Gin,
I suspect you mean the aquaplaning (or hydroplaning) speed
There are two equations for this, one for spin up aquaplaning at touch down and another for spin down aquaplaning during braking.
The exams usually go for spin down which is
Vp in Kts = 9 x (square root of tyre pressure in psi / SG
of the surface contaminant)
If the contaminant is water the SG is 1 so the equation becomes:
Vp in Kts = 9(sqr root tyre pressure in psi)
The spin up equation is similar but uses 7.7 in place of 9.