# CAD Forum -- Specifying Arc Length

There are several ways to specify an arc in AutoCAD, knowing just the length of the arc and the radius. September 24, 2006

Question
I need to be able to draw an arc in ACAD2005 that is a specific length and radius. These will be the only two known variables. I can't use a chord measurement. We are jigging a known radius between points. I will be able to measure the jig's radius (measuring the chord will be too inaccurate). Can anyone give us any help with this?

Forum Responses
From contributor A:
I come across this fairly often. The only way that I have come up with to tackle this problem is a little convoluted. I first draw an arc at the given radius but at some arbitrary length. I then use the Lengthen command and select T for Total. Then just select the arc and it will extend or shrink to the length you gave.

From contributor A:
I forgot a step. After you select T for Total, you will prompted to give a value. At that point you will type in the length you want the arc to be.

From the original questioner:
Thanks - that works.

From contributor B:
You can use some math to get angle:
angle= (length*180)/PI*R
now just draw arc with center, radius, angle
PI=3.141593

From contributor C:
1. In ought-to-CAD draw a circle of the desired radius and list the circumference.
2. Divide the desired radius by the circumference.
3. Multiply the result from #2 by 360 to obtain the circle section for the required arc.
4. Draw two lines of the calculated angle from the center of the circle to the circle.
5. There’s your arc of the desired length.

From contributor C:
Correction:
2. Divide the desired length (not radius) by the circumference.

From contributor D:
It's simple - draw the circle first with the radius that you have, then select draw point and then "measure" command to the length that you have. The points are going to be spaced to that # in circle - and just break the rest of that and you have your arch.

From contributor E:
We are using '06 and do this on the pointer but on the command line don't you get the options - center, end?
try;
arc
select a start point
E for end - put in your fixed length