Rock Drill Bit Sizes Explained: The Complete DTH Bit Size Guide (With Chart)

Drill Bit Sizing

DTH Bits

Rock drill bit sizes typically range from 2-3/4″ (70mm) to 42″ (1067mm) in diameter, with the correct size determined by three factors: your DTH hammer class, your required borehole diameter, and your application — whether that’s water well drilling, mining, construction, or quarrying. Choosing the wrong size doesn’t just slow the job down. An under-gauge bit produces holes too small for your casing program, while an oversized bit starves the hammer of energy, increases air consumption, and drives up your cost per foot.

At Drill King International, we manufacture DTH drill bits across one of the widest size ranges in the industry, and one of the most common questions our team answers is deceptively simple: “What size bit do I actually need?” This guide breaks down how rock drill bit sizing works, provides a practical DTH hammer-to-bit size chart, and walks through the right sizes for the most common drilling applications.

How Rock Drill Bit Sizes Are Measured

The gauge buttons — not the bit body — determine the effective drilling diameter

Rock drill bit size refers to the effective drilling diameter — the measurement across the gauge buttons that determines the final hole size, not the body of the bit itself. This is the number that matters when matching a bit to your casing, screen, or blast hole design.

Two measurement systems are used side by side in the drilling industry:

Imperial (inches) — standard across North America and much of the oilfield-adjacent drilling world. Bits are specified as 3-1/2″, 4″, 6″, 8″, and so on.

Metric (millimeters) — standard in Europe, Asia, Australia, and most international tenders. Common examples include 90mm, 115mm, 152mm, and 203mm.

Where Confusion Creeps In

DTH hammers are almost always referred to in inches (a “4-inch hammer” or “6-inch hammer“), even on projects where the bit diameter is specified in millimeters. A 6″ DTH hammer doesn’t drill a 6” hole — it drills a range of hole sizes depending on the bit fitted to it. Getting comfortable converting between the two systems prevents costly specification and procurement errors, especially on international projects.

Bit Diameter vs. Shank Size

The bit diameter determines the size of the hole you drill; the shank determines whether the bit physically fits your hammer. These are separate specifications, and both must be correct.

The shank is the splined rear connection that transfers percussive energy from the DTH hammer’s piston into the bit. Each hammer class uses a defined shank profile, and industry-standard shank series — such as DHD, QL, SD, Mission, and COP — are not interchangeable. A 4″ bit with a DHD340 shank will not fit a hammer built for a QL40 shank, even though both are “4-inch” tools.

Before You Order

Always confirm two things: the bit diameter your hole design requires, and the exact shank series of the hammer it will run on. If you’re unsure which shank your hammer uses, our technical team can identify it from the hammer model — it’s a two-minute conversation that avoids an expensive mismatch.

DTH Bit Size Chart: Hammer Class to Bit Diameter

The chart below shows the typical bit diameter range for each standard DTH hammer class. Use it as a first-pass reference — the exact working range varies slightly by hammer model and shank series.

DTH Hammer Class	Typical Bit Diameter (in)	Typical Bit Diameter (mm)	Common Applications
2″ – 2.5″	2-3/4″ – 3-1/2″	70 – 90mm	Micro-piling, anchors, tight-access geotechnical work
3″	3-1/2″ – 4″	85 – 105mm	Blast holes, anchors, small water bores
3.5″	3-3/4″ – 4-1/2″	95 – 115mm	Quarry blast holes, geotechnical investigation
4″	4-1/4″ – 5″	110 – 130mm	Blast hole drilling, water wells, geothermal loops
5″	5-1/4″ – 6-1/2″	134 – 165mm	Mining production holes, water wells
6″	6-1/4″ – 8″	156 – 203mm	Water wells, geothermal, construction piling
8″	8-1/2″ – 10″	216 – 254mm	Large water wells, foundation drilling
10″	10-5/8″ – 12-1/4″	270 – 311mm	Deep water wells, large-diameter foundations
12″ +	12-1/4″ – 42″	311 – 1067mm	Large-diameter piling, caissons, shaft drilling (often with hole openers)

Two practical notes on reading this chart. First, every hammer performs best in the lower-to-middle portion of its bit range — running the largest possible bit on a hammer works, but penetration rates drop because the same impact energy is spread across a larger face. Second, for holes beyond roughly 12″ – 15″, the standard approach shifts from a single large bit to a pilot bit followed by a DTH hole opener, which enlarges the pilot hole in stages. Drill King manufactures hole openers from 6″ up to 36″ diameters for exactly this kind of staged enlargement.

Choosing the Right Bit Size by Application

The correct bit size is driven by engineering requirements — casing programs, yield targets, blast designs — not by preference. Here’s how sizing typically breaks down across the major drilling sectors.

Matching bit diameter and shank series to the hammer assembly is application-specific

01

Most residential and agricultural water wells are drilled with bits in the 6″ – 10″ (152 – 254mm) range. The governing factor is the casing and screen diameter needed to achieve the target yield: the borehole must be large enough to accommodate the casing plus an annular space for gravel pack and grouting. A common configuration is a 6-1/4″ or 6-1/2″ bit for 4″ – 5″ casing, stepping up to 8″ – 10″ bits where 6″ casing or higher-yield screens are specified. Municipal and irrigation wells frequently push into the 10″ – 12″+ range. If you’re sizing a bit for a cased well, work backwards from the casing outside diameter and add the annular clearance your completion design requires.

02

Bench blast holes in mining and quarrying most commonly use bits between 3-1/2″ and 7″ (90 – 178mm). Hole diameter here is dictated by the blast design — burden, spacing, and explosive charge calculations all key off the hole size — as well as bench height and rig capability. Smaller quarry operations typically run 3-1/2″ – 4-1/2″ bits on 3″ – 4″ hammers, while larger open-pit production drilling moves up to 5″ – 7″ holes.

03

Construction drilling spans the widest size spectrum of any sector. Geotechnical investigation and micro-piling work sits at the small end, often 2-3/4″ – 4″ bits, while rock-socketed piles, caissons, and foundation shafts can require anything from 12″ to 42″ diameters, achieved with large-diameter bits, hole openers, or casing advance systems. Where unstable overburden must be supported during drilling, casing advancement systems drill and case simultaneously — with the casing size selected first and the pilot bit and ring bit sized to match.

04

Closed-loop geothermal boreholes are typically drilled at 4-1/2″ – 6″ (115 – 152mm), balancing enough diameter for the loop pipe and grout against the cost per foot of drilling. Deeper commercial geothermal wells follow sizing logic closer to water well practice, stepping down in diameter through cased sections.

What About Casing Sizes?

Casing size and bit size are designed together, not separately. In any cased hole — water wells, foundation work, unstable overburden — the drill bit must produce a hole with sufficient clearance over the casing’s outside diameter, typically at least 1″ – 2″ of total annular space depending on depth and grouting requirements.

In casing advance drilling, this relationship is built into the system: the pilot bit and ring bit are engineered to cut a hole precisely sized for the casing being simultaneously installed. If your project specifies casing first (as most do), your bit size is effectively decided for you — which is why we always recommend confirming the full casing program before ordering bits.

Getting the Size Right: A Quick Checklist

Before you order your next rock drill bit, run through these five checks:

1

Confirm the required hole diameter from the casing program, blast design, or engineering specification.

2

Match the bit to your hammer class — the bit diameter should sit comfortably within the hammer’s working range, ideally not at its maximum.

3

Verify the shank series (DHD, QL, SD, Mission, COP, etc.) against your hammer model.

4

Check mm/inch conversions twice on international specifications.

5

Consider the formation — face design and button configuration should match the rock, but only after diameter and shank are locked in.

Selecting the correct rock drill bit size comes down to matching three things: the hole your project requires, the hammer you’re running, and the shank that connects them. Drill King International manufactures DTH drill bits from 2-3/4″ (70mm) to 42″ (1067mm), with popular sizes held in stock for fast delivery and full customisation available for specialised requirements.

Frequently Asked Questions

What sizes do rock drill bits come in?

Rock drill bits for DTH drilling range from around 2-3/4″ (70mm) up to 42″ (1067mm) in diameter. The most commonly used sizes fall between 3-1/2″ and 10″, covering the bulk of blast hole, water well, geothermal, and construction drilling. Diameters above roughly 15″ are usually achieved with DTH hole openers enlarging a pilot hole rather than a single bit.

What size drill bit do I need for a water well?

Most water wells use bits between 6″ and 10″ (152 – 254mm), sized to fit the casing and screen diameter plus annular clearance for gravel pack and grout. A typical residential well running 4″ – 5″ casing is drilled with a 6-1/4″ – 6-1/2″ bit, while wells with 6″ casing or larger generally require 8″ – 10″ bits.

Does a 6-inch DTH hammer drill a 6-inch hole?

Not exactly — the hammer class describes the tool, not the hole. A 6″ DTH hammer typically runs bits from about 6-1/4″ up to 8″, so the actual hole diameter depends on the bit fitted. Always specify the bit diameter you need for the hole, then confirm it falls within your hammer’s working range.

How do I know which shank my drill bit needs?

The shank must match your DTH hammer’s series — common standards include DHD, QL, SD, Mission, and COP profiles, and they are not interchangeable. Your hammer’s model designation identifies its shank; if it’s unclear, a bit supplier can confirm it from the hammer make and model before you order.

Are rock drill bit sizes measured in mm or inches?

Both systems are in active use. Inches dominate in North America, while millimeters are standard across Europe, Asia, and Australia. DTH hammers are almost universally described in inches even on metric projects, so double-checking conversions between the two systems is essential to avoid ordering errors.

If you’re unsure which size or shank configuration suits your rig and application, our technical team is ready to help you specify it correctly the first time.