Screw Size Converter (Gauge to Metric)

What it does

Screw sizing has three parallel naming systems that all describe the same physical part — and switching between them is a daily headache for anyone working across regional hardware standards. The US uses gauge numbers (#0, #2, #4, #6, #8, #10, #12 — strange legacy of the Wire Gauge Standard from the 1840s) for screws under 1/4-inch and fractional inches above (1/4", 5/16", 3/8"). Europe, Japan, and most of the world use metric (M2, M3, M4, M5, M6, M8, M10) where the number is the major diameter in millimeters. UK has historic Whitworth and BSF systems, mostly obsolete except in restoration work. A #8 wood screw, an M4 machine screw, and a 0.164-inch diameter rod are all roughly the same thickness — but try finding that conversion in your head while standing in a hardware aisle.

The converter takes any one of those three naming inputs and shows the other two, alongside the relevant thread pitches (UNC coarse vs UNF fine for imperial; standard metric pitch in mm), common available lengths for that size, head-type compatibility (Phillips, flat, Pozidriv, Torx, hex, square), and the rough strength rating. Useful for: matching replacement screws when the original packaging is gone, ordering metric hardware for an imperial project (or vice versa), figuring out which pilot-drill-bit size to pre-drill, and double-checking before you order 100 of something that turns out to be the wrong standard.

Reference points: #6 ≈ M3.5 ≈ 0.138" (small wood screws, drywall screws). #8 ≈ M4 ≈ 0.164" (the most common all-purpose size for cabinetry, framing, and assembly). #10 ≈ M5 ≈ 0.190" (heavier wood and machine screws). 1/4" ≈ M6 ≈ 0.250" (the boundary between “screw” and “bolt” in many catalogs). Above that, fractional and metric standards diverge meaningfully and mixing them causes problems — never try to force an M8 bolt into a 5/16" threaded hole; the pitches don’t match and you’ll strip both.

How to use it

1. Enter your known size — gauge number (#8), metric (M4), or fractional inch (5/32").
2. Read the equivalents in the other two naming systems.
3. Check the listed thread pitches (UNC/UNF for imperial, standard metric pitch).
4. Cross-reference common lengths and head types available for that size.
5. Use the pilot-drill-bit recommendation for pre-drilling wood or soft materials.

When to use this tool

• Replacing a lost screw and need to identify size from a sample.
• Ordering hardware in metric for a project that originally used imperial (or vice versa).
• Sourcing screws from international vendors (AliExpress, McMaster-Carr) where naming may differ.
• Pre-drilling pilot holes — the right bit size depends on screw size and material.

When not to use it

• Precision threaded fasteners for engineering applications — use the actual standard (ANSI B18, ISO 261) not approximations.
• Mating different-standard threads — even when sizes nearly match, thread pitch differs and forcing fasteners damages threads.
• Specialty fasteners (security screws, captive screws, weather-rated marine fasteners) where the head/material/coating matters more than nominal size.
• Whitworth/BSF restoration — those use a different thread angle (55° vs UN’s 60°) and aren’t cross-compatible with modern fasteners.

Common use cases

• Onboarding a colleague who needs the same calculation/conversion
• Verifying a number or output before passing it on
• Quick conversion during a typical workday
• Pre-decision sanity-check on inputs and outputs