Engineering drawing symbols: what each one means on a part drawing
Engineering drawing symbols are the shorthand a part drawing uses to say what to make and how to check it. This guide draws each symbol a machine shop meets, with its meaning and the standard that governs it.
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Why drawings use symbols
WisTech Open is the open textbook consortium of the Wisconsin Technical College System. Its blueprint reading text notes a shift toward symbols over words and abbreviations as prints are used more widely. Older drawings still carry the abbreviations: DIA for diameter, CBORE for counterbore, CSK for countersink and DP for depth.
The symbols come from standards. ASME describes Y14.5 as establishing the symbols, rules and definitions for stating and interpreting GD&T on engineering drawings. ISO 1101 defines the symbol language for the geometrical specification of workpieces. ISO 129-1 sets the general principles for presenting dimensions and tolerances on technical drawings.
Check the title block or the general notes for the standard a drawing follows. Editions differ, so a symbol you do not recognize is a question for the customer before it is a line on your quote.
Dimensioning symbols
These symbols sit in front of a dimension value and say what kind of feature it describes. The meanings come from WisTech Open’s chapter on print symbols and notes. They also draw on the GD&T symbol notes from Florida International University’s engineering drawing course.
DiameterThe diameter of a round feature, such as a hole or a turned diameter. Older drawings write DIA.
RadiusThe radius of an arc. An inside radius is a fillet, and an outside radius is a round.
Spherical diameterThe diameter of a spherical feature. The S comes before the diameter symbol.
Spherical radiusThe radius of a spherical surface, written SR before the value.
Controlled radiusA radius held between its limits as a fair curve, with no flats or reversals.
SquareA square feature with four equal sides, so one dimension gives both.
CounterboreA hole enlarged at one end, often so a fastener head sits flush or below the surface. Older drawings write CBORE.
SpotfaceThe counterbore symbol with SF: a flat, round seat around a hole, often on a casting or a molded part.
CountersinkA cone-shaped recess at the end of a hole for a flat-head fastener. Older drawings write CSK.
DepthThe depth of a hole, slot or recess, followed by the value. Older drawings write DP.
Number of placesThe feature or dimension repeats the stated number of times.
Arc lengthThe dimension is measured along a curved outline, with the arc drawn above the value.
Basic dimensionA theoretically exact value. Its tolerance comes from a feature control frame, not the title block.
Reference dimensionShown for information, without a tolerance, and not inspected.
Not to scaleAn underlined value marks a dimension that is not drawn to scale.
Reading a hole callout
A hole callout strings several symbols into one note. The section below shows two common ones, drawn in millimetres.
Left: two through holes, each counterbored. Right: a through hole with a 90° countersink. The numbers are an example, not a standard size.
Read the first callout from left to right: two holes, 6.6 mm in diameter, through the part. Each is counterbored to 11 mm in diameter and 6.5 mm deep. WisTech Open’s text describes the same order: the counterbore symbol, then the diameter of the counterbore, then its depth.
The second callout is a countersink, 13 mm in diameter at the surface, with a 90° included angle. WisTech Open gives 90° as the included angle of a metric flat-head fastener and 82° for an inch one. Check that the angle on the drawing matches the screw it is meant for.
GD&T symbols
WisTech Open describes geometric dimensioning and tolerancing (GD&T) as a set of rules and symbols that communicate the intent of a design. Its focus is the function of the part. Each characteristic below controls one aspect of a feature’s form, orientation, location or runout. Form tolerances do not reference datums.
Form
StraightnessA line on a surface, or an axis, is straight within the tolerance.
FlatnessThe whole surface lies between two parallel planes.
CircularityEach cross-section of a round feature is round, not oval or out of round.
CylindricityA cylindrical feature is round enough and straight enough along its axis, both at once.
Profile
Profile of a lineA two-dimensional zone around each line of a feature, usually a curved one.
Profile of a surfaceA three-dimensional zone around a whole surface that the surface must stay within.
Orientation
AngularityA feature sits at a stated angle to a datum.
PerpendicularityA surface or axis sits at 90° to a datum.
ParallelismA surface or axis stays equidistant from a datum at every point.
Location
PositionThe zone within which a feature’s axis or centre plane may vary from its true position.
ConcentricityTwo or more features share a common axis.
SymmetryA feature sits symmetrically about the centre plane of a datum feature.
Runout
Circular runoutHow much a circular feature varies at one location as the part turns 360° about a datum axis.
Total runoutHow much a whole surface varies as the part turns 360° about a datum axis.
Modifiers
Modifiers sit in the feature control frame after the tolerance or a datum letter, and change how the tolerance applies.
Maximum material conditionThe size with the most material: the smallest hole or the largest shaft.
Least material conditionThe size with the least material: the largest hole or the smallest shaft.
Projected tolerance zoneThe zone extends beyond the surface, often to check that a fastener clears a mating part.
Free stateRefers to the distortion of a part once the forces applied during manufacture are removed.
Tangent planePlaced after the tolerance in the frame, it indicates a tangent plane.
Statistical toleranceTolerances assigned to related parts of an assembly on a statistical basis, used only where statistical process control applies.
All aroundA circle at the bend of the leader applies the tolerance to surfaces all around the part.
The feature control frame
A GD&T requirement is written in a feature control frame: a row of boxes read from left to right. WisTech Open’s text lists its parts as the symbol, the tolerance, any modifier and the datum references.
1Characteristic. The first box holds the geometric symbol, here position.
2Tolerance. The second box holds the tolerance, with a diameter symbol when the zone is round.
3Modifier. A circled M or L after the tolerance applies it at maximum or least material condition.
4Datum references. The letters name the datums the tolerance is measured from.
5Datum feature symbol. A square frame with the datum letter, joined to the surface by a triangle.
The modifier changes what a shop can accept. In WisTech Open’s example, a ⌀0.625 hole may measure from ⌀0.620 to ⌀0.630. Its position tolerance is ⌀0.03 at the smallest size, ⌀0.620, and grows by the same amount as the hole grows, to ⌀0.04 at ⌀0.630.
A basic dimension, the value in a rectangular frame, locates the feature exactly. It takes no tolerance from the title block, because the feature control frame carries it.
The surface finish symbol
The surface texture symbol is a check mark whose point rests on the surface, with the roughness value above the point. WisTech Open’s text gives the value as an average reading in microinches or micrometres: 63 means 0.000063 in, and 1.6 means 1.6 µm. A general note may read FAO, finish all over.
Basic symbolA check mark with its point on the surface. It sets the finish without naming the method.
Material removal requiredA bar across the symbol: the surface is machined to reach the finish.
No material removalA circle in the V: the finish comes without removing material, as in a casting or a molding.
The value hints at the process. WisTech Open’s roughness chart gives drilling a range of 250 to 63 microinches Ra, and grinding a range of 63 to 4.
ASME describes Y14.36 as establishing the method to designate surface texture, with symbols for roughness, waviness and lay. ISO 21920-1:2021 specifies the rules for indicating surface texture by profile methods with graphical symbols. ISO lists it as the revision of ISO 1302:2002, which is withdrawn, so a drawing that cites ISO 1302 points to a withdrawn standard.
The weld symbol
A welding symbol is built on a horizontal reference line, with an arrow that points to the joint. Open Washington’s Introduction to Welding text explains the rule that matters most. Information below the reference line applies to the side of the joint the arrow points to. Information above it applies to the other side.
1Arrow. Points to the joint the weld is made on.
2Reference line. Always horizontal, and read from left to right.
3Weld symbol. A fillet weld here. Below the line, it applies to the arrow side of the joint. Above the line, it applies to the other side.
4Size and length. The size sits to the left of the weld symbol and the length to the right.
5Tail. Notes such as the welding process, or TYP for a typical weld.
6Field weld flag. The weld is made in the field, during installation, and the flag points toward the tail.
7Weld-all-around circle. The weld goes all the way around the joint.
The circle and the flag both sit where the arrow meets the reference line, as Open Oregon’s text on supplementary welding symbols shows. Both matter to a quote: all around adds weld length, and a field weld moves the work to the installation site.
AWS describes A2.4 as a method for specifying welding, brazing and nondestructive examination information by means of symbols. ISO 2553 is the international standard for symbolic representation of welded joints. ISO notes two approaches to marking the arrow side and the other side: one on a dual reference line, one on a single reference line. The standard covers both.
Which standard governs which symbol
The table sums up the families of symbols on this page and the standards that cover them, as each publisher describes its own standard.
This page describes each standard only from its publisher’s public page. For contract work, read the edition the drawing names.
Symbols on drawings you make
If you start a drawing from a 3D model, the STEP to DXF converter lays out three views with overall dimensions. The callouts, tolerances and finishes on this page are what you add to make it a part drawing. The title block template gives you the sheet and the general tolerance note.
When the product is configured from rules, the drawing can come from the same rules. Turning your Excel builder into a CPQ app generates the quote drawing in your own title block, with the dimensions the customer checks. A named person approves it.
Questions people ask
What are the symbols on an engineering drawing?
They are shorthand for features and requirements. Dimensioning symbols mark diameters, radii, counterbores, countersinks and depths. GD&T symbols control form, orientation, location and runout. The surface texture symbol sets the finish, and the welding symbol describes each weld.
What are the 14 GD&T symbols?
Straightness, flatness, circularity and cylindricity control form. Profile of a line and profile of a surface control profile. Angularity, perpendicularity and parallelism control orientation. Position, concentricity and symmetry control location, and circular and total runout control runout.
What does the counterbore symbol look like?
A square U shape, open at the top. In a hole callout it comes before the diameter and the depth of the counterbore. With SF inside it, the same symbol calls for a spotface.
What is the difference between R and SR on a drawing?
R marks the radius of an arc. SR marks the radius of a spherical surface, and S followed by the diameter symbol marks the diameter of a spherical feature.
What does a circle on a weld symbol mean?
A circle where the arrow meets the reference line means the weld goes all around the joint. A flag at the same point means the weld is made in the field, during installation.
Which standard defines engineering drawing symbols?
ASME Y14.5 and ISO 1101 cover GD&T, and ISO 129-1 covers how dimensions are presented. ASME Y14.36 and ISO 21920-1 cover surface texture symbols. AWS A2.4 and ISO 2553 cover welding symbols.
