Punch force formula
The fetched Omni punch force calculator uses the standard perimeter-based punching equation:
F = P x S x t
S = 0.7 x T when shear strength is estimated from tensile strength
Using tensile or shear strength
Omni notes that when the shear strength is unknown, it is commonly estimated as 50-70% of tensile strength. The page then uses 0.7 x T for the combined formula.
Source example
With a 15 mm perimeter hole, 5 mm sheet thickness, and Al7075-T6 shear strength of 331 MPa, the required punching force is about 24.825 kN. Those are the defaults loaded here.