Why a tire size calculator? I needed this for a project car and I couldn't find anything like it anywhere. I measured the amount of space I had in the wheel wells so I knew what tire sizes would fit in inches, but I then needed to know which real tire sizes that corresponded to. In addition, I wanted to put wider tires on the rear, but keep them within 0.1" of the front tire height so I could have a universal spare tire. Here's my answer.
Note: I have not accounted for the "rule of thumb" that tire section width changes by around 0.2" for every 0.5" difference between the rim width and tire width.
Input desired tire min and max sizes in inches. The "Variation" field lets you say how much of a difference in tire sizes (in inches) is allowable to report the tires as the 'same'. Since tire sizes are not made by actual height (they're based on the ratio of the sidewall height to the width of the tire), you may not find two different width tires with exactly the same height.
I generally feel a difference of 0.1" is fine for any tire combination. You could expand it if, for instance, you wanted 26" front tires and up to 28" rear tires or reduce it if you wanted to be even more certain for a 4x4 vehicle.