# How to draw a precise circular arc in Adobe Illustrator?

Say, an arc with radius r centered at o, from angle a to angle b.

I don't see how to use Arc tool in Adobe Illustrator this way. What's the best way to draw it?

AI sucks for this. Illustrator doesn't draw "real" circles (ie., using centerpoint and radius) either, it makes an `Ellipse` using bezier curves.

Your best bet is to make a `Ellipse` with equal width & height (ie., a circle) and use 2 `Line Segment` shapes to intersect the `Ellipse`. It takes 3-4 steps instead of 1, but it will allow you to use specific dimensions.

Use the pie graph tool! It's a bit of a hack but it works.

Example: Creating a 30° angle.

1. Create a pie graph with the dimensions you want.
2. Calculate the angle in terms of revolutions of a full circle. 30°/360° = 1/12 revolutions.
3. Create a pie graph with the values 1 and 11 (1/12 + 11/12 = 1 or 100%)
4. Ungroup the graph object and select the wedge that corresponds the angle.
• Or, as an easier alternative to step 2 (revolutions of a full circle) if you don't have a calculator handy, enter the angle you want in the first cell, then 360 minus that number in the second. So, `30` and `330` for 30°, `85` and `275` for 85°, etc. And when you're done, select the centre point with the direct selection tool (`a`) and delete it, leaving just the arc instead of a wedge. Feb 11, 2013 at 15:48

Farray's answer is the standard way. Another way is to use scripting. One script that could help is the "Circle" script from here http://shspage.com/aijs/en/#ovalize. It will create a circle with a given number of equal segments, want a 60° arc, make six segment circle and delete all but one segment. Another option (if you have some programming skills) could be to creating your own script. Scriptographer is a scripting plugin for Illustrator that adds some constructors to allow you to create arcs in your own scripts.

Unless things were improved, you have to still draw a symmetrical ellipse (circle) then use the "c" (scissors) to to cut it to size. The "scientific" way takes about as many steps and is less visual. A black hat is a hat black (French proverb.) It would be nice if you could specify and angle and radius then move the results into position.

• By eye? The question is about precise specification.
– e100
Nov 1, 2012 at 13:38

Faster, precise and much simpler: With the Ellipse tool selected, hold down Shft+Alt and draw a perfect circle to the diameter of your choice. (You can add fill but remove stroke).

Select the Line Segment Tool, select the circle's center anchor point (not the path), draw a vertical segment overlapping the top edge of the circle a few pixels (overlapping is important).

press R on keyboard (rotating Tool).

Then hold Alt and select the segment's bottom anchor point. The rotating tool windows pops up. Now, on the box where you enter the angle's rotation, enter 360/N ** (N being the number for how many equal segments of circle you wish to divide the circle into)** and still on the the menu, click Copy.

Press Ctrl+D (N-2 times on keyboard).

Get the Select Tool and select everything on the screen and that you have it...your circle perfectly divided in symmetric segments as you chose. You can proceed and do whatever with the image.

Here is how you can achieve a precise arc with any radius (in AI): Let us assume we want an arc of 250 °.

1. Draw a line with length 360 px (line 1).

2. Draw another line with the length corresponding to your arc angle. In our case 250 px. (line 2)

3. Now place the start point of line  2 over start point of line 1 (overlap both lines).

4. Select line 1 (360 px), and make its opacity 0 %.

5. Open the brushes window (window → brushes).

6. Select both lines, drag and drop into the brushes window to create a new brush.

7. Now draw a circle of whatever radius you want (give a stroke and fill transparent).

8. Select your created brush from the brush definition in tool box.