I've seen a formula for doing this, but can't find it now. Still looking into new wheels for my car. Some list "offset" figure and some just list "backspacing." I need to be able to convert one to the other.

English please? Where is the question?

Here's some more info on backspacing and offset

BACKSPACING


The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace.



OFFSET


The distance from the centerline of the wheel to the mounting surface of the wheel.

Thanks for your help Ammi. I guess my English isn't so bad after all.

Back Space to offset conversion:
take overall rim width including outer lip (usually listed rim width + 1"), then divide by 2, then subtract that number from the measured backspace, then convert that number to metric by multiplying by 25.4.

so: 17x8 inch wheel with 5.5 inch BS
8"+1" = 9" /2 = 4.5"; 5.5" - 4.5" = 1" *25.4= 25.4 offset

Offset to Backspace conversion:
Take offset, divide by 25.4, then add the overall rim width divided by 2

so: 17x8 with 30 mm offset
30mm / 25.4 =1.18" + (((8"+1")/2)= 4.5") = 5.68" BS

hope that makes sense