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A 3/8 inch bolt takes a 9/16 inch wrench — 0.5625 inches, or 14.29 mm, nearest metric wrench 14 mm. Remember the rule this whole chart runs on: the wrench fits the bolt head, not the shank diameter, so a 3/8 inch bolt never takes a 3/8 inch wrench. Pick your bolt in the calculator or print the full table below.
Choose a bolt from the list — SAE fractional sizes or metric M-sizes — and the calculator returns the hex-head wrench size, its decimal inches, its millimetres, and the nearest size in the other system. The wrench size is the distance across the flats of the bolt head, which is always larger than the bolt's shank diameter.
Use the print button above for a clean copy. Wrench size is the distance across the flats of the hex head. One inch is exactly 25.4 mm, so every millimetre figure below is the inch figure times 25.4, rounded to two decimals.
| Bolt | Wrench size | Wrench decimal (in) | Wrench (mm) |
|---|---|---|---|
| SAE 1/4 in | 7/16 in | 0.4375 | 11.11 |
| SAE 5/16 in | 1/2 in | 0.5000 | 12.70 |
| SAE 3/8 in | 9/16 in | 0.5625 | 14.29 |
| SAE 7/16 in | 5/8 in | 0.6250 | 15.88 |
| SAE 1/2 in | 3/4 in | 0.7500 | 19.05 |
| SAE 9/16 in | 13/16 in | 0.8125 | 20.64 |
| SAE 5/8 in | 15/16 in | 0.9375 | 23.81 |
| SAE 3/4 in | 1-1/8 in | 1.1250 | 28.58 |
| Metric M6 | 10 mm | 0.39 | 10.00 |
| Metric M8 | 13 mm | 0.51 | 13.00 |
| Metric M10 | 16 mm * | 0.63 | 16.00 |
| Metric M12 | 18 mm * | 0.71 | 18.00 |
| Metric M14 | 21 mm * | 0.83 | 21.00 |
| Metric M16 | 24 mm | 0.94 | 24.00 |
| Metric M20 | 30 mm | 1.18 | 30.00 |
* These three sizes vary by standard: M10 heads take 16 or 17 mm wrenches, M12 heads take 18 or 19 mm, and M14 heads take 21 or 22 mm, depending on the fastener standard and manufacturer. Some M10 fasteners in particular ship with the alternate 17 mm head, so check the head with a caliper or a test wrench before buying a single wrench for the job.
Wrench size = distance across the flats of the hex head, taken from the standard hex-head tables for SAE and metric fasteners. Millimetres = inches × 25.4 exactly; inches = millimetres ÷ 25.4. Nearest equivalents are the closest size in the other system's standard wrench set, not an exact conversion: a 14.29 mm wrench has no exact metric twin, so 14 mm is the nearest stocked size, and a 16 mm wrench (0.63 in) sits closest to a 5/8 in wrench (0.625 in).
Setup: 3/8 in bolt.
Line 1 — the wrench. The standard hex head for a 3/8 in bolt is 9/16 in, not 3/8 in — the head is wider than the shank.
Line 2 — the decimal. 9 ÷ 16 = 0.5625 in.
Line 3 — the millimetres. 0.5625 × 25.4 = 14.29 mm.
Line 4 — the nearest metric wrench. The closest stocked metric size to 14.29 mm is 14 mm, close enough to work in a pinch but not an exact fit.
A 1/2 in bolt. It takes a 3/4 in wrench: 0.7500 in, 19.05 mm, nearest metric 19 mm. This is the single most common size confusion in home repair — people reach for the 1/2 in wrench and wonder why it will not fit.
An M10 bolt. It takes a 16 mm wrench (0.63 in, nearest SAE 5/8 in), but check first: some M10 fasteners use the alternate 17 mm head. If the 16 mm wrench feels loose, try 17 mm before forcing anything.
An M20 bolt. It takes a 30 mm wrench, 1.18 in, nearest SAE 1-3/16 in. At this size, use a six-point socket rather than an open-end wrench: the torque is high and a rounded head at M20 is a miserable problem.
| Output | Formula | What it means |
|---|---|---|
| Wrench size | standard hex-head table | 9/16 in at the defaults — the wrench that fits a 3/8 in bolt head. |
| Decimal inches | fraction as a decimal | 0.5625 in at the defaults — the same wrench size, for calipers and comparison. |
| Millimetres | inches × 25.4 | 14.29 mm at the defaults — the same wrench size in metric units. |
| Nearest equivalent | closest stocked size, other system | 14 mm at the defaults — the closest metric wrench, not an exact twin. |
The tables give standard hex-head sizes, which cover the great majority of common bolts, but not every fastener: heavy-hex, structural, and 12-point heads differ, and some manufacturers use non-standard heads. The three starred metric sizes vary by standard (M10: 16/17 mm, M12: 18/19 mm, M14: 21/22 mm), so always verify a starred size on the actual fastener. Nearest equivalents are approximations for emergencies, not replacements: a loose-fitting wrench rounds bolt heads, especially under high torque. Error policy: an unlisted bolt size is reported as an error rather than modeled.
Bright Side Kit. "Wrench Size Chart: SAE & Metric Bolt Guide." https://brightoolbox.com/wrench-size-chart/ Accessed October 24, 2026.Data sourcesA 9/16 inch wrench. That is 0.5625 inches or 14.29 mm, nearest metric wrench 14 mm. The wrench fits the head, which is wider than the 3/8 inch shank.
A 3/4 inch wrench. That is 19.05 mm, nearest metric wrench 19 mm. Reaching for the 1/2 inch wrench is the most common mistake with this size.
Usually 16 mm, sometimes 17 mm. M10 heads vary by standard, so check the actual fastener: if a 16 mm wrench feels loose, try 17 mm before forcing it.
Usually 18 mm, sometimes 19 mm. Like M10 and M14, M12 heads come in two standard sizes depending on the fastener standard and manufacturer.
Because they measure different parts. The bolt size is the shank diameter; the wrench size is the distance across the flats of the hex head, which must be wider than the shank to have any material around it.
In a pinch, if it is the nearest equivalent and fits snugly. A 14 mm wrench on a 9/16 inch head is close, but a loose fit rounds heads, so use the exact size whenever you have it, especially under high torque.
Multiply inches by 25.4. A 9/16 inch wrench is 0.5625 × 25.4 = 14.29 mm. Divide millimetres by 25.4 to go the other way.
A 30 mm wrench. That is 1.18 inches, nearest SAE size 1-3/16 inches. Use a six-point socket at this size to avoid rounding the head.