An IADC code is a shorthand that describes a drill bit’s design so bits from different manufacturers can be compared. There are two separate systems: a 3-digit code for roller-cone (tricone) bits — cutting structure, formation hardness, bearing — and a 4-character code for fixed-cutter bits (PDC and diamond) — body material, cutter density, cutter size, profile. This page decodes both, digit by digit, and corrects a misreading of the roller-cone code that several widely read pages repeat.
Read left to right: the first digit gives the cutting structure and the broad formation class (the “series”), the second digit divides that class into four hardness steps, and the third digit gives the bearing and gauge design. An optional fourth letter marks extra features.
| Series | Cutting structure | Formation class |
|---|---|---|
| 1 | Milled (steel) tooth | Soft formations — low compressive strength, high drillability. |
| 2 | Milled tooth | Medium to medium-hard formations — higher compressive strength. |
| 3 | Milled tooth | Hard, semi-abrasive to abrasive formations. |
| 4 | Tungsten-carbide insert (TCI) | Very soft formations — low compressive strength, high drillability. |
| 5 | TCI | Soft to medium formations — low compressive strength. |
| 6 | TCI | Medium-hard formations — high compressive strength. |
| 7 | TCI | Hard, semi-abrasive to abrasive formations. |
| 8 | TCI | Extremely hard, abrasive formations. |
Several widely read pages decode the first digit as one continuous hardness ladder from 1 to 8, which would make series 5 a hard-formation bit — and at least one popular page reads the digits out of order, taking the final 7 in 537 as meaning a very hard formation. Both readings are wrong. In the IADC system, series 1–3 cover the full soft-to-hard range for milled-tooth bits, and the scale then restarts at soft with series 4 for insert bits. Series 5 is therefore a TCI bit for soft to medium formations of low compressive strength — one of the most commonly run insert classes — not a hard-rock bit. Formation is carried by the first two digits; the third digit is always the bearing.
| Type | Meaning |
|---|---|
| 1 | Softest formations within the series. |
| 2 | Soft-to-medium within the series. |
| 3 | Medium-to-hard within the series. |
| 4 | Hardest formations within the series. |
| Digit | Bearing / gauge design |
|---|---|
| 1 | Open (non-sealed) roller bearing. |
| 2 | Roller bearing, air-cooled — for air drilling. |
| 3 | Open roller bearing with gauge protection. |
| 4 | Sealed roller bearing. |
| 5 | Sealed roller bearing with gauge protection. |
| 6 | Sealed friction (journal) bearing. |
| 7 | Sealed friction bearing with gauge protection. |
A fourth letter, when present, marks an additional design feature — for example A for air application, C for a center jet, D for deviation control, E for extended jets, X for chisel-shaped inserts, Y for conical inserts.
Worked example — 537. The 5 says: TCI bit, built for soft to medium formations of low compressive strength. The 3 says: the harder end of that soft-to-medium band (third of four steps). The 7 says: sealed friction bearing with gauge protection — the premium bearing package. So a 537 is an insert bit for the firmer side of soft-to-medium rock, with sealed journal bearings and protected gauge.
Contrast — 517 and 637.A 517 shares the series and the bearing package but is type 1: the softest formations series 5 covers. A 637 keeps the type and bearing but moves up a full series: series 6 means medium-hard rock of high compressive strength. Moving 517 → 537 is a small step within one formation class; moving 537 → 637 is a step into genuinely harder rock.
Fixed-cutter bits — PDC, natural diamond, and TSP — use a separate IADC system: one letter followed by three digits.
| Character | What it describes | Values |
|---|---|---|
| 1st (letter) | Body material and cutter family | M = matrix-body PDC; S = steel-body PDC. D and T mark natural-diamond and TSP matrix bits. |
| 2nd (digit) | Cutter density | PDC bits use 1–4, light set to heavy set (on a mid-size bit, roughly 30 or fewer cutters at 1, up to 50 and more at 4). Surface-set diamond bits use 6–8, graded by stone size. |
| 3rd (digit) | Cutter size | 1 = large (over 24 mm), 2 = medium (14–24 mm), 3 = small (about 13 mm — the common standard cutter), 4 = smallest (about 8 mm). |
| 4th (digit) | Bit profile | 1 = flat or fishtail, 2 = short profile, 3 = medium taper, 4 = long profile. |
So an M323 is a matrix-body PDC with a medium-density set of medium-size (14–24 mm) cutters on a medium-taper profile.
The roller-cone code is quoted worldwide on quotations and packing lists. The fixed-cutter code is not: in practice, PDC bits are bought and sold by the specification itself — blade count, cutter diameter, body type, connection — rather than by the 4-character class. If you are ordering a PDC bit, state the specification directly; our PDC page lists exactly what to give.
The series digit is the fastest filter when matching a bit to an interval. Typical rock types per series:
| Series | Bit type | Typical formations |
|---|---|---|
| 1 | Milled tooth | Soft shales, clays, unconsolidated sands. |
| 2 | Milled tooth | Firmer shales, sandy shales, softer limestone. |
| 3 | Milled tooth | Hard, abrasive sandstone; hard limestone and dolomite. |
| 4 | TCI | Very soft, sticky, low-strength formations. |
| 5 | TCI | Soft-to-medium shales, limestone, soft sandstone; sands and anhydrite with harder layers mixed in. |
| 6 | TCI | Medium-hard limestone, dolomite, harder sandstone and shale sequences. |
| 7 | TCI | Hard, semi-abrasive rock — hard limestone, dolomite, hard sandstone. |
| 8 | TCI | Extremely hard, abrasive rock — quartzite, granite, chert. |
Milled-tooth and TCI series overlap on purpose: a series-1 tooth bit and a series-4 or 5 insert bit can both drill soft rock. The tooth bit usually drills faster and costs less; the insert bit lasts longer. Which wins depends on rock abrasiveness and how much a trip to change bits costs on your rig.
For a tricone bit, three items make a complete, quotable specification: IADC code + bit size + connection. A request like “8½ inch, IADC 537, 4½ inch API Regular pin” can be priced by any qualified factory without a single follow-up question. If you do not know the code, send the formation description instead — rock type and hardness — and the series table above is how it gets translated back into a code.
Two practical notes. First, quote the code from the bit that performed well, not the one that happened to be on the last invoice — the code preserves the design that worked even when you switch manufacturers. Second, always state the connection: bits of the same diameter ship with different pins, and a mismatch stops the job. Our bit-to-connection chart lists the standard pairings. When ready, send the code, size, and connection on WhatsApp and we return a matched bit, priced CFR your port.