6.8 mm Drill Bit Size | Metric To Inches, Uses & M8 Tapping

6.8 mm equals 0.2677 inches, a metric drill bit size commonly used as a tap drill for M8 threads.

A 6.8 mm drill bit sits in a useful middle ground of metric sizes: it’s not a standard step up from 6.5 mm, but it’s a workhorse for anyone cutting M8 threads. The size matters most in metalworking, where matching the drill to the thread pitch is the difference between a clean thread and a stripped one. Here’s what the number means, where it’s used, and what to watch for.

6.8 mm Drill Bit In Inches: Fractional Equivalent

The decimal conversion is exact and simple: divide millimeters by 25.4. That puts 6.8 mm at 0.2677 in. For fractional bits, most conversion charts round this to 17/64 in. The catch is that 17/64 in equals 6.747 mm — about 0.053 mm smaller than 6.8 mm.

When you’re choosing between the two, it comes down to how precise the hole must be. For tapping M8 threads, the exact 6.8 mm bit is the safer pick. The fractional bit drills a hole roughly 0.05 mm undersized, which changes how much material the tap has to cut.

Why 6.8 mm Is A Common M8 Tap Drill

For ISO metric threads, the rule is straightforward: tap drill size = major diameter – pitch. M8 threads commonly use a 1.25 mm pitch, so 8 – 1.25 = 6.75 mm. The nearest available metric bit to that is 6.8 mm, which explains why it shows up so often in tapping charts.

That formula points to the one caveat that matters: M8 threads also come in other pitches. M8 x 1.0 and M8 x 1.25 are the common ones, and the formula changes the drill size for each. Always confirm the pitch on the fastener before you drill, and verify the thread engagement you need — softer materials often call for a larger hole to avoid tapping issues.

Specs And Options For 6.8 mm Bits

Commercial 6.8 mm bits come in a range of point angles, shank types, and lengths. McMaster-Carr’s listings show common configurations: 118° standard points for general-purpose drilling, 135° split points for starting on curved surfaces without wandering, and round shanks across a range of overall lengths. Examples from their catalog include a 109 mm overall length with a 55.4 mm drilling depth, a 156 mm length with a 102 mm depth, and shorter 74 mm screw-machine bits with shallower depths.

Manufacturers vary in how they build the same nominal size:

• Cleveland makes a jobber-length version with a 118° point, HSS oxide finish, and DIN 338 / ASME B94.11M compliance.
• Dormer Pramet’s Series 2A 6.8 mm bit runs 109 mm overall with a 69 mm flute length, 2 flutes, and a straight HSS shank.
• Walter offers a screw-machine style with a 74 mm overall length and an H8 diameter tolerance for tighter fit control.

When you’re shopping for a specific job, match the bit’s features to the task: a split point helps on curved or uneven starts, longer overall lengths reach deeper holes, and the H8 tolerance on some models matters when you need a precise fit. If you’re ready to compare models side by side, our roundup of the best 6.8 mm drill bits breaks down the top options for tapping and general drilling.

Workaround: Fractions Vs. The Exact Metric Bit

When charts aren’t handy, the conversion formula — divide millimeters by 25.4 — gets you the decimal, and then you find the nearest smaller and larger fractional bit to bracket the size. For 6.8 mm, that’s 17/64 in close by on one side and 11/32 in (8.73 mm) further out on the other.