How to draw an arc in AutoCAD: 11 methods, the counter-clockwise rule and the radius trap

How to draw an arc in AutoCAD comes down to one command, ARC, aliased to A. In a building drawing set, arcs turn up in three familiar places: the swing of a door leaf, the rim of a basin or tub, and the rounded corner of a walkway or balcony.

This command has more construction methods than any other drawing command. The command line offers only two options, but the Arc button menu lays out eleven. This article is a hands-on session: every section is something you can do on a blank drawing. To learn modelling and documentation end to end, see the AutoCAD for construction course.

The arc command in AutoCAD: three prompts, eleven methods

Type A and press Enter. The prompts are short.

Those three lines are the default method: pick three points and the arc passes through all of them. Every other method lives in the drop-down under the Arc button on the Draw panel.

You do not have to memorise all eleven. One rule is enough: each method is named after the order in which the command asks for things. Pick Start, Center, End and it asks for the start point, then the centre, then the end point. Pick Center, Start, Angle and it asks for the centre first, then the start point, then the included angle.

Reading the menu by group is easier still. The four groups begin with three points, with Start, with Center, and finally Continue. In real documentation work, four methods cover almost everything.

Exercise 1: draw an arc through 2 points and one in between

Open a blank drawing. First build a frame to snap to.

1. Type REC and Enter. Click any point, type D and Enter, type 3600 and Enter, type 2400 and Enter, then click to place it.
2. Type Z and Enter, then E and Enter, so the whole frame is on screen.
3. Turn object snap on with F3.

Now draw the three-point arc. This is the default, and the quickest answer to how to draw an arc in AutoCAD when three marks already exist on the drawing.

1. Type A and Enter.
2. Snap to the bottom left corner of the frame — that is the first point.
3. Click a point near the middle of the frame, a little higher up — the second point.
4. Snap to the bottom right corner — the third point. The arc appears, passing through all three.

The three-point method is the only one that needs neither a centre nor a radius. In exchange, it gives you no control over the radius: the radius is a result, not something you state.

A great many people go looking for a way to draw an arc through 2 points. The ARC command has no such menu entry, because two points do not define an arc. A third condition is needed, and the menu offers four: another point on the arc, the centre, the radius, or the tangent direction. Which one you pick depends on what you already know.

Snapping is what decides accuracy here. If the cursor does not lock onto the frame corner, the arc lands a few units off and nothing looks wrong. Turning snapping on and choosing modes is covered in Object snap in AutoCAD.

Exercise 2: a 900 door swing with Center, Start, Angle

This is the arc drawn most often on a floor plan. The door is 900 wide, the hinge sits at one jamb, and the leaf opens 90 degrees.

1. Draw a 900 line for the opening: type L, click the first point, type @900,0 and Enter, then Enter again to finish.
2. Type A and Enter, then C and Enter. The command switches to asking for the centre.
3. Snap to the left end of the line as the centre — that is the hinge.
4. Snap to the right end of the line as the start point of the arc.
5. Type A and Enter, type 90 and Enter.

A 90 degree arc appears. Repeat step 5 with 45 and you get a half-open leaf, which is how some interior drawing sets show doors.

The counter-clockwise rule, where most arcs go wrong

With exercise 2 finished, swap steps 3 and 4: take the right end as the centre and the left end as the start point. The arc still sweeps 90 degrees, but it curves to the opposite side.

The cause fits in one sentence, and Autodesk states it in the command reference: "Arcs are drawn in a counterclockwise direction by default." An arc is always built counter-clockwise from its start point to its end point. Same centre, same two points, reversed order, and you get the complementary arc.

When an arc curves the wrong way, do not delete it and start again. Two fixes are quicker:

• Swap the start and end points and call the command again.
• Hold Ctrl while you drag. The command reference is explicit: holding Ctrl sweeps the arc clockwise.

The second is handier mid-chain, because it does not force you to abandon the command. The ARC (Command) page carries both statements.

The radius trap: type a number and nothing appears

Start, End, Radius is the tidiest method when the drawing set already states a radius. It also fails in a very quiet way: you type the number, press Enter, and the command ends without adding anything. There is no error message.

Try it and see. Mark two points 2400 apart, call A, pick them as start and end, then type R and 900:

The reason is pure geometry. The radius must be greater than half the chord, because no smaller circle can pass through both points. A 2400 chord has a half-chord of 1200, so asking for 900 is asking for something that does not exist.

The same method has one more behaviour that few people know: a negative radius still draws, but gives the major arc, meaning the rest of the circle. Typing -1500 instead of 1500 is the fastest way to get the outer arc without swapping points.

Continue: the fastest way to draw kerbs and handrails

Continue, the last entry on the menu, is the one worth using most and noticed least. Draw a line, then call A and press Enter immediately at the first prompt. The arc takes the end of that line as its start point and the line's direction as its tangent, so only the end point is left to pick.

Try it:

1. Type L, draw a horizontal line about 3000 long, Enter to finish.
2. Type A and Enter immediately, without picking anything.
3. Click a point up and to the right. The arc appears, tangent to the line.
4. Type L and Enter immediately. The new line is tangent to the arc in turn.

Repeat the last two steps and you get a smooth alignment: line, curve, line, curve. This is the quickest way to draw a kerb line, a curved balcony edge, a handrail or a crane rail, because every junction is smooth without any tangent arithmetic.

One caution when chaining: a continued arc follows the segment just drawn, not the one currently selected. If another command runs in between, draw the lead-in segment again.

Measure arc length in AutoCAD after you draw

Once the geometry is there, the numbers have to go on the sheet. An arc carries three of them — radius, included angle and arc length — and each needs a different command.

Arc length uses DIMARC, aliased to DAR. It is neither the radius nor the chord: it is the distance travelled along the arc, so the number is always larger than the chord joining the two ends.

1. Draw an arc of radius 1200 sweeping exactly 90 degrees.
2. Type DAR and Enter. The prompt reads Select arc or polyline arc segment:. Click the arc.
3. Place the dimension line outside. The program prints Dimension text = 1885.

Check it by hand: a quarter of the circumference is pi times 1200 divided by 2, which is 1884.96. The number matches.

The small arc symbol in front of the figure is what stops a reader confusing arc length with chord length. The full set of drawing conventions is in Construction drawing symbols.

There is also a way to read a radius without leaving anything on the drawing. DIMRADIUS prints the number on its second line, before it asks where to put the dimension.

So you click the arc, read the radius off the command line, and press Esc. Nothing is added to the drawing and you still have the number. DIMLINEAR works the other way round, printing its figure only at the end.

Frequently asked questions

What is the alias for the arc command?

Type A and Enter. The default method is three points; for any other, type the letter of the option, such as C for centre, or pick it from the Arc button menu.

Why does my arc curve the wrong way?

Because an arc is always built counter-clockwise from start point to end point. Swap the two points, or hold Ctrl while you drag.

I typed a radius and no arc appeared. Why?

The radius is smaller than half the chord. Measure the distance between the two points again and type a number larger than half of it.

How do I divide an arc in AutoCAD into equal parts?

Use DIVIDE, select the arc and type the number of segments. If nothing seems to happen, the command has not failed: the division points exist but are drawn in the default point style, which is a single pixel. Type PTYPE and choose a larger style to see them.

How do I continue an arc from a line?

Draw the line, then call A and press Enter immediately without picking anything. The arc joins the end of the line and is tangent to it, leaving only the end point to pick.

What is the difference between an arc and a polyline?

An arc is a separate object. A polyline can hold both straight and curved segments in one continuous object. DIMARC measures either, exactly as its first prompt promises: Select arc or polyline arc segment:.

Related articles

• What is AutoCAD — a handbook for people working in construction
• Object snap in AutoCAD — turning snapping on so arcs land on the corner
• The ZOOM command in AutoCAD — zooming, and three jobs confused with it
• Construction drawing symbols — the conventions used across a drawing set