Counterbore vs countersink: symbols, geometry, and which screw wants which
Both features exist to get a fastener head to sit at, or below, the surface of the work. They differ in one geometric fact that drives everything else: a countersink is a cone, a counterbore is a cylinder [1] [2]. The cone matches the angled underside of a flat head screw; the cylinder swallows the parallel sides of a socket head cap screw, a hex bolt head, or a screw head you intend to bury under a wood plug. This page covers the drawing symbols, the hole geometry, the screw-head mapping, the spotface and counterdrill variants, and when to use each. For the bit-side numbers, diameters per screw size and the 82/90 degree angle tables, see our countersink size chart, which owns that ground; this page is the concept and geometry reference.
The one-minute table
| Feature | Countersink | Counterbore | Sources |
|---|---|---|---|
| Shape | conical, funnel shaped opening | cylindrical, flat bottomed pocket | [1] [2] |
| Drawing symbol (ASME Y14.5) | wide V, with angle and surface diameter | open topped bracket, with diameter and depth | [3] [4] |
| Fastener it seats | flat (countersunk) head screws, rivets | socket head cap screws, hex heads, washer seats, wood plugs | [1] [5] |
| Depth in the callout | usually none; angle and rim diameter define the cone | always given; the pocket must clear head height | [3] [6] |
| Typical tool | countersink bit, or combination pilot-plus-flare bit | counterbore bit, end mill, or Forstner bit in wood | [1] [7] |
| Load path | angular, into the cone surface | parallel to the bolt axis, onto the flat shoulder | [1] [2] |
The symbols, and how to read a callout
On a modern drawing both features are specified with symbols rather than words, per ASME Y14.5 in North America and the ISO 129 family internationally [3] [4]. The counterbore symbol is a shallow, open topped bracket; the countersink symbol is a wide V; the depth symbol is a downward arrow placed before any depth value [4] [8]. A hole note reads in the order the machinist works: pilot drill first, then the enlargement, then depths. A counterbored through hole reads as drill the pilot through, counterbore to a stated diameter down to a stated depth; a countersunk hole reads as drill the pilot through, countersink to a stated surface diameter at a stated included angle [3] [8]. That is the key structural difference in the callouts: the counterbore must carry a depth because its cylinder has length, while the countersink usually omits depth because the cone is fully defined by its angle and rim diameter [6] [8].
Three field variations are worth knowing. Older drawings spell the symbols out as text, CBORE and CSINK, which the standards discourage but tolerate [6] [9]. A spotface uses the counterbore symbol, sometimes with SF inside the bracket since the 2009 revision of Y14.5, and often carries no depth at all because the machinist just cleans the surface flat [6] [5]. And drawings made outside North America follow ISO 129-1 and the fastener-hole standards in the ISO 15785 family rather than Y14.5, so symbol sets and dimension placement differ slightly across the two traditions [4] [9].
Screw heads: which recess fits which head
The head geometry decides the recess, not preference. A flat head screw (also called countersunk head) has a conical underside, 82 degrees included on US inch standard screws per the ANSI B18.6 family and 90 degrees on metric flat heads per ISO 2009, ISO 7046, and ISO 10642, so it seats only in a matching-angle countersink [7] [10]. A socket head cap screw has a cylindrical head with a flat bearing face, so a cone cannot seat it at all; it needs a counterbore sized to head diameter plus clearance and deep enough for head height [1] [11]. Hex bolt heads, lag screw heads, and nut seats follow the same rule as the socket cap screw: flat underside, cylinder required [5] [2].
| Head type | Underside | Recess required | Why | Sources |
|---|---|---|---|---|
| Flat head (countersunk) screw | conical, 82 deg inch / 90 deg metric | countersink, matching angle | flush finish for sliding or show surfaces | [7] [10] |
| Socket head cap screw | flat, cylindrical sides | counterbore | head below surface; strong axial clamp | [1] [11] |
| Hex bolt or lag screw | flat | counterbore | wrench clearance or buried head | [5] [2] |
| Wood screw under a plug | either | counterbore then pilot | pocket filled with a glued wood plug | [12] [5] |
| Bolt on a casting or curved bar | flat | spotface | flat, perpendicular landing on rough stock | [6] [5] |
| Rivets (airframe) | conical, 100 deg typical | countersink | flush skins, aerodynamics | [7] [10] |
One strength note that follows from the geometry: a counterbored socket cap screw clamps with its force parallel to the bolt axis onto a flat shoulder, spreading load evenly, while a countersunk head seats on a cone and concentrates stress at the rim, which is why heavily loaded joints prefer the counterbore and why aerospace practice is exacting about countersink angle match [1] [7].
When to use each
Use a countersink when the head is conical and the goal is flush: flat head wood screws in furniture and trim, flat head machine screws where parts must slide over the joint, and riveted skins [7] [10]. Use a counterbore when the head is cylindrical or the goal is buried: socket head cap screws in machined assemblies, hex heads that need wrench clearance below the surface, washers that need a flat seat, and the wood plug look, where the counterbore is cut with a Forstner bit (sizes on our Forstner bit sizes page) and capped with a glued plug [1] [12]. In wood specifically, the standard sequence for a plugged screw is counterbore for the plug, then pilot hole for the shank, then optionally a small countersink to clean the cone for a flat head; the pilot hole chart covers the middle step [12] [5].
The wrong choice fails differently. A counterbore under a flat head screw leaves a cylindrical gap around the cone with nothing bearing on it: the screw bottoms or the joint rocks [2] [7]. A countersink under a socket cap screw leaves the head standing proud on the cone's rim with a knife edge contact that can crack the part or gall the head [1] [7].
The variants: spotface and counterdrill
Four related hole features appear beside the two main ones, and the naming is where confusion concentrates. A spotface is simply a very shallow counterbore whose purpose is a flat, perpendicular seat for a bolt head or nut on a surface that is neither, castings, forgings, curved bars; the symbol is the counterbore's, optionally with SF inside, and the depth is commonly unspecified or a nominal 1.5 mm (0.06 in) class cleanup because only the flat matters [6] [5]. A counterdrill is a stepped hole in which a larger drilled diameter transitions to a smaller one through a conical shoulder, typically at the roughly 120 degree angle a standard drill point leaves behind, so the cone is incidental tool geometry rather than a fastener seat; it is dimensioned by the large diameter and the depth of the full diameter section, and its angle may be omitted for exactly that reason [6] [9].
| Feature | Geometry | Symbol | Specified by | Purpose | Sources |
|---|---|---|---|---|---|
| Countersink | cone at hole mouth | wide V | surface diameter plus included angle | seat a conical head flush | [3] [6] |
| Counterdrill | cylinder then conical transition to smaller hole | countersink symbol with depth | large diameter plus depth of full diameter | clearance over smaller fastener or assembly step | [6] [9] |
| Counterbore | cylinder with flat bottom | open bracket | diameter plus depth | bury a cylindrical head | [3] [6] |
| Spotface | very shallow counterbore | counterbore symbol, sometimes SF | diameter only; depth often nominal | flat seat on rough or curved stock | [6] [5] |
The common thread: as the list runs from countersink to spotface, the feature gets less conical and more cylindrical, and the specified depth matters less while the flat matters more. Xometry's type-of-hole guide adds one more variant, the counterdrilled-and-plugged hole, a countersink further drilled parallel so a cap hides the fastener entirely, useful only where the screw will never need to come back out [5] [1].
Metal shop numbers, briefly
The inch-series counterbore diameters for socket head cap screws trace to ASME B18.3 appendix data as republished by the reference charts: a 1/4 inch screw takes a 7/16 inch counterbore, 3/8 takes 5/8, 1/2 takes 13/16, 3/4 takes 1-3/16, and 1 inch takes 1-5/8, each with a corresponding pilot clearance hole in close, normal, and loose fits [11] [13]. Engineers Edge and Carbide Depot agree value for value across the whole #0 through 2 inch range, which is the two-source check this site runs on every factual row [11] [13]. The countersink side of the ledger, head diameters and cone angles per screw size, lives on the countersink size chart page, which also carries the wood screw gauge tables.
Related
For the bit-side dimensions of every countersink per screw size, wood and machine both: countersink size chart. For cutting the wood counterbore before the plug: Forstner bit sizes. For the pilot hole under all of it: pilot hole chart. For the sheet-metal screws that most often wear countersunk heads: self tapping vs self drilling screws. For drilling the holes themselves at the right speed: drill speed chart.
Sources
- Xometry, Countersink vs Counterbore Holes in Machining: cone vs cylinder geometry, socket head seating, load distribution differences, counterdrill and spotface descriptions (xometry.com/resources/machining/countersink-vs-counterbore-holes/)
- Xometry, What are Countersink Holes in Engineering: angle conventions by fastener family, flush seating logic, counterdrilled-and-plugged variant (xometry.com/resources/machining/what-are-countersink-holes/)
- Engineer's Bible, Types of Holes: counterbore and countersink symbols, pilot-through assumption, spotface as shallow counterbore with SF notation (engineersbible.com/types-of-holes/)
- Drafting Manual (GIE), Dimensioning and Tolerancing Symbols section 6: counterbore symbol combined with diameter symbol, countersink symbol placement, depth symbol usage (brlcad.org/design/drafting/Drafting_Manual_DimensioningAndTolerancingSymbols_6-1.pdf)
- Xometry, Types of Holes in Engineering: spotface definition and use on castings and curved surfaces, counterbore purposes by head type (xometry.com/resources/machining/types-of-holes-in-engineering/)
- Engineering Drawing unit guide (rathasochenda.com PRI Unit 11, Guide Holes Callouts PDF): counterbore/spotface/countersink/counterdrill callout conventions, C'BORE and C'SINK legacy text, SF symbol from ASME Y14.5-2009, counterdrill 120 deg transition and full-diameter depth rule (rathasochenda.com/wp-content/uploads/2020/04/Guide-Holes-Callouts.pdf)
- Severance Tools, Countersink Angles Explained: 82 deg ANSI B18.6 inch flat heads, 90 deg ISO metric, 100 deg aerospace, 120 deg chamfer, angle-mismatch failure modes (severancetools.com/resources/countersink-angle-guide.html)
- TechDraw AI, Dimension Symbols on Engineering Drawings: hole callout reading order, counterbore and countersink callout examples with depth symbol (techdrawai.com/blog/engineering-drawing-dimension-symbols)
- Engineering Essentials, Dimensioning and Locating Advanced Features: counterbored hole specification rules (drill dia, cbore dia, depth), countersunk specification (dia, angle), spotface on curved surfaces, ASME Y14.5-2009 applicability (engineeringessentials.com/ege5/files/ege/dim/dim_page4d.htm)
- Eugene Fastener, Flat Head Socket Screw Countersink Guide: ASME B18.3 82 deg inch vs DIN 7991 / ISO 10642 90 deg metric, non-interchangeability of the two angles (eugenefast.com/home/page/resources/flat-head-socket-screw-countersink-guide)
- Engineer's Bible, Counterbore Hole Size Chart for Socket Cap Head Fasteners (ANSI Inch): pilot hole close/normal/loose fits and counterbore diameters and depths #0 through 2 inch (engineersbible.com/counterbore-socket-ansi-inch/)
- W.L. Fuller, Wood Screw Chart: counterbore diameters for plugs by screw gauge, pilot drill pairing, C-series countersink numbers (wlfuller.com/html/wood_screw_chart.html)
- Carbide Depot, Counterbored Holes Dimensions (inch): USA socket head cap screw counterbore diameters, depths, and clearance holes #0 through 2 inch (carbidedepot.com/formulas-cb-inch.htm)