How do you convert vp-p to Decibel's

How do you convert Volts peak to peak into decibels?

My Vout is 12 Vp-p and my Vin is 42 Vp-p.

The equation that I have is 20 x log (Vout/Vin)

Is this the correct equation or do I have to convert the Voltage into Vrms, Vp or something else first?

The gain is 20 x log (12/42) = -10.9 dB, the minus sign means that you have signal loss not gain.By the way this equation is only correct when both the input and output impedances are equal. And as the other respondents have said you can use peak, peak to peak voltages or R.M.S. voltage for V in and V out but the both V in and V out must be in the same units.

You can use the peak-to-peak values and you will get the same answer as if you used rms values.

Your equation is correct. Just be sure to use compatable units (as you have), Vp/Vp, Vrms/Vrms, etc.