Since she PM'd this to like eleventy billion people it seems, I thought I'd post the solution for everyone.
First, the question:
Quote:
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Originally Posted by sti deede
You are super smart, so I'm hopeing you'll be able to help.
I've been hunting for the formula for a the length of a chord online and I can't find it. I need it to finish a site plan I'm working on. 
I know the radius of the circle and the length of the arc that I need the chord for.
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Now the solution:
Part one is simply the definition for relating arc length, radius, and central angle (in radians).
Part two is how to use that definition with the known radius and arc length to find the chord length.
Just make sure your calculator is in radians mode when using the final equation:
chord length = 2r sin(s/2r)