, which is a rounded comparison rather than an exact equivalent.
- 9/16 inch converts to exactly 14.2875 mm; the nearest whole-millimeter value is 14 mm, which is a rounded comparison rather than an exact equivalent.
- Always match your ratchet drive size to the job for power and safety.
Tips & Notes
- Wrench size corresponds to bolt head size, NOT bolt diameter. For example, a 3/8"-16 bolt uses a 9/16" wrench.
- When comparing SAE and metric tool sizes, use the exact conversion values as measurements only. A nearby labeled size may not fit correctly or be safe to substitute.
- 7/16 inch converts to 11.1125 mm, and 9/16 inch converts to 14.2875 mm. These are exact measurement conversions, not guarantees that differently labeled tools are interchangeable.
- Use the recommended socket drive size for your wrench size range to avoid damaging tools or fasteners.
- Torque wrenches are precision tools - choose a drive size appropriate to your torque range and avoid adapters when possible for accuracy.
- Metric is standard for most modern and foreign vehicles; SAE is still common in American-made or older equipment. Having both sets can save time.
SAE to Metric Wrench and Socket Sizes in Conclusion
SAE and metric wrench sizes describe measurements in different units, but the relationship is straightforward: multiply inches by 25.4 to get millimeters. Use the exact decimal conversion when precision matters, and treat a rounded whole-millimeter value as an approximation rather than an interchangeable tool size. The chart and calculator above are intended to make both measurement systems easier to recognize and understand.
Happy wrenching!