An arc is a fraction of the way around a circle, so its length is that same fraction of the circumference. These free printable worksheets keep to the curved edge alone.
If they get stuck
The answer key prints on a separate page.
The step from a whole circle to a piece of one is a single multiplication, and it is the step a formula sheet hides. The circumference is 2πr; the arc is that times the angle over 360. A child who writes the fraction down first — 90 over 360, which is a quarter — has usually finished before one who reaches for the formula.
Every angle here divides 360 exactly, and the radii are chosen so the answer lands on two decimal places. So the paper never says round to the nearest tenth and the key can be used to check rather than to compare. π is 3.14 throughout.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Arc length, step by step