The most important dimensions of helical gear geometry can be calculated online according ISO 21771. For test dimensions like diametral dimensions over balls iterative calculations are necessary, therefore it is difficult to do these calculations manually. The free online calculation shows the radial or diametral dimensions over balls, the base tangent length (or span) and the required profile shift to avoid undercut in addition to the most important diameters. Also the tooth form is shown graphically, so that errors like undercut or minimal topland are quickly seen.
According ISO 21771 for internal gears only the number of teeth is negative. All diameters are shown as positive values. This is different to standards like DIN 3960.