How to measure an arc in AutoCAD is trickier than measuring a straight line, because one arc carries three different numbers and each of them needs its own command. People routinely mistake the chord for the arc length, or go hunting for a command that does not exist.
This article is a hands-on session: every section is a measurement you can make on an arc you draw yourself. To learn modelling and dimensioning end to end, see the AutoCAD for construction course.
Three different numbers on one arc
First draw something to measure. Type A and Enter, then C and Enter, click a point as the centre, type @1200,0 as the start point, then type A and 90. There are eleven ways to construct an arc, covered in How to draw an arc in AutoCAD.
That arc carries three numbers, and they are nothing like each other.
| Figure | Value | Command |
|---|---|---|
| Radius | 1200 | DIMRADIUS |
| Chord | 1697 | no command of its own |
| Arc length | 1885 | DIMARC |
The chord is the straight line joining the two ends. The arc length is the distance travelled along the arc, so it is always greater than the chord. Check it by hand: a quarter of the circumference is pi times 1200 divided by 2, which is 1884.96, matching what the program prints.
Deciding which of the three a drawing actually needs is the real first step in how to measure an arc in AutoCAD. The rest of this article takes them one at a time.
Measure arc length in AutoCAD with DIMARC
The command that dimensions arc length is DIMARC, aliased DAR. Its prompts are short.
| Command line | What to do |
|---|---|
Select arc or polyline arc segment: | Click the arc |
Specify arc length dimension location or [Mtext/Text/Angle/Partial]: | Drag it clear and click |
Once placed, the program prints Dimension text = 1885. In front of the figure sits a small arc symbol. That is not decoration: it is what stops a reader confusing arc length with chord length. The full set of drawing conventions is in Construction drawing symbols.
Two things few tutorials mention, and both come up often.
Dimension part of an arc rather than all of it. Call DAR again, click the arc, but at the placement prompt type P and Enter. The command then asks for two points on the arc. Pick one end and the midpoint and the figure comes out as 942, exactly half of 1885. Notice the prompt now carries an extra Leader option, for when the piece of arc is too short to hold the text.
It also dimensions an arc segment inside a polyline. The first prompt promises exactly that: Select arc or polyline arc segment:. A wall corner rounded with FILLET on a polyline does not have to be exploded first. The details are on the DIMARC (Command) page.
Measure a radius in AutoCAD, then press Esc
DIMRADIUS, aliased DRA, dimensions a radius. It behaves differently from DIMLINEAR, and that difference is the part worth remembering.
| Command line | Note |
|---|---|
Select arc or circle: | Click the arc |
Dimension text = 1200 | The figure is already here |
Specify dimension line location or [Mtext/Text/Angle]: | Press Esc and you are done |
The figure prints on the second line, before the command asks where to put it. That makes this command a quick measuring tool: click the arc, read the radius off the command line, press Esc. Nothing is added to the drawing and you still have the number.
DIMDIAMETER, aliased DDI, asks exactly the same questions and differs only in the prefix on the figure. Choosing radius or diameter follows drawing convention rather than preference: round columns, pipes and holes take a diameter; fillets and door swings take a radius.
When the centre of the arc is off the sheet
Kerb lines and curved balconies often have radii of tens of metres. The centre falls outside the sheet, so dimensioning with DIMRADIUS gives a correct figure and an unusable drawing.
Try it on an arc of radius 20000 sweeping 30 degrees. Dimension it plainly, then ask how much room that dimension occupies: its bounding box comes back 21,072 units tall. To dimension an arc that fits inside a doorway, the dimension line has run 21 metres downward, out of the title block and off the sheet.
DIMJOGGED, aliased DJO, exists to fix exactly that. It shortens the dimension line and puts a jog in the middle of it.
The second prompt is the one people misread. It asks for an override centre so the command has somewhere to draw to, and that point takes no part in the measurement. Put the override more than 18 metres from the true centre and the program still prints Dimension text = 20000. The reference states plainly that the override replaces the true centre point, on the DIMJOGGED (Command) page.
One detail to get right first time: put the override centre in line with the direction of the true centre and there is no jog at all. The jog is the fold between two branches, so the override has to sit deliberately off to one side. The jog angle comes from the dimension style in use; measured on the 2027 release with ISO-25 it is 45 degrees.
Measuring an included angle: two figures from one pair of lines
DIMANGULAR, aliased DAN, dimensions angles. Where you place the dimension arc decides the figure, and that is the easiest trap in this command.
The test uses two lines running from a shared point. Select them and place the arc inside the angle, and the program prints Dimension text = 30. Call the command again, select the same two lines, but place the arc outside the vertex, and the figure becomes 150.
Both are correct, and this is not a fault. Two crossing lines create four angles, and the command dimensions whichever one the arc passes through. So read the figure after placing it rather than clicking to be done. Parallel lines cannot be dimensioned this way, because they have no vertex to measure from.
Measure a curve in AutoCAD when it is not an arc
Plenty of people want to measure a curve in AutoCAD when that curve is not a circular arc at all. This is where things go wrong, because DIMARC accepts only arcs and polyline arc segments. Select a spline and the command refuses it.
So how to measure an arc in AutoCAD and how to measure a free curve are two different jobs, and they need different tools.
Two ways to read a figure off any curved object, neither of which leaves anything on the drawing:
- The Properties palette, opened with
Ctrland1. Select the object and read the length field. Quickest when you only need one number. - The
LISTcommand, aliasedLI. Select the object and press Enter, and the program opens the text window showing the object type, its layer and its properties. For a closed polyline this is where both the perimeter and the area can be read.
MEASUREGEOM needs stating accurately. It measures radius, angle, distance, area and volume, but it has no arc length option. The list of options is on the MEASUREGEOM (Command) page.
While we are here, one widely repeated name needs correcting: there is no command called ARCLENGTH. On the Dimension menu, Arc Length is only a button label; take that route and the program prompts you to type the letter L. The command you can type directly is DIMARC, aliased DAR.
Divide an arc in AutoCAD, and the invisible-point trap
DIVIDE, aliased DIV, splits an arc into equal parts. Three steps: type DIV, select the arc, type the number of segments and press Enter.
Almost everyone hits the same thing the first time: nothing appears to happen. The command has not failed. It has placed the division points along the arc, but the default point style is a single pixel, so the eye cannot catch them.
The fix: type PTYPE and Enter to open the Point Style dialog, then pick a larger style, such as a cross inside a circle. The division points appear at once. The two settings in that dialog are stored in PDMODE for the style and PDSIZE for the size.
One choice there is worth getting right immediately. Set the size relative to the screen and the points keep their size as you zoom; set it in absolute units and they grow and shrink with the zoom. For documentation work, the first keeps the screen readable. The details are on the Point Style Dialog Box page.
The command also carries a Block option: instead of points, it places an existing block at each division. Answer Yes to the alignment question and the block's X axis follows the curvature of the arc. That is how you space lights along a curved balcony edge, or repeat a symbol along an alignment, without rotating each one by hand.
To divide by a given spacing rather than a number of parts, use MEASURE, aliased ME. Both commands place points; they differ in the question they ask. DIVIDE asks for the number of segments, MEASURE asks for the length of each one.
Frequently asked questions
Which command measures an arc in AutoCAD?
It depends what you are measuring. Arc length uses DIMARC, aliased DAR; radius uses DIMRADIUS, aliased DRA; included angle uses DIMANGULAR, aliased DAN.
Is there an ARCLENGTH command in AutoCAD?
No. On the Dimension menu that is only a button label. The command you can type is DIMARC; via the button you type the letter L when prompted.
How do I measure an arc without dimensioning the drawing?
Call DIMRADIUS, click the arc, read the figure off the command line, then press Esc. Or select the object and read the Properties palette on Ctrl and 1.
I divided an arc and no points appeared. Why?
The default point style is too small to see. Type PTYPE and choose a larger style, and the division points show up.
Why do the same two lines give 30 one time and 150 the next?
Because two crossing lines create four angles. The command dimensions whichever one the arc passes through, so the last click decides the figure.
Can I dimension an arc segment inside a polyline?
Yes. The DIMARC prompt reads Select arc or polyline arc segment:, so a corner rounded with FILLET does not need the polyline exploded.
Related articles
- How to draw an arc in AutoCAD — eleven construction methods and the radius-against-chord trap
- Object snap in AutoCAD — snapping to arc endpoints and centres while dimensioning
- Construction drawing symbols — the conventions used across a drawing set
- What is AutoCAD — a handbook for people working in construction