About this tool
This calculator gives the wrench or socket size (the width across flats) for standard metric hex bolts from M3 to M48 and SAE hex bolts from 1/4" to 1". Its main differences from a plain chart: it works backwards from a measured nut or bolt head, and when you only own the other measurement system it lists the nearest sockets and marks each one as a fit, slightly loose, or too small to seat.
How to use
- Choose Bolt → wrench size to start from the thread size printed on the bolt or nut, or Wrench → bolt size if you only have a measurement.
- For Bolt → wrench size, pick Metric or SAE and select the thread size, such as M8 or 3/8". The wrench size appears beside the inputs.
- For Wrench → bolt size, enter the width across flats in millimetres or inches (fractions such as 9/16 and 1-1/8 are accepted). The matching bolt sizes appear, closest first.
- Read the "sockets to try" list if you own only the other system. Prefer a socket marked Fits; avoid any marked Too small, will not seat.
- Use Copy result to save the answer, or the size chart below to scan every size and open any row.
How the lookup works
Wrench size is not calculated from the thread diameter: it is set by standards tables. Metric hex bolts and nuts follow ISO 4014/4017 (bolts) and ISO 4032 (nuts); inch hex bolts follow ASME B18.2.1. This tool stores those tables, so every answer is a lookup, not an approximation. The cross-system suggestions are the only computed values: the tool converts the exact wrench size to millimetres, compares it with a standard set of metric or SAE sockets, and reports the closest one plus the nearest socket on either side.
Reading the fit labels
Each substitute socket shows its difference from the exact size. A socket that is smaller than the fastener (marked Too small: will not seat) cannot go over the head at all, however close it looks on paper. A socket slightly larger (Slightly loose) will fit with some play, which is acceptable for light or temporary work but can round the corners of a hex head under high torque. A difference within about 0.1 mm is shown as Fits. For example, an M8 bolt needs 13 mm; the nearest SAE socket, 1/2", is 0.30 mm too small, while 9/16" is 1.29 mm too large, so neither is a good match, and a 13 mm socket is worth borrowing or buying.
Finding a bolt size from a measured nut or head
Measure across two opposite flat faces with calipers, not corner to corner, which reads about 15% larger on a hexagon. Enter the number and the tool lists every standard size within 0.5 mm. A 13 mm reading matches M8 exactly and 5/16" SAE (a 1/2" wrench) as a close second; a 17 mm reading matches no current ISO size but is the older DIN size for M10. If nothing is within 0.5 mm the tool shows only the nearest size and asks you to re-measure, because a worn, painted, or damaged head can read well off the standard. This lookup identifies the wrench size class of a fastener; it does not read thread pitch or length.
The M10, M12, and M14 exception
ISO 272:1982 reduced the across-flats size for M10, M12, and M14 from the older DIN 931/933 values of 17, 19, and 22 mm to the current 16, 18, and 21 mm. Both sets are still in circulation: older equipment, some automotive fasteners, and budget hardware may use the larger size. If the current size feels loose on one of these three sizes, try 1 mm larger. The reverse lookup flags these legacy matches explicitly.
Bolts, nuts, and which fasteners are not covered
A standard hex nut uses the same across-flats size as the hex bolt of the same nominal diameter, so an M8 nut and an M8 bolt head both take 13 mm. The tool does not cover flange bolts or nuts, heavy-hex structural bolts, hex-key (Allen) or Torx drives, square-drive fasteners, or hardware made to non-ISO national standards. For those, measure the fastener directly or check the manufacturer’s datasheet.
Privacy
The size you select and any measurement you enter are processed in your browser. This tool does not send them to a server, and it does not store them after you close or reload the page.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
- Allen Key Size CalculatorDriving a socket head, button head, or set screw instead? Find the Allen (hex) key size.
- Clearance Hole CalculatorDrilling a hole for this bolt to pass through? Find the clearance hole size.
- Tap Drill Size CalculatorDrilling a hole for this same bolt size? Look up the correct tap drill bit before threading it.
Assumptions and limitations
- Covers standard hex bolts and nuts only: ISO metric M3–M48 and ASME inch 1/4"–1". Flange, heavy-hex, structural, Allen/hex-key, Torx, and square-drive fasteners are not included.
- Sizes follow current ISO and ASME tables. M10, M12, and M14 hardware made to the older DIN 931/933 standard needs a socket 1 mm larger than the size shown.
- The metric-to-SAE socket suggestions are the nearest standard sizes, not certified equivalents. Confirm any substitute seats fully before applying significant torque.
- The reverse lookup accepts measurements from 4 mm to 80 mm. Damaged, painted, or worn heads can measure off the standard, so treat a "nearest size only" result as a prompt to re-measure.
- Standard socket sets stop at 1-1/2" (SAE) and 75 mm (metric); the chart shows "-" where no socket within 10% exists.
- This is a reference chart, not engineering advice. It does not give torque values, thread pitch, or material grade.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What size socket do I need for an M8 bolt?
A 13 mm socket or wrench. If you only have SAE sockets, 1/2" (12.7 mm) is the nearest size but is 0.3 mm too small to seat, and 9/16" is about 1.3 mm too large, so a 13 mm socket is the better choice.
How do I find the bolt size from a nut I measured?
Switch to Wrench → bolt size and enter the width across flats: the distance between two opposite flat sides, measured with calipers. The tool lists every standard size within 0.5 mm, including older DIN sizes such as 17 mm for M10.
Why does my M10, M12, or M14 bolt not fit the size shown?
It was probably made to the older DIN 931/933 standard, which used 17, 19, and 22 mm for M10, M12, and M14 instead of today’s 16, 18, and 21 mm. Try a socket 1 mm larger.
Is a 13 mm socket the same as 1/2 inch?
No. A 1/2" socket is 12.7 mm, so it is 0.3 mm smaller than 13 mm. It may turn a loose or worn 13 mm fastener but will not seat on a new, tight one.
Is it safe to use a metric socket on an SAE bolt?
Only if it is the same size or very slightly larger. A smaller socket will not fit, and a much larger one can round the corners of the head. Use the closest socket marked Fits for light work, and buy the correct size for anything you will torque hard.
Is a nut the same wrench size as the bolt?
Yes for standard hex fasteners: a nut and a bolt head of the same nominal diameter (M10 or 3/8", for example) share the same across-flats size.
Does this work for Allen keys, Torx, or flange bolts?
No. This tool covers standard external hex heads and nuts only. Torx, square-drive, flange, and heavy-hex structural fasteners use different sizes. For socket screws driven by an Allen key, use the Allen Key Size Calculator.