Six classic length illusions — Müller-Lyer, Ponzo, Ebbinghaus, Delboeuf, vertical-horizontal and Jastrow — drawn live, with a strength slider, a measure-it overlay printing true pixel lengths and a bias self-test.
Drag the slider until the right-hand figure looks the same size as the left. Then reveal the answer — your error is printed as a percentage. Repeat a few times for an average bias.
Everything here is drawn with plain canvas maths and runs offline. Illusion strength varies a lot between people and is not a measure of eyesight or intelligence.