Tire width on your rim

A tire has its nominal width on its design rim. The ISO 5775 standard sets the internal width of that rim by nominal width: 17 mm for 23 and 24 mm, 19 mm for 25 to 28 mm, 21 mm for 29 to 34 mm, 23 mm for 35 to 46 mm, 25 mm for 47 to 57 mm and 30 mm from 58 mm. If a maker measures on a different rim, set it yourself. Source: Vittoria

If the internal width of your rim differs from the design rim, the tire gets 0.4 mm wider or narrower per millimeter. Continental gives this rule of thumb for tires with a bead, with or without a tube: a 28 mm tire measured on a 19 mm internal width is about 29.6 mm wide on 23 mm. As it is an estimate, the result says “approx.”. Source: Continental

The calculator does not check whether a tire is approved for your rim. The rim maker’s tire approval tells you that.