End of 2007 an ISO geometry standard for cylindrical gears was published which is mostly identical to DIN3960. There is one major difference for internal gears, which are chanracterized by a negative number of teeth in the calculation as it is dome in DIN3960.

Different as in DIN3960 where all diameters and the center distance for internal gears a negative values, these values are positive in ISO21771. The online calculation on these pages is also using the sign convention according to ISO21771.

A positive sign for diameters and center distance avoids irritations about the tolerances. What does a tolerance h7 mean for a negative center distance? Is the backlash increased or deacreased? How is it interpreted in manufacturing? The positive value in the calculation fits better tothe drawing where diameters and distances are usually shown as numbers whithout a sign.