The IADC dull grading system is an 8-position code recorded when a bit is pulled out of the hole. In eight short entries it describes how worn the cutting structure is, what kind of damage it shows and where, the condition of the bearings and the gauge, and why the bit came out. It is the standard language for describing a used bit — the same code works for roller-cone (tricone) and fixed-cutter (PDC) bits.
A dull grade is written as eight entries in a fixed order, for example 1-2-WT-A-X-I-NO-TD. The first four describe the cutting structure; the last four describe the rest of the bit and the reason the run ended.
| Position | Records | How it is graded |
|---|---|---|
| 1 — Inner cutting structure | Wear of the inner rows of teeth or cutters. On a fixed-cutter bit, the cutters in the inner two-thirds of the bit radius. | Scale 0 to 8: 0 means no loss of tooth or cutter height, 8 means the cutting structure is completely gone. Each tooth or cutter is judged and the average is reported, so 4 means about half the height is worn away. |
| 2 — Outer cutting structure | Wear of the outer rows — the outer one-third of the radius on a fixed-cutter bit, the gauge-side rows on a roller cone. | Same 0 to 8 scale as position 1. |
| 3 — Dull characteristic | The main physical change from new condition — the single most prominent one. | A two-letter code from the dull characteristic table below (BT, WT, HC and the rest). |
| 4 — Location | Where on the bit the main dull characteristic sits. | Roller cone: N nose row, M middle row, G gauge row, A all rows, plus the cone number where useful. Fixed cutter: C cone, N nose, T taper, S shoulder, G gauge, A all areas. |
| 5 — Bearings and seals | Bearing condition — roller-cone bits only. | Non-sealed bearings: 0 to 8, the share of bearing life used. Sealed bearings: E seals effective, F seals failed. Always X for a fixed-cutter bit, which has no bearings. |
| 6 — Gauge | Whether the bit still cuts a full-diameter hole. | I means in gauge. If the bit is undergauge, the amount is recorded in sixteenths of an inch — a 2 means 2/16 inch under nominal diameter. |
| 7 — Other dull characteristic | A second wear or damage feature worth recording. | A two-letter code from the same table as position 3. NO if there is nothing else to report. |
| 8 — Reason pulled | Why the bit came out of the hole. | A two- or three-letter code from the reason-pulled table below (TD, PR, HR and the rest). |
Positions 3 and 7 use the same list of two-letter codes. Some apply to any bit; the cone-related codes apply only to roller-cone bits.
| Code | Meaning |
|---|---|
| BC | Broken cone (roller cone) |
| BF | Bond failure — the braze holding a cutter has failed (fixed cutter) |
| BT | Broken teeth or cutters |
| BU | Balled up — bit packed with formation |
| CC | Cracked cone (roller cone) |
| CD | Cone dragged — a cone stopped turning (roller cone) |
| CI | Cone interference (roller cone) |
| CR | Cored — the center of the cutting structure is worn away |
| CT | Chipped teeth or cutters |
| ER | Erosion — bit body or tooth material washed away by the fluid |
| FC | Flat-crested wear — even height loss across the tooth face |
| HC | Heat checking — fine surface cracks from repeated heating and cooling |
| JD | Junk damage — mechanical damage from metal debris in the hole |
| LC | Lost cone (roller cone) |
| LN | Lost nozzle |
| LT | Lost teeth or cutters |
| OC | Off-center wear |
| PB | Pinched bit — bit squeezed under gauge |
| PN | Plugged nozzle or flow passage |
| RG | Rounded gauge — gauge row or gauge pads worn to a rounded profile |
| RO | Ring out — a concentrated ring of lost cutting structure |
| SD | Shirttail damage (roller cone) |
| SS | Self-sharpening wear |
| TR | Tracking — teeth striking the same bottom-hole pattern |
| WO | Washed-out bit — fluid has cut through the bit body |
| WT | Worn teeth or cutters — smooth, even wear |
| NO | No dull characteristic to report |
Position 8 explains why the run ended. It matters because it says whether the bit was pulled for its own condition or for a reason that had nothing to do with the bit.
| Code | Meaning |
|---|---|
| BHA | Change of bottom-hole assembly |
| CM | Condition mud |
| CP | Core point reached |
| DMF | Downhole motor failure |
| DP | Drill plug |
| DSF | Drill string failure |
| DST | Drill stem test |
| DTF | Downhole tool failure |
| FM | Formation change |
| HP | Hole problems |
| HR | Hours — the planned bit-hour limit was reached |
| LOG | Run logs |
| PP | Pump pressure change |
| PR | Penetration rate — drilling rate no longer acceptable |
| RIG | Rig repair |
| TD | Total depth — the planned interval or well depth was reached |
| TQ | Torque — erratic or rising torque |
| TW | Twist-off |
| WC | Weather conditions |
| WO | Washout in the drill string |
Note that WO appears in both tables with different meanings: as a dull characteristic it is a washed-out bit; as a reason pulled it is a washout in the drill string above the bit.
A dull grade is a record, but its value is forward-looking: it says whether the bit matched the rock and the parameters, and what to correct if it did not. Three examples, read conservatively.
1-2-WT-A-X-I-NO-TD (fixed cutter). Read out: light wear on the inner rows (1 of 8), slightly more on the outer rows (2 of 8); the main characteristic is smooth worn cutters, seen across all areas; X because a PDC bit has no bearings; the bit is still in gauge; nothing else to report; pulled because the section reached total depth. Slightly faster outer wear is expected — outer cutters travel a longer path on every revolution. A bit that reaches total depth in gauge with low, even wear was well matched to the formation; the same bit type is a sound starting point for the next similar section.
2-4-HC-S-X-I-CT-PR (fixed cutter). Read out: moderate wear, heaviest on the shoulder; the main characteristic is heat checking on the shoulder, with chipped cutters as the secondary; pulled because penetration rate fell. Heat checking is fine surface cracking that develops when a cutter face overheats against the rock and is then cooled by the mud, over many cycles — bit-maker grading guides attribute it to a bit starved of cooling flow or run with too much weight for the conditions. The usual response is to review hydraulics first — flow rate and nozzle sizing — and moderate weight and rotary speed, before changing the bit design itself.
4-6-WT-A-F-2-BT-HR (roller cone, sealed bearing). Read out: inner rows half worn (4 of 8), outer rows three-quarters worn (6 of 8); worn teeth across all rows, with some broken teeth as the secondary; seals failed; 2/16 inch undergauge; pulled on hours. The outer rows wearing faster than the inner rows, together with the undergauge reading, is typically read as the gauge side doing the hardest work — an abrasive formation working the outside of the bit — so the next bit generally carries more gauge protection or a cutting structure specified for harder, more abrasive rock. Seals graded F are generally treated as the end of a sealed bit's reliable running life, so the hour limit should not be extended on the next run. The undergauge reading also warns the crew that the next full-size bit meets a slightly tight hole near bottom.
The reason-pulled code keeps interpretation honest. A heavily worn bit pulled at TD tells a different story from the same wear pulled for PR — and a barely worn bit pulled for a motor failure (DMF) says nothing about the bit at all. Before changing bit type, check position 8: if the run ended for reasons unrelated to the bit, the wear record may not justify any change.
Yes — and the 8-position code on this page is still the one in general use. In 2021, IADC's Advanced Rig Technology committee, working jointly with SPE, began an upgrade of the dull grading system: finer damage categories, more bit-face zones, grading for the bottom-hole assembly, and data-management practices designed for automated analysis. A first revised recommended practice was circulated for industry comment in mid-2022, and in April 2026 IADC published the resulting Bit and BHA Dull Grading Manual as a set of recommended practices, free to download from IADC. It has not been announced as a replacement for the 8-position code, which remains the format bit records are written in across the industry. Learn the 8-position system first; the new manual extends it rather than retiring it.
Dull grading is the working language of our bit supply model. When you order through Ironstone, the dull grades from your recent runs travel back to the factory with the next order — so cutter grade, blade count, gauge protection, or hydraulics are corrected against what the last bit actually experienced, instead of the next bit being an uncorrected repeat of the last order. Send hole size, formation, and your dull grades on WhatsApp; the grade is what turns a repeat order into a corrected one.