Question 1

How is the "nearest" cross-system match found?

Accepted Answer

Every fastener size, in every system, is converted to its major diameter in millimeters. The nearest match in another system is simply the entry whose diameter is closest to that value. Metric, UNC, UNF, and wood/sheet-metal screw gauge don't line up exactly, so this is always an approximation — there is no bolt that is "exactly" both M6 and 1/4-20, only two sizes that are close (6mm vs 6.35mm).

Question 2

How is tap drill size calculated?

Accepted Answer

For metric threads: tap drill (mm) = major diameter − thread pitch. For UNC/UNF threads: tap drill (in) = major diameter − 1/TPI. Both are the standard approximation formulas for roughly 75% thread engagement, which is the typical target for general-purpose tapped holes. Machinists usually round the result to the nearest standard numbered, lettered, or fractional drill bit they have on hand.

Question 3

Why is there no tap drill size for wood screws?

Accepted Answer

Wood isn't tapped with a fixed thread the way metal is — a wood screw cuts its own thread as it's driven in, and the right pilot hole size depends on the wood's hardness, not a standardized thread spec. For that reason this tool doesn't show a tap drill size for wood/sheet-metal screw gauge input; as a rule of thumb, a pilot hole close to the screw's minor (root) diameter works for most softwoods, and closer to the major diameter shown here for hardwoods.

Question 4

What is the difference between UNC and UNF threads?

Accepted Answer

UNC (Unified Coarse) and UNF (Unified Fine) are both standard US inch thread series. For the same nominal diameter (e.g. 1/4"), UNC has fewer, deeper threads per inch (1/4-20) while UNF has more, shallower threads per inch (1/4-28). UNF bolts generally resist loosening from vibration better and are common in automotive and precision applications; UNC is the more common general-purpose choice.

Question 5

Is a metric M6 bolt the same as a 1/4-20 UNC bolt?

Accepted Answer

No, but they're close enough that people often substitute one for the other in non-critical applications. M6 is 6.0mm (0.236") in diameter, while 1/4-20 is 0.25" (6.35mm) — a difference of about 0.35mm. They will not interchange in a tapped hole or nut sized for the other.

Question 6

Does wood screw gauge number mean the same thing as a machine screw number?

Accepted Answer

The gauge numbers (like #6, #8, #10) use the same numbering convention and the same major diameters for both wood screws and small machine screws — a #8 wood screw and a #8-32 machine screw both have a 0.164" major diameter. The difference is the thread: machine screws have a fixed, standardized TPI cut for a nut or tapped hole, while wood screw threads are coarser and self-tapping, with no fixed TPI standard.

Screw, Bolt & Fastener Size Converter

Select a Size

Equivalent Sizes

Frequently Asked Questions

How is the "nearest" cross-system match found?

How is tap drill size calculated?

Why is there no tap drill size for wood screws?

What is the difference between UNC and UNF threads?

Is a metric M6 bolt the same as a 1/4-20 UNC bolt?

Does wood screw gauge number mean the same thing as a machine screw number?

Related Tools

Length Converter

Weight Converter

Knitting Needle & Crochet Hook Size Converter

Tire Size & Speedometer Error Calculator