Metric ↔ SAE Wrench, Socket & Bolt Size Converter

Convert any metric wrench, socket, or bolt size to its SAE (inch / fractional) equivalent and back. 8 mm ≈ 5/16", 10 mm ≈ 3/8", 13 mm ≈ 1/2", 17 mm ≈ 11/16", 19 mm ≈ 3/4". The interactive tool below does the math — the chart and explainer underneath cover when a swap actually fits and when the 0.4 mm of slop will round the head.

Quick mm → SAE wrench size reference

8 mm = 5/16" (exact = 7.94 mm) • 10 mm ≈ 3/8" (exact = 9.53 mm, 0.47 mm loose) • 11 mm ≈ 7/16" (exact = 11.11 mm, snug) • 13 mm = 1/2" (exact = 12.70 mm, 0.30 mm loose — use 12-point) • 14 mm ≈ 9/16" (exact = 14.29 mm) • 16 mm ≈ 5/8" (exact = 15.88 mm) • 17 mm ≈ 11/16" (exact = 17.46 mm, will slip on rust) • 19 mm = 3/4" (exact = 19.05 mm, near perfect) • 21 mm ≈ 13/16" (lug nut size) • 22 mm = 7/8" (exact = 22.23 mm).

When can you actually substitute SAE for metric (and vice versa)?

Substitution is safe when the size difference is under 0.3 mm and you are using a 6-point socket on a sharp-edged bolt. The classic safe swaps: 19 mm = 3/4", 13 mm = 1/2", 8 mm = 5/16". Marginal: 10 mm / 3/8", 17 mm / 11/16" — these will round a rusted or already-rounded bolt head. Never substitute: anything Toyota or Honda — OEM specs metric and the head clearances are tight; an SAE socket on a 12 mm Honda axle bolt will strip the head and you will be drilling it out.

Why do mm and inch sockets exist at all?

Cars and equipment built for the US market through the 1980s used SAE (Society of Automotive Engineers) inch fasteners. Imports and most modern equipment use metric. After ~1992 nearly every passenger vehicle sold in the US switched to metric — including the Big Three. Today the only places you still find SAE bolts in volume are older domestic trucks, lawn/garden equipment, some appliances, and ham-radio/electronics hardware. If your project is post-1995 and not lawn equipment, assume metric.

Common search shortcut: what is 10 mm in SAE?

10 mm is closest to 3/8" (which equals 9.53 mm — about 0.47 mm smaller). The 3/8" will go on a sharp-edged 10 mm bolt head but will slip on anything rusted. The next size up is 7/16" at 11.11 mm — too loose. The honest answer: buy a 10 mm socket. Same logic for 13 mm (use 1/2" if you must, but real 13 mm is better), 14 mm (use 9/16"), and 17 mm (use 11/16" carefully).

Metric bolt diameter vs wrench size — do not confuse them

M10 = a bolt with 10 mm shank diameter, but the wrench size for an M10 hex head is typically 17 mm (sometimes 15 mm on flange bolts). M8 = 13 mm wrench, M6 = 10 mm wrench, M12 = 19 mm wrench, M14 = 22 mm wrench. The converter above handles both — pick "bolt diameter" to look up wrench size, or pick "wrench size" to look up the bolt that fits.

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