Every pedometer's distance is steps × stride — and almost everyone uses a guess for the stride. Walk a distance you know once, and stop guessing.
Longer is better: at 100 m, one miscounted step moves the answer under 1%. Below ~50 m, don't bother.
The standard formula — height × 0.415 — is a population average wearing your clothes. Real strides vary with legs, pace, and habit by easily ±10%, which means every distance your pedometer reports inherits that error. Measuring once fixes it: the calibrator counts your steps over a distance you trust, divides, and hands the result straight to the pedometer. Two minutes on a running track and your kilometres are actually kilometres.
Stride also shrinks a little at slower speeds and on treadmills — if you mostly walk one way, calibrate that way. The optional height field on the result compares your measured stride to the formula, mostly for the small satisfaction of beating an average.