Formula Help

I am a computer program and not a mathematician.  I use the equations
below to calculate Gallons when I have the Radius, Depth and Length of
an oil tank.  However, sometimes I need to calculate Depth when I have
the Radius, Length and Gallons.  Any help in reworking these formulas
would be greatly appreciated.


If iDepth <= iRadius Then

            iGals = ((iRadius * iRadius * Acos((iRadius - iDepth) /
iRadius)) - ((iRadius - iDepth) * Sqrt(2 * iRadius * iDepth - iDepth *
iDepth))) * iLength / 231

Else
            iGals = ((PI * iRadius * iRadius) - ((iRadius * iRadius *
Acos((iRadius - (2 * iRadius - iDepth)) / iRadius)) - ((iRadius - (2 *
iRadius - iDepth)) * Sqrt(2 * iRadius * (2 * iRadius - iDepth) - ((2 *
iRadius - iDepth) * (2 * iRadius - iDepth)))))) * iLength / 231

End If