Metric vs SAE Bolt Sizes: Complete Conversion Chart

The hardware store aisle has two worlds living side by side. One uses millimeters, the other uses fractional inches. A bolt from each world can look identical from across the workbench — and swapping them is one of the most common causes of stripped threads in home garages and small shops.

This guide is the one chart you keep bookmarked: every common size in both systems, thread pitches side by side, and a warning section on the pairs that are deceptively close.

The Metric System (ISO)

Metric bolts are specified as M[diameter] × [pitch] × [length], all in millimeters. The M means “metric”. Pitch is the axial distance between thread crests. A metric bolt can be coarse (standard) or fine; coarse is default and used in roughly 90% of applications.

Examples: M6 × 1.0 × 20, M10 × 1.5 × 40, M12 × 1.75 × 50.

The SAE / UNC / UNF System

Imperial bolts come in two main pitch flavors: UNC (Unified Coarse) and UNF (Unified Fine). Diameter is listed either as a fractional inch (1/4", 3/8") or as a gauge number for small sizes (#6, #8, #10). Pitch is given in TPI — Threads Per Inch.

Examples: 1/4-20 × 1" (UNC), 3/8-24 × 1-1/2" (UNF).

The Full Conversion Chart

Below are the most common sizes you’ll encounter, with coarse pitch for each system and the measured diameter difference.

The Dangerous Lookalikes

Three metric/imperial pairs cause most of the cross-threading disasters in home shops. Memorize these three.

M6 vs 1/4-20 — 0.35 mm Apart

Often confused because both are the “small but not tiny” bolt used on electronics chassis, brackets, and camera tripods. The 1/4" bolt will feel loose in an M6 hole; the M6 will jam partway into a 1/4-20 hole. A caliper tells you instantly which one you have.

M8 vs 5/16-18 — 0.06 mm Apart (The Worst One)

This is the pair that fools experienced mechanics. M8 is 8.00 mm; 5/16" is 7.94 mm. That’s a 60-micron difference — less than a human hair. You cannot measure this by eye. If you feel an M8 bolt “almost threading” into a 5/16-18 hole, stop. The pitches (1.25 mm vs 1.41 mm) are incompatible and you’ll pop crests off the bolt in seconds.

M10 vs 3/8-16 — 0.47 mm Apart

The most common automotive mistake. Import vehicles use M10 everywhere; domestic pre-1980 use 3/8". Post-1980 domestics are almost all metric, but you’ll still find 3/8" on trailer hitches, aftermarket accessories, and older farm equipment.

How to Tell Metric from SAE Without Measuring

• Head markings — numeric stamps (8.8, 10.9) = metric. Radial lines = SAE.
• Country of origin — vehicles and equipment from Japan, Germany, Korea, China are metric. Older US or UK gear is often imperial.
• Application — aerospace, plumbing, and woodworking in the US are typically imperial. Automotive, electronics, and industrial globally are metric.
• Wrench fit — if a 13 mm wrench fits perfectly it is metric; a 1/2" wrench that feels tight but sloppy suggests an M12 head (19 mm AF).

When Does It Really Matter?

For a wall-mounted shelf bracket, substituting a slightly oversize bolt and a new hole gets the job done. For anything that sees cyclic load, vibration, or clamping torque — engine mounts, suspension, brake calipers, structural brackets — the correct bolt is non-negotiable. Wrong pitch means partial thread engagement, which means a bolt rated for 10,000 lbs might shear at 2,000.