Drill Bit Size Chart: mm to Inches & Fractions

Short answer: 1 inch is exactly 25.4 mm, so divide a metric size by 25.4 to get decimal inches. The sizes people ask about most: 5 mm = 0.197” (≈ 13/64”), 6 mm = 0.236” (≈ 15/64”), 8 mm = 0.315” (≈ 5/16”), 10 mm = 0.394” (≈ 25/64”) and 12 mm = 0.472” (≈ 15/32”). Below are two full charts — metric to inch (3–40 mm) and inch to metric (1/16”–1-1/2”) — with the actual difference in millimetres, which is the number that decides whether a substitute bit is good enough.

Last Updated: 2026-10-24

Why this chart has a “difference” column

Most conversion charts stop at “8 mm ≈ 5/16”“. That answer is fine for a pilot hole in pine. It is not fine when the hole has to hold a wall plug, a concrete screw or an expansion anchor, because those are designed around one hole diameter and lose holding power when the hole is a few tenths of a millimetre too big.

For a masonry drill bit, the practical question is rarely “what is 8 mm in inches”. It is “I have an inch anchor and a box of metric bits — will it work?” The difference column answers that: positive means the inch bit drills bigger than the metric bit, negative means smaller.

Two more details matter for masonry work:

• Masonry and SDS bits use a coarser inch series than twist drills. HSS twist drills come in 1/64” steps up to 1/2”. Carbide masonry and SDS bits are normally sold in 1/8, 5/32, 3/16, 7/32, 1/4, 5/16, 3/8, 7/16, 1/2 and so on. So the nearest inch masonry bit is often further from your metric size than the nearest fraction.
• A carbide bit drills a hole slightly larger than its nominal size. The tip is ground a little over nominal so the hole still fits the anchor after the tip wears. How much over is controlled by test specifications such as PGM, which is why anchor makers ask for certified bits. A worn or badly ground bit adds more on top of the conversion error.

Metric to inch drill bit chart (3–40 mm)

The last column shows which of our series stock that exact metric diameter (SDS Plus = DH01, SDS Max = DH02, Hex shank = DH03, straight-shank impact masonry = DH04). “—” means it is not a standard size in our catalogue.

Metric bit	Decimal inch	Closest fractional size (twist-drill steps)	Closest common masonry inch size	Difference vs masonry size	Dehui series stocked
3 mm	0.1181”	1/8” (+0.17 mm)	1/8” (3.17 mm)	+0.17 mm	Straight
3.5 mm	0.1378”	9/64” (+0.07 mm)	1/8” (3.17 mm)	−0.33 mm	—
4 mm	0.1575”	5/32” (−0.03 mm)	5/32” (3.97 mm)	−0.03 mm	SDS Plus, Straight
4.5 mm	0.1772”	11/64” (−0.13 mm)	3/16” (4.76 mm)	+0.26 mm	—
5 mm	0.1969”	13/64” (+0.16 mm)	3/16” (4.76 mm)	−0.24 mm	SDS Plus, Straight
5.5 mm	0.2165”	7/32” (+0.06 mm)	7/32” (5.56 mm)	+0.06 mm	SDS Plus
6 mm	0.2362”	15/64” (−0.05 mm)	1/4” (6.35 mm)	+0.35 mm	SDS Plus, Hex, Straight
6.5 mm	0.2559”	1/4” (−0.15 mm)	1/4” (6.35 mm)	−0.15 mm	SDS Plus
7 mm	0.2756”	9/32” (+0.14 mm)	1/4” (6.35 mm)	−0.65 mm	SDS Plus, Straight
7.5 mm	0.2953”	19/64” (+0.04 mm)	5/16” (7.94 mm)	+0.44 mm	—
8 mm	0.3150”	5/16” (−0.06 mm)	5/16” (7.94 mm)	−0.06 mm	SDS Plus, SDS Max, Hex, Straight
8.5 mm	0.3346”	21/64” (−0.17 mm)	5/16” (7.94 mm)	−0.56 mm	—
9 mm	0.3543”	23/64” (+0.13 mm)	3/8” (9.52 mm)	+0.52 mm	SDS Plus
9.5 mm	0.3740”	3/8” (+0.02 mm)	3/8” (9.52 mm)	+0.02 mm	—
10 mm	0.3937”	25/64” (−0.08 mm)	3/8” (9.52 mm)	−0.48 mm	SDS Plus, SDS Max, Hex, Straight
11 mm	0.4331”	7/16” (+0.11 mm)	7/16” (11.11 mm)	+0.11 mm	SDS Plus
12 mm	0.4724”	15/32” (−0.09 mm)	1/2” (12.70 mm)	+0.70 mm	SDS Plus, SDS Max, Hex, Straight
13 mm	0.5118”	1/2” (−0.30 mm)	1/2” (12.70 mm)	−0.30 mm	SDS Plus, Hex
14 mm	0.5512”	9/16” (+0.29 mm)	9/16” (14.29 mm)	+0.29 mm	SDS Plus, SDS Max, Hex, Straight
15 mm	0.5906”	19/32” (+0.08 mm)	9/16” (14.29 mm)	−0.71 mm	SDS Plus
16 mm	0.6299”	5/8” (−0.12 mm)	5/8” (15.88 mm)	−0.12 mm	SDS Plus, SDS Max, Hex, Straight
16.5 mm	0.6496”	21/32” (+0.17 mm)	5/8” (15.88 mm)	−0.62 mm	SDS Plus
17 mm	0.6693”	21/32” (−0.33 mm)	11/16” (17.46 mm)	+0.46 mm	—
18 mm	0.7087”	23/32” (+0.26 mm)	11/16” (17.46 mm)	−0.54 mm	SDS Plus, SDS Max, Hex, Straight
19 mm	0.7480”	3/4” (+0.05 mm)	3/4” (19.05 mm)	+0.05 mm	SDS Plus
20 mm	0.7874”	25/32” (−0.16 mm)	13/16” (20.64 mm)	+0.64 mm	SDS Plus, SDS Max, Hex, Straight
22 mm	0.8661”	7/8” (+0.22 mm)	7/8” (22.22 mm)	+0.22 mm	SDS Plus, SDS Max, Hex
23 mm	0.9055”	29/32” (+0.02 mm)	7/8” (22.22 mm)	−0.78 mm	SDS Plus
24 mm	0.9449”	15/16” (−0.19 mm)	1” (25.40 mm)	+1.40 mm	SDS Plus
25 mm	0.9843”	31/32” (−0.39 mm)	1” (25.40 mm)	+0.40 mm	SDS Plus, SDS Max, Hex
26 mm	1.0236”	1” (−0.60 mm)	1” (25.40 mm)	−0.60 mm	SDS Plus
28 mm	1.1024”	1-1/8” (+0.57 mm)	1-1/8” (28.57 mm)	+0.57 mm	SDS Plus, SDS Max, Hex
30 mm	1.1811”	1-3/16” (+0.16 mm)	1-1/8” (28.57 mm)	−1.43 mm	SDS Plus, SDS Max, Hex
32 mm	1.2598”	1-1/4” (−0.25 mm)	1-1/4” (31.75 mm)	−0.25 mm	SDS Plus, SDS Max, Hex
35 mm	1.3780”	1-3/8” (−0.08 mm)	1-3/8” (34.92 mm)	−0.08 mm	SDS Max, Hex
38 mm	1.4961”	1-1/2” (+0.10 mm)	1-1/2” (38.10 mm)	+0.10 mm	SDS Max, Hex
40 mm	1.5748”	1-1/2” (−1.90 mm)	1-1/2” (38.10 mm)	−1.90 mm	SDS Max, Hex

How to read it

• 8 mm, 9.5 mm, 16 mm and 19 mm sit almost on an inch size (5/16”, 3/8”, 5/8”, 3/4”). These are the safest swaps.
• 6 mm, 10 mm and 12 mm — three of the most common anchor and plug sizes — are each 0.35–0.7 mm away from the nearest inch masonry bit. Swapping them is where most “the anchor spins in the hole” complaints start.
• Above 20 mm the gaps vary between 0.1 and 0.7 mm. At these sizes you are usually drilling for chemical anchors, rebar or pipe penetrations, where the anchor or system data sheet gives a hole size you should follow exactly.

Inch to metric drill bit chart (1/16”–1-1/2”)

Use this when a drawing, anchor box or customer order gives an inch size and you stock metric bits. The last column is the nearest diameter we make as a standard masonry size.

Inch bit	Decimal inch	Exact mm	Closest metric bit	Difference	Closest masonry size we stock
1/16”	0.0625”	1.587 mm	1.5 mm	−0.09 mm	3 mm (+1.41 mm)
1/8”	0.1250”	3.175 mm	3 mm	−0.17 mm	3 mm (−0.17 mm)
5/32”	0.1562”	3.969 mm	4 mm	+0.03 mm	4 mm (+0.03 mm)
3/16”	0.1875”	4.762 mm	5 mm	+0.24 mm	5 mm (+0.24 mm)
7/32”	0.2188”	5.556 mm	5.5 mm	−0.06 mm	5.5 mm (−0.06 mm)
1/4”	0.2500”	6.350 mm	6.5 mm	+0.15 mm	6.5 mm (+0.15 mm)
5/16”	0.3125”	7.938 mm	8 mm	+0.06 mm	8 mm (+0.06 mm)
3/8”	0.3750”	9.525 mm	9.5 mm	−0.02 mm	10 mm (+0.48 mm)
7/16”	0.4375”	11.112 mm	11 mm	−0.11 mm	11 mm (−0.11 mm)
1/2”	0.5000”	12.700 mm	12.5 mm	−0.20 mm	13 mm (+0.30 mm)
9/16”	0.5625”	14.287 mm	14 mm	−0.29 mm	14 mm (−0.29 mm)
5/8”	0.6250”	15.875 mm	16 mm	+0.12 mm	16 mm (+0.12 mm)
11/16”	0.6875”	17.462 mm	17 mm	−0.46 mm	18 mm (+0.54 mm)
3/4”	0.7500”	19.050 mm	19 mm	−0.05 mm	19 mm (−0.05 mm)
13/16”	0.8125”	20.637 mm	21 mm	+0.36 mm	20 mm (−0.64 mm)
7/8”	0.8750”	22.225 mm	22 mm	−0.22 mm	22 mm (−0.22 mm)
1”	1.0000”	25.400 mm	25 mm	−0.40 mm	25 mm (−0.40 mm)
1-1/8”	1.1250”	28.575 mm	29 mm	+0.43 mm	28 mm (−0.57 mm)
1-1/4”	1.2500”	31.750 mm	32 mm	+0.25 mm	32 mm (+0.25 mm)
1-3/8”	1.3750”	34.925 mm	35 mm	+0.08 mm	35 mm (+0.08 mm)
1-1/2”	1.5000”	38.100 mm	38 mm	−0.10 mm	38 mm (−0.10 mm)

When a near size is fine, and when it is not

Job	Is a ±0.3 mm substitute OK?	Why
Pilot holes in wood, clearance holes in sheet metal	Yes	The screw thread or bolt does not depend on the hole wall
Plastic wall plugs in brick and block	Usually, if it is smaller — not larger	A larger hole lets the plug spin; a slightly smaller one just needs a firmer tap
Concrete screws (Tapcon-type and metric)	No	The thread cuts into the hole wall; the bit size is part of the screw’s rating
Wedge, sleeve and drop-in anchors	No	Expansion depends on the hole diameter; oversize holes reduce pull-out load
Chemical anchors and rebar	Only within the range the resin data sheet gives	Oversize holes need more resin and change the bond result

For anchor-by-anchor hole sizes, see our anchor hole size chart. If your project specifies certified holes, our PGM certification guide explains what that tests.

Inch sizes for distributors who sell both systems

Distributors in markets that mix European and American tools (the Gulf, parts of Southeast Asia, Latin America) often carry both. These pairs cover most of the overlap:

• 5/32” ↔ 4 mm (−0.03 mm) and 5/16” ↔ 8 mm (−0.06 mm): treat as the same size.
• 3/16” ↔ 5 mm (+0.24 mm): close enough for plugs, not for 5 mm concrete screws.
• 1/4” ↔ 6 mm / 6.5 mm: 1/4” is 6.35 mm, between the two. Stock both metric sizes if you sell into anchor work.
• 3/8” ↔ 10 mm (−0.48 mm) and 1/2” ↔ 12 / 13 mm: not interchangeable for anchors. Keep separate SKUs.
• 5/8” ↔ 16 mm and 3/4” ↔ 19 mm: near-exact, a single SKU can serve both markets for general drilling.

Our standard range is metric: SDS Plus bits from 4 to 32 mm, SDS Max bits from 8 to 40 mm, straight-shank masonry bits from 3 to 20 mm and hex shank bits from 6 to 40 mm. If your market works in inches, put the inch sizes in your quote request and we will tell you what we can supply.

How the numbers were calculated

Every value is calculated from 1 inch = 25.4 mm (the international inch, exact since 1959). Decimal inches are rounded to four places; differences to 0.01 mm. “Closest fractional size” uses 1/64” steps up to 1/2”, 1/32” steps to 1” and 1/16” above, which matches common twist-drill sets. “Closest common masonry inch size” uses the sizes masonry and SDS bits are normally sold in. The stocked-size column is generated from our current catalogue of 417 SDS Plus, SDS Max, hex and straight-shank SKUs.