The formulas to calculate the diameters of inside and outside diameters are well-known trigonometry formulas based on a ‘Unit Circle.’ With the number of segments, a segment edge length is calculated that will make a segmented ring that if you put it on your lathe and remove the minimum amount of wood to make the outside diameter smooth, it will be the diameter calculated by the software.

The inside diameter is then calculated based on the Board Width and again, if you remove just enough wood from the inside to make it smooth, the inside diameter will be as calculated.

The formulas in Woodturner PRO haven’t been changed in 23 years and they are correct.

If your ring has a different inside diameter than calculated, the board width you used was narrower than what was called for in the software.

If you click on the Ring View or the Segment View, you will see a calculation that only shows on those two screens. It shows the segment edge length of the inside (short side) of the segment. On the Ring View, you will have to scroll down to see a box with ALL calculations available. If you cut a test segment, you can print out either the ring view or the segment view and it will be the exact size that the segment should be (this may not be 100% correct depending on your printer). You can lay your segment on it. It is easier, though, just to measure the short edge with a micrometer and compare it to the calculation.

I hope that helps.

Lloyd