Measure objects in centimeters

Line the object up with the 0, not with the end of the ruler. Every ruler has a little margin before its zero — measure from the end and the answer comes out a centimeter or two long, every time.

Now follow the toothbrush to its far end and read the mark it stops on. Nothing is worked out here — the numbers along a ruler have already counted the centimeters from 0, so the mark at the end IS the measurement.

So the toothbrush is 13 cm.