Is a 21mm the same as 13/16?
A 21 mm measurement is not exactly the same as 13/16 inch. In practice, 13/16" equals about 20.64 mm, while 21 mm equals about 0.8268 inches. The two sizes are close, but not interchangeable for precise fittings.
Understanding metric and imperial sizing
Tool sizes come in two dominant systems: metric (millimeters) and imperial (inches). The numbers reflect different standardizations, and manufacturing tolerances mean that even near matches may not fit perfectly. When mixing systems, expect a small but real difference that can affect fit and torque.
Converting between millimeters and inches
Conversion basics: 1 inch equals 25.4 millimeters. Therefore, 13/16 inch is 0.8125 inches, which equals 20.6375 millimeters. Conversely, 21 millimeters equals 0.82677 inches. These figures show why the sizes are close but not identical, which is important for proper tool selection.
Key size relationships you should know when shopping for tools or swapping parts.
- 13/16 inch = 20.6375 mm (about 20.64 mm)
- 21 mm = 0.82677 inches (about 0.827 in)
- The difference between 21 mm and 13/16" is roughly 0.3625 mm (about 1.75%)
In practical terms, a 21 mm tool may feel slightly looser on a 13/16" fastener, and a 13/16" tool may feel tight on a true 21 mm fastener. The impact grows with precision requirements and torque specifications.
Practical guidance for matching sizes
When possible, use the exact size specified by the fastener. If you must substitute, opt for the size that fits snugly without forcing the tool onto the fastener, and avoid using the next size up if it won’t seat properly.
- Prefer the exact size that matches the fastener (21 mm for a 21 mm fastener, 13/16" for a 13/16" fastener).
- If substitution is unavoidable, choose the closer size that can be fitted without forcing, then test for engagement and seating.
- Be mindful of manufacturing tolerances; adjacent sizes can sometimes work in non-critical tasks, but not in torque- or safety-critical assemblies.
- When in doubt, measure the fastener with calipers or refer to the manufacturer’s specification.
Bottom line: for critical or load-bearing work, matching the exact size is essential. While 21 mm and 13/16" are close, they are not interchangeable in a precise, reliable way.
Summary
In short, 21 mm is not the same as 13/16 inch. 13/16" equals about 20.64 mm, while 21 mm equals about 0.827 inches. The difference of roughly 0.36 mm (around 1.7%) means they are not identical, and exact sizing should be used for proper fit and safety. When in doubt, measure the fastener and consult size charts to choose the correct tool.
What can I use instead of a 21mm?
Wrench Conversion Chart – Metric to Standard (SAE/Inches)
| Metric Size (mm) | Closest Standard Size | Difference |
|---|---|---|
| 19mm | 3/4″ | -0.002″ |
| 20mm | 25/32″ | +0.006″ |
| 21mm | 13/16″ | +0.014″ |
| 22mm | 7/8″ | -0.009″ |
Is a 13/16 socket the same as 21mm?
No, a 13/16 inch socket is not exactly the same as a 21mm socket, but they are very close and often considered interchangeable, as 13/16 inch is approximately $20.65$mm. Because of the small difference, one size may fit a nut or bolt more snugly than the other depending on manufacturing tolerances. The 13/16" socket often provides a tighter fit on many $21$mm lug nuts.
- 13/16" conversion: 13/1613 / 1613/16 inch is equal to approximately 20.6520.6520.65 mm.
- 21mm conversion: 212121 mm is equal to approximately 13/1613 / 1613/16 inch, or 0.8270.8270.827 inches.
- Practical application: While they are theoretically different, many tools and parts manufacturers list them as equivalent, making them effectively interchangeable in practice.
- Best fit: If you have both a 13/16" and a 21mm socket, it is best to try both to see which one fits the fastener most securely without being too loose or too tight. A tighter fit is generally preferred to avoid rounding the fastener head.
Is 13/16 bigger than 21mm?
21mm = just over 13/16 inch. 22mm = almost 7/8 inch. 23mm = 29/32 inch. 24mm = just over 15/16 inch.
What is a 21mm socket equivalent to?
A 21mm socket's equivalent in standard (SAE) size is 13/16 inch, though it is slightly larger at 20.65mm20.65 m m20.65𝑚𝑚 versus 21mm21 m m21𝑚𝑚. You can also use a 21mm socket on a 13/16" nut in many cases, but the reverse may not work as well because of the size difference.
Key differences
- Metric: 21mm21 m m21𝑚𝑚 is an exact measurement in the metric system.
- Standard (SAE): 13/16"13 / 16 "13/16" is the closest standard equivalent.
- Measurement: 13/16"13 / 16 "13/16" is approximately 20.65mm20.65 m m20.65𝑚𝑚, making it slightly smaller than a 21mm21 m m21𝑚𝑚 socket.
Which one to use
- For a 21mm fastener: Use a 21mm21 m m21𝑚𝑚 socket for the best fit.
- For a 13/16" fastener: Use a 13/16"13 / 16 "13/16" socket.
- When a perfect fit is not critical: A 21mm socket will often work on a 13/16" nut because the size difference is small.
