Wrench Size Chart: Metric (mm) to SAE (Inches)
Use the wrench-specific calculator below to find the closest SAE wrench for any metric size. Open-end wrenches have looser tolerance than box-end wrenches, so match closely for box ends.
Metric Wrench (mm) → SAE Wrench
- Decimal inches
- 0.5512"
- Closest SAE
- 9/16"
- Difference
- -0.0113"
- Interchangeable
- Use with Caution
What Is a Wrench Size Chart?
A wrench size chart maps metric (mm) wrench sizes to their closest SAE (inch) equivalents. It is a workshop reference used by mechanics, technicians, and DIY builders to pick the right tool when only one system is on hand. The numbers represent the distance across the flats of a hex fastener: a 17mm wrench fits a fastener with 17mm between two opposing flat sides.
Metric to SAE Wrench Conversion Table
| Metric | Closest SAE | Decimal Inches | Wrench Tolerance |
|---|---|---|---|
| 3.18mm | 1/8" | 0.125" | Exact match |
| 4mm | 5/32" | 0.156" | Tight match |
| 5mm | 3/16" | 0.188" | Caution |
| 6mm | 7/32" | 0.219" | Avoid |
| 7mm | 1/4" | 0.250" | Avoid |
| 8mm | 5/16" | 0.313" | Tight match |
| 9mm | 11/32" | 0.344" | Caution |
| 10mm | 3/8" | 0.375" | Avoid |
| 11mm | 7/16" | 0.438" | Tight match |
| 12mm | 15/32" | 0.469" | Tight match |
| 13mm | 1/2" | 0.500" | Caution |
| 14mm | 9/16" | 0.563" | Caution |
| 15mm | 19/32" | 0.594" | Tight match |
| 16mm | 5/8" | 0.625" | Tight match |
| 17mm | 21/32" | 0.656" | Caution |
| 18mm | 11/16" | 0.688" | Avoid |
| 19mm | 3/4" | 0.750" | Tight match |
| 20mm | 25/32" | 0.781" | Caution |
| 21mm | 13/16" | 0.813" | Caution |
| 22mm | 7/8" | 0.875" | Caution |
| 24mm | 15/16" | 0.938" | Caution |
| 27mm | 1-1/16" | 1.063" | Tight match |
| 30mm | 1-3/16" | 1.188" | Caution |
| 32mm | 1-1/4" | 1.250" | Caution |
| 36mm | 1-7/16" | 1.438" | Avoid |
| 41mm | 1-5/8" | 1.625" | Caution |
| 46mm | 1-13/16" | 1.813" | Tight match |
| 50mm | 2" | 2.000" | Avoid |
| 55mm | 2-3/16" | 2.188" | Caution |
| 60mm | 2-3/8" | 2.375" | Caution |
| 63mm | 2-1/2" | 2.500" | Avoid |
Open-End Wrench Sizing vs Box-End Wrench Sizing
Open-end wrenches contact only two flats of a fastener and tolerate a slightly oversized fit. Box-end wrenches surround all six flats (or all twelve points) and need precise size matching, since even 0.010 inch of slack can lead to rounding when high torque is applied. As a rule, use box ends for the first crack on a tight fastener and open ends for fast spin-off once the fastener is loose.
A 12-point box end is easier to position in tight spaces, which helps in engine bays and brake assemblies, but it grips the corners of the fastener rather than the flats, raising the rounding risk if the wrench is the wrong size or worn.
Combination Wrench Sizes: Metric and SAE
A combination wrench has a box end and an open end of the same size. Common metric combination wrench sets cover 8mm through 19mm at minimum, with full sets running to 32mm. SAE sets typically cover 1/4" through 1" with full sets going to 1-1/4". A useful master set covers both systems and includes the in-between sizes (15mm, 21mm) that hex socket sets often skip.
Brand Comparison: Tolerance and Fit
Premium brands like Snap-on, Wera, and GearWrench manufacture to tighter tolerances than budget brands like Pittsburgh or Neiko. The practical difference shows up when using a metric wrench on an SAE fastener, or the reverse: a Snap-on 19mm wrench fits a 3/4" bolt snugly, while a budget 19mm wrench may spin freely on the same bolt. For torque-critical work, such as head bolts, suspension, and brake calipers, premium tools pay back in fewer rounded fasteners. Tekton and Sunex sit in the middle on price and tolerance, a common choice for a home mechanic who wants better fit than budget tools without paying professional-line prices.
Spanner Sizes for UK and European Mechanics
Spanner is the standard term for wrench in the UK, Ireland, Australia, and most Commonwealth countries. Modern UK spanners are metric, identical in size to continental European spanners. Older British vehicles built before 1980 used Whitworth and BSF (British Standard Fine) thread standards with their own spanner sizes; see the thread size chart for the conversion.
Reading a Worn or Unmarked Wrench
When a wrench's stamped size has worn away, measure the jaw opening of an open-end wrench, or the internal hex opening of a box end, with calipers across two opposite flats. Match that decimal reading against the socket conversion chart to identify both the size and its closest equivalent in the other system. A quick field check without calipers: try the wrench on a fastener of known size first, since a snug fit with no play confirms the size faster than guessing from the wrench's worn markings.
Wrench Conversion FAQ
What is the metric equivalent of a 1/2 inch wrench?
A 1/2 inch wrench is 12.7mm exactly. The closest metric wrench is 13mm, which is 0.012 inch larger, so use it with caution on tight fasteners.
Are open-end and box-end wrenches the same size?
Yes. A 17mm combination wrench has a 17mm open end and a 17mm box end. Box ends grip all six flats and resist rounding better.
What metric wrench is closest to 9/16?
14mm is the closest, at 0.012" smaller. It will fit a 9/16" fastener with some slop.
Do premium wrench brands fit better?
Yes. Snap-on, GearWrench, and Wera hold tighter manufacturing tolerances than budget brands, so a slightly cross-system fit is more secure.
What is a spanner?
Spanner is the British and European term for what Americans call a wrench. Sizes are identical; only the terminology differs.
Can I use a wrench and a socket interchangeably on the same bolt?
Yes, for turning the fastener, since both measure the same distance across the flats. A socket is generally faster with a ratchet, while a wrench works better in tight clearance where a socket and ratchet head won't fit.