BQ, NQ, HQ, and PQ are the standard wireline core sizes used in diamond drilling, part of a letter-coded system that runs from small (AQ) to large (PQ). Each letter specifies both the hole diameter and the core diameter the barrel recovers — NQ (47.6 mm core) and HQ (63.5 mm core) are the two sizes used in the large majority of mineral exploration programs today.
Every diamond drilling article, core photo, and equipment quote you encounter will use these two-letter codes as shorthand, so knowing what BQ, NQ, HQ, and PQ actually stand for — and why a program picks one over another — is basic fluency for anyone working around core. This guide covers where the naming system comes from, the full size chart, the trade-offs behind choosing a size, and where triple-tube variants fit in.
Where do the letters BQ, NQ, HQ, PQ come from?
The codes belong to the “Q wireline” system, a standard originally developed by the Diamond Core Drill Manufacturers Association (DCDMA) and now used industry-wide, including by major suppliers like Boart Longyear. Each size has a single letter — A, B, N, H, P, S — that identifies a matched set of drill rod, core barrel, bit, and reaming shell, all designed to fit together. The “Q” simply denotes that these are wireline tools, meaning the inner tube can be retrieved by a wireline winch through the rod string rather than pulling the entire rod string out of the hole to recover core.
The letters run smallest to largest: AQ, BQ, NQ, HQ, PQ, and SQ. There’s no memorable acronym behind the letters — they’re simply the DCDMA’s designations — but the ordering is consistent and universally recognised, so “stepping down from HQ to NQ” or “drilling PQ for metallurgical samples” means the same thing on any drill site in the world.
Standard core size chart
Hole diameter is the width of the borehole the bit cuts; core diameter is the width of the intact rock cylinder recovered inside it. The difference between the two is the bit’s wall thickness plus clearance.
| Size | Hole diameter (mm / in) | Core diameter (mm / in) | Typical use |
|---|---|---|---|
| AQ | 48.0 mm / 1.890″ | 27.0 mm / 1.062″ | Shallow, early-stage or geotechnical holes |
| BQ | 60.0 mm / 2.360″ | 36.5 mm / 1.432″ | Lightweight, cost-effective; often a telescoped tail size |
| NQ | 75.7 mm / 2.980″ | 47.6 mm / 1.875″ | Industry-standard for most mineral exploration |
| HQ | 96.0 mm / 3.782″ | 63.5 mm / 2.500″ | Larger core for better recovery and more sample volume |
| PQ | 122.6 mm / 4.827″ | 85.0 mm / 3.345″ | Metallurgical testwork, bulk sampling, geotechnical programs |
Two triple-tube variants worth knowing: HQ3 keeps the same 96.0 mm hole but a slightly reduced 61.1 mm core, and PQ3 keeps the same 122.6 mm hole with an 83 mm core. The small reduction in core diameter is the trade-off for the triple-tube system’s much better recovery in broken ground (more on that below). A less common large size, SQ, exists at 146.0 mm hole / 102.0 mm core for specialised bulk sampling, but it sees limited use outside dedicated large-diameter programs.
Why does core diameter actually matter?
A wider core isn’t just “more rock” — it changes what a program can do with the hole:
- Sample volume. A PQ core (85 mm) has roughly 3.2 times the cross-sectional area of an NQ core (47.6 mm), which matters directly for metallurgical testwork and bulk sampling programs that need a minimum mass of material.
- Structural and textural detail. Larger core is easier to log for fine structural features, vein relationships, and geotechnical measurements like RQD, simply because there’s more rock face to examine and less of it is lost to grinding at the core edge.
- Recovery in broken ground. Larger-diameter barrels generally hold together better through fractured or weathered zones, though the triple-tube design (see below) matters more for recovery than diameter alone.
- Rig size, weight, and cost. Every step up in diameter means a heavier rod string, more torque and pump capacity required from the rig, and a higher cost per metre. HQ rods and barrels are substantially heavier to handle than NQ, which shows up directly in drilling day-rates.
That last trade-off — sample quality against weight, torque, and cost — is the whole story behind why a program chooses one size over another.
NQ vs. HQ: how do you choose?
NQ and HQ account for the majority of mineral exploration drilling worldwide, so the practical decision most programs face is between these two.
Choose NQ when:
- The hole is deep, and rod weight and torque capacity are the limiting factor
- The rig is smaller or transport to a remote pad is difficult
- Standard assay-grade sampling is the goal and extra core volume isn’t needed
- Cost per metre needs to stay down across a large, multi-hole program
Choose HQ when:
- More sample volume is needed for structural logging, geotechnical measurement, or metallurgical splits
- Ground conditions are broken or fractured and better recovery is a priority (especially paired with HQ3 triple-tube)
- The rig has the torque and pump capacity to handle the heavier string
There’s no universal “better” size — an NQ hole that reaches the target with good recovery beats an HQ hole that runs out of rig capacity before it gets there. Many programs default to NQ for standard resource-definition drilling and reserve HQ for geotechnical holes, metallurgical intervals, or ground known to be difficult.
Telescoping: starting big, finishing small
Deep or long holes are rarely drilled at one diameter start to finish. A common practice is to telescope down — start the hole at a larger size, then reduce diameter partway down (running a smaller barrel and rods inside casing left from the larger size) as depth and rod weight become the limiting factor.
A striking real-world example: a deep underground exploration hole drilled in South Australia to 2,254 m — reportedly the deepest hole on record for that setting — began at HQ, stepped down to NQ, and finished its tail in BQ to reach the target. The same underground drilling capability has been used to complete NQ2 holes beyond 2,000 m and BQ holes to roughly 3,000 m from underground platforms. The logic is straightforward: as a hole gets deeper, the accumulated weight and torsional load of the rod string grows, and stepping down to a lighter, smaller-diameter string lets the rig keep advancing well beyond what a single large diameter could reach.
The reverse — reaming a hole out to a larger diameter partway down — is far less common and much more difficult, which is why programs typically start at (or above) the largest diameter they might need, rather than starting small and hoping to step up later.
What are triple-tube systems (HQ3, PQ3, NQ3), and why use them?
Standard (“double-tube”) wireline core barrels have an outer tube that rotates with the drill string and an inner tube that holds the core, isolated from rotation but still exposed to some vibration and torque at the point of core entry.
Triple-tube systems add a third element: a split inner liner that sits inside the inner tube and remains stationary while the outer barrel rotates around it. This isolates the core almost completely from rotational disturbance the moment it enters the barrel, which dramatically reduces core loss, grinding, and disturbance in broken, weathered, or poorly consolidated ground — the conditions where standard double-tube recovery often falls short.
The trade-off is a slightly smaller core diameter for the same hole size (HQ3’s 61.1 mm core versus HQ’s 63.5 mm) and added system complexity. In practice, many exploration and geotechnical programs run triple-tube as their default in anything but the most competent, unbroken rock, because the recovery gain reliably pays for itself in usable data.
How does core size affect handling and logging in the field?
Core size has practical downstream effects well beyond the drill rig:
- Core trays. Standard core trays hold a fixed length of core per tray, and that length shrinks as diameter grows — an NQ tray commonly holds around 5–6 m of core, while a PQ tray of the same physical footprint holds closer to 3 m. Larger core means more trays, more storage volume, and more core-shed space for the same metreage drilled.
- Sample mass. For a given sampled interval, larger core delivers proportionally more sample mass to the lab — useful when assay methods or metallurgical tests need extra material, but wasteful (and costlier to ship and crush) if the program doesn’t need it.
- Cutting and splitting. Wider core is generally easier to split cleanly with a saw, since there’s more material to work with relative to the blade kerf, and it’s easier to photograph clearly for structural or textural detail.
None of this changes how logging is structured — depth-interval data is depth-interval data regardless of core diameter — but it does change how much physical material and storage a program is committing to per metre drilled.
FAQ
What’s the most common core size used in mineral exploration? NQ (47.6 mm core diameter) is the most widely used size for general resource-definition drilling worldwide, because it balances sample quality, rig weight, and cost per metre better than the alternatives for the majority of programs. HQ is the next most common, generally reserved for geotechnical work, metallurgical sampling, or difficult ground where extra core volume or recovery matters.
What does the “3” mean in HQ3 or PQ3? It denotes a triple-tube core barrel — a design with a stationary split inner liner that isolates the core from rotation, improving recovery in broken or poorly consolidated ground. HQ3 and PQ3 use the same hole diameter as standard HQ and PQ but recover a slightly smaller core diameter as a trade-off for that improved recovery.
Can you change core size partway through a hole? Yes — this is called telescoping, and it’s standard practice on deep or long holes. The hole starts at a larger diameter and steps down to progressively smaller sizes (for example HQ to NQ to BQ) as depth increases and rod weight becomes the limiting factor. Stepping the diameter back up partway down a hole is far more difficult and rarely done.
Is a bigger core diameter always better? No. Larger core gives more sample volume and can improve structural logging detail, but it comes with a heavier rod string, more required rig torque and pump capacity, higher cost per metre, and more physical core to store and transport. The right size is the smallest one that reliably delivers what the program’s geology and sampling requirements actually need.
Does core diameter affect assay results? Not directly — the assay measures grade in the sample sent to the lab, regardless of the core it came from. Core diameter affects how much material is available to sample (larger core yields more mass per interval) and how representative a half- or quarter-core split is, which matters more for coarse or nuggety mineralization where sample mass has a bigger influence on assay variability.
From core size to the database: where this fits in the workflow
Core diameter is one of the first decisions a drill program locks in, and it belongs on the collar record alongside dip, azimuth, and drilling method — not buried in a driller’s daily report. In Blue Butterfly, core diameter (and any size changes partway down a telescoped hole) is a standard collar field, so every drillhole carries its size history alongside its depth-interval data. Strip logs and 3D visualisation pull from the same synced database, so a geologist reviewing recovery or planning a metallurgical sample split can see core size right next to the logged intervals — no cross-referencing a separate drilling report to check what diameter a given depth was drilled at.
Sources
- Wikipedia — Exploration diamond drilling (standard Q wireline bit size table: AQ through SQ, HQ3/PQ3 dimensions, citing Hartman & Mutmansky’s Introductory Mining Engineering and Boart Longyear’s drilling reference): https://en.wikipedia.org/wiki/Exploration_diamond_drilling
- Precision Drilling Australia — Understanding Core Drilling Sizes (core size chart, triple-tube and thin-wall variants, size selection guidance): https://www.precisiondrillingaustralia.com.au/blog/core-drilling-sizes/
- Sonic Edge Drilling — HQ, HQ3, and NQ Coring (rod diameters, triple-tube mechanism, NQ vs. HQ selection guidance): https://sonic-edge.ca/geotechnical-drilling/hq-hq3-and-nq-coring/
- UG Mining Tech — The deepest underground exploration holes! (2,254 m telescoped HQ–NQ–BQ hole example; NQ2 and BQ depth capability from underground platforms): https://ugminingtech.com/2022/07/04/the-deepest-underground-diamond-exploration-holes/
- PCT Plastic Core Trays — Core Box & Core Tray Size Guide: HQ, NQ, PQ and More (typical core-per-tray length by size): https://plasticcoretrays.com/core-box-core-tray-size-guide-hq-nq-pq-and-more/
- Robertson Geo — Common Coring Drill Bit Sizes (reference chart, DCDMA standard): https://www.robertson-geo.com/wp-content/themes/robertson/downloadfiles/geoinfo/1538562948.pdf