you take the Pitch Diameter of two different gears, add them together and divide two(2), and that will determine the on-center spacing between their center points

this tool does not work on a specific standard, as it doesn't need to. as long as you reference all calculated units in the tool by the same standard (mm, inches, ft, miles, etc) then everything will remain equivalently proportional

It's 2USD you tight arse!

​@@biddysingh2 Issues is it once had a free option and many people used it, the greed took over. It's cheap but will kill the site.. It once was a blessing to the youth in the maker/inventor community. I cant recommend the site to anyone anymore dew to giving Card info to a privet individual. Youth doesn't have credit cards and young engineers used that site it for many things. and parents are to skeptical Its hard starting out. I want the future generations to find success without a 2$ deterrent holding them back, I found a set of every gear anyone could need that I can scale and add the holes I need per project. Lucky I'm a professional 3d modeler and can give back to the community at every opportunity.