Three Ways to Find a Chord
A chord connects two points on a circle with a straight line, and machinists usually arrive at it from one of three directions: knowing the central angle the arc subtends, knowing how long the arc itself is, or knowing the sagitta, the bulge height between the chord and the arc. This calculator works from whichever of those three you already have.
Sagitta Comes Up More Than You'd Think
Sagitta is what you measure when you lay a straightedge across a curved feature and read the gap at the midpoint with a feeler gauge or depth mic. It's a fast way to verify a radius on a part without needing to access the true center, which is often the case on large radii or partial arcs where the center point is off the part entirely.
Checking Your Numbers
All three modes should agree with each other for the same arc: feeding the angle this calculator outputs back in as the "radius + angle" mode should return the same chord you started with. Use that as a quick sanity check when working from field measurements that might have some error in them.