IADC Rock Bit & PDC Classification
The codes stamped on every roller cone and PDC bit before it ever goes in the hole — formation type, cutting structure, bearing type, bit profile, and body material, decoded.
The first digit is the series (formation type family); the second digit is the sub-series, a finer grade within that same formation range.
| Series | Structure | Formation |
|---|---|---|
| 1 | Milled tooth | Soft formations, low compressive strength — very soft shale to medium lime/shale |
| 2 | Milled tooth | Medium to medium-hard formations, high compressive strength — medium lime/shale to medium-hard lime/sand/slate |
| 3 | Milled tooth | Hard, semi-abrasive formations — hard lime through hard dolomite |
| 4 | TCI / insert | Soft formations, low compressive strength — very soft shale to medium lime/shale |
| 5 | TCI / insert | Soft to medium-hard formations, low compressive strength — very soft shale/sand to medium soft shale/lime |
| 6 | TCI / insert | Medium to hard formations, high compressive strength — medium lime/shale to medium-hard lime/sand/slate |
| 7 | TCI / insert | Hard, semi-abrasive and abrasive formations — hard lime/dolomite to hard dolomite |
| 8 | TCI / insert | Extremely hard and abrasive formations — hard chert to hard granite |
| Digit | Bearing Type |
|---|---|
| 1 | Standard roller bearing, non-sealed |
| 2 | Roller bearing, air cooled, non-sealed |
| 3 | Roller bearing, gauge protected, non-sealed |
| 4 | Sealed roller bearing |
| 5 | Sealed roller bearing, gauge protected |
| 6 | Sealed friction (journal) bearing |
| 7 | Sealed friction bearing, gauge protected |
| 8 | Sealed friction bearing, gauge protected, additional gauge/body protection |
| Letter | Feature |
|---|---|
| A | Air application |
| B | Special bearing bit |
| C | Center jet |
| D | Deviation control |
| E | Extended jets (full-length nozzles) |
| G | Extra gauge / body protection |
| H | Horizontal / steering application |
| J | Jet deflection |
| L | Lug pads |
| M | Motor application |
| S | Standard steel tooth model |
| T | Two-cone bit |
| W | Enhanced cutting structure |
| X | Predominantly chisel-shaped inserts |
| Y | Predominantly conical inserts |
| Z | Other-shaped inserts |
| Letter | Body Material |
|---|---|
| M | Matrix |
| S | Steel |
| D | Diamond (matrix variant, added diamond content) |
| Digit | Formation |
|---|---|
| 1 | Very soft, highly drillable — clay, marl, gumbo, unconsolidated sand |
| 2 | Soft, soft sticky — clay, marl, gumbo, unconsolidated sand with some sticking tendency |
| 3 | Soft to medium — low compressive strength sand, shale, anhydrite with harder streaks |
| 4 | Medium — moderate compressive strength sand, chalk, anhydrite, shale |
| 6 | Medium-hard — higher compressive strength, non- to semi-abrasive sand, shale, lime, anhydrite |
| 7 | Hard — high compressive strength, sharp sand or siltstone layers |
| 8 | Extremely hard and abrasive — dense, sharp formations such as quartzite or volcanic rock |
| Digit | For Formation Types 1–4 |
|---|---|
| 2 | 19 mm PDC cutters |
| 3 | 13 mm PDC cutters |
| 4 | 8 mm PDC cutters |
| Digit | For Formation Types 6–8 |
|---|---|
| 1 | Natural diamond |
| 2 | TSP (thermally stable polycrystalline) |
| 3 | Combination |
| 4 | Impregnated diamond |
| Digit | Profile |
|---|---|
| 1 | Short fishtail |
| 2 | Short profile |
| 3 | Medium profile |
| 4 | Long profile |
Reading a Full Code
Why the Code Is Worth Reading, Not Just Matching
A classification code is a compressed engineering decision — formation strength assessment, cutter technology choice, bearing selection, and bit geometry, all reduced to three or four characters so a bit can be selected, ordered, and compared across manufacturers without a phone call. Reading it rather than just pattern-matching a familiar string means catching, for example, that a series-7 roller cone bit and a formation-type-7 PDC bit are answering the same "hard, semi-abrasive to abrasive formation" question with two structurally different tools — one crushing with carbide inserts on rotating cones, the other shearing with fixed cutters.
Bit type also feeds directly into rig-floor decisions once the bit is actually made up — see the bit thread and make-up torque lookup for how bit size and connection type translate into the correct make-up torque, and the drill string page for where the bit sits in the rest of the string and the WOB it's designed to take.
Once a bit of a given classification has actually drilled a run, what it comes back looking like gets recorded in an entirely separate code — see the IADC dull bit grading reference for that system.
Frequently Asked Questions
What do the three digits on a roller cone bit actually mean?
First digit is series — formation type and hardness range, 1 through 8, with 1-3 milled tooth and 4-8 TCI/insert. Second digit is the sub-series within that series, a finer gradation of the same formation range. Third digit is bearing type, 1 through 8, running from a basic non-sealed roller bearing up to a sealed, gauge-protected friction bearing. A trailing letter, if present, is a manufacturer feature code — air application, motor application, extra gauge protection, and so on.
Why do milled tooth and TCI bits split at series 3 and 4?
They are fundamentally different cutting structures for different formation strengths. Milled tooth bits (series 1-3) cut with steel teeth machined into the cone and suit softer, more drillable formations where a sharp edge does the work. TCI, tungsten carbide insert, bits (series 4-8) press carbide buttons into the cone instead and hold up to the abrasion and impact loading of harder, more abrasive formations that would round off a milled tooth quickly.
What does the PDC body material letter change about the bit?
Matrix (M) bodies are cast from a tungsten carbide powder infiltrated with a binder, giving strong erosion and wear resistance suited to abrasive formations, at the cost of being more brittle and harder to repair. Steel (S) bodies are machined steel — tougher and easier to weld-repair, but less erosion resistant, so more dependent on hardfacing in an abrasive application. Diamond (D) bodies are a matrix variant with additional diamond content for the most severe abrasive service.
How does PDC cutting structure classification change between soft and hard formation codes?
For formation types 1-4 (soft to medium), the cutting structure digit describes PDC cutter size — larger 19 mm cutters down to smaller 8 mm cutters. For formation types 6-8 (medium-hard to extremely hard), the cutter technology itself changes to natural diamond, TSP, a combination, or impregnated diamond, since standard PDC cutters do not hold up in that range. The same digit position means a different thing depending on which formation-type range the first digit falls in.
Is IADC classification the same thing as dull grading?
No — they are companion systems for opposite ends of a bit's life. Classification is the code stamped on a new bit describing what it is and what it is built to drill. Dull grading is the separate eight-position code assigned after the bit comes out of the hole describing the condition it came back in. See the dull bit grading reference for that side.