Oilfield Drill Bit Sizes & API Pin Connection Chart

Oilfield and water-well drill bit sizes are not the same as the fractional or numbered sizes on a workshop twist-drill chart. A rock bit is sized by the diameter of the hole it drills, stated in inches with fractions (for example 8 1/2" or 12 1/4") and in millimetres, and each size range comes with a standard API threaded connection that screws the bit onto the drill string. This page gives the common bit sizes, the matching API Regular pin connections and how bit size is chosen from the casing program, cross-checked with the Volgaburmash catalogue.

How Rock Bit Sizes Are Expressed

Bit size is the gauge diameter: the diameter of the circle swept by the outermost cutting elements. In the oilfield it is written in inches and fractions of an inch, with the millimetre value obtained by multiplying by 25.4 (8 1/2" = 215.9 mm). Many catalogues, including Volgaburmash, list both, and some sizes that come from metric well designs, such as 83 mm or 300 mm, have odd fractional equivalents.

API Specification 7-1 sets manufacturing tolerances on bit diameter. Roller cone bits are made on nominal size or slightly oversize (plus tolerance only), while fixed-cutter PDC bits are made on nominal size or slightly undersize (minus tolerance only). That is one reason a PDC bit run after a roller cone bit of the same nominal size does not need to ream the hole.

Standard Oilfield Bit Sizes and Typical Hole Sections

Well design works from the bottom up: the production casing or liner size sets the smallest hole, and every section above it must be large enough to run the next casing string through it. The table shows the most common bit size and casing combinations used worldwide, with links to the Dubai stock list filtered by size.

Water wells, blast holes and construction holes follow their own logic: the bit size is chosen for the casing and gravel pack or for the explosive column, and pilot holes are often drilled small (5 1/2" to 8 1/2") and reamed afterwards. Stock availability changes; the links above open the live stock list, and the most stocked sizes are summarised at the top of the stock page.

API Regular Pin Connection Chart by Bit Size

Rock bits normally have an API Regular (Reg) pin, a male tapered thread, at the top. The connection size is linked to the bit diameter because a larger bit needs a stronger shank. The ranges below follow the API recommendation used across the industry:

Make-up torque values are the recommended torques for these thread connections from the Volgaburmash Oil & Gas catalogue; the catalogue also lists NC 31 (9.6–11.6 kN·m) and NC 50 (15.0–17.0 kN·m). Always use the value on the datasheet of the specific bit.

The Volgaburmash range follows the same pattern. SlimHolePro (SLHP) roller cone bits, 4 1/8" to 6 3/4", use 2 3/8", 2 7/8" and 3 1/2" Reg pins; Motor (MTR) bits, 7 1/2" to 12 1/2", use 4 1/2" and 6 5/8" Reg; Grand (GRD) bits from 13 3/4" to 26" use 6 5/8" and 7 5/8" Reg. PDC bits of the FastDrill (FD) series, 3 9/32" to 17 1/2", cover every pin from 2 3/8" to 7 5/8" Reg. Boundary sizes can differ between manufacturers and models, so treat the chart as a guide and confirm the thread on the bit datasheet before ordering.

Pin, Box, Reg, FH and NC: What the Connection Name Means

• Pin and box: the pin is the male thread, the box the female. Almost all drill bits are pin-up; the bit sub or the bottom of the motor has the matching box.
• Reg (Regular): the standard rotary-shouldered connection for bits. The number (for example 4 1/2") is a nominal size, not the actual thread diameter.
• FH (Full Hole) and NC (Numbered Connection): used mostly on drill pipe and collars, and on some special tools. In the Volgaburmash range, WiperTrip (WT) reamers are also offered with box connections including NC 26, NC 31 and NC 50, CoreBits (CB) with box 6 5/8" FH or NC 50, and some AIRJET and AIRX blast-hole bits with a 4 1/2" FH or 6" BECO pin.
• Crossover subs: when the bit pin does not match the string, a bit sub or crossover sub converts between connections.

Checking Bit Gauge Before and After a Run

A bit is only the size on its label while its gauge is intact. Before running a bit, check its diameter with a ring gauge of the nominal size and record any difference, keeping in mind the plus tolerance of roller cone bits and the minus tolerance of PDC bits. After pulling the bit, check it again. If a roller cone bit has lost diameter, the bottom of the hole is under gauge and the next bit must ream that interval rather than being run straight to bottom, or it can be pinched and damaged. The IADC dull grade records this as the gauge measurement in sixteenths of an inch, so a worn bit is reported as, for example, 1/16 or 2/16 under gauge. Recording gauge wear together with the depth drilled gives a much better basis for choosing the next bit size, series and gauge protection than footage alone.

How to Choose the Right Bit Size

1. Start from the casing program. The bit must pass through the drift diameter of the casing already set and leave enough annular clearance for the next casing string and cement.
2. Check the connection. Match the bit pin to the bit sub, motor or drill collar box on the rig.
3. Check the stock or lead time for that exact size. Non-standard sizes such as 8 3/8" or 8 3/4" may be available only in some series.
4. Choose the bit type for the formation once the size is fixed: roller cone or PDC, IADC code or cutter layout. See how to choose a drill bit.

Common Mistakes with Bit Sizes and Connections

• Ordering by millimetres rounded from inches (for example 216 mm instead of 215.9 mm) and receiving a non-standard size.
• Assuming every 5 1/2" or 8 1/2" bit has the same pin; boundary sizes differ between series.
• Under-torquing the connection at make-up, which can let the bit back off downhole, or over-torquing it and damaging the shoulder.
• Running a PDC bit into a hole drilled by a worn, under-gauge bit without reaming first.

Browse roller cone bits and PDC bits by series, or read how cutter size affects PDC design in PDC cutter: sizes, grades and wear.

Drill Bit Size FAQ

What are the standard oilfield drill bit sizes?

The most common sizes are 26", 17 1/2", 12 1/4", 8 1/2" and 6" (660.4, 444.5, 311.1, 215.9 and 152.4 mm), with alternatives such as 9 7/8", 7 7/8" and 5 1/2" depending on the casing program.

What API connection does an 8 1/2" bit have?

Bits from 7 1/2" to 9 3/8" normally have a 4 1/2" API Regular pin, so an 8 1/2" bit uses 4 1/2" Reg. A 12 1/4" bit uses 6 5/8" Reg and a 6" bit uses 3 1/2" Reg.

How do I convert bit size from inches to millimetres?

Multiply the size in inches by 25.4. For example, 12 1/4" = 12.25 × 25.4 = 311.15 mm, which catalogues list as 311.1 mm.

Why is a PDC bit slightly smaller than a roller cone bit of the same size?

API tolerances give roller cone bits a small plus tolerance and fixed-cutter bits a small minus tolerance, so a PDC bit can follow a roller cone bit of the same nominal size without reaming the hole.