Connect two cylinders and gas flows from the fuller one, the donor, into the other until both gauges read the same. The calculator finds that end pressure and the new mix in the receiving cylinder, for nitrox and trimix, with the real properties of oxygen, helium and nitrogen.
It also shows how much gas moved and how much is left in each cylinder, as mass and as free gas. The simple rule, averaging by cylinder size, sits beside it. Real gases don’t follow that rule exactly at cylinder pressures, and the gap can reach several bar.