Does anyone know how to calculate the Ripple Voltage on a half-wave rectifier circuit and on a full-wave?

I found the following formulas but when I compare them with what I get on the oscilloscope, the difference is huge!

Ripple (half-wave)= 4,5 x I(mA)/C( μ F)

Ripple (full-wave)= 1,7 x I(mA)/C( μ F)

Cheers

David Mendes