Converting in dB or vice-vers is simple. This is just the ratio of two AC signals, expressed in dB, log. scale.
If we have, say 1Vrms input, and output is 0,707Vrms (attenuation), then we can express this in dB like that:

A = 20 * log(V1/V2)
First, compute (on calculator) V1/V2 = 1/0,707 = 1,4142
Second, press "log" (from 1,4142), and this is 0,15
Third - multiply this (0,15) with 20 and you`ll get 3dB.

Note that the result would be the same (3dB) if we`ve had amplification, from 0,707 to 1V rms. It is the ratio.

Converting from dB to the amplification ratio, backwards:

-we have, say 3dB amplification, or A = 20 * log V1/V2 = 3 dB
-from that, log V1/V2 = 3/20 or log V1/V2 = 0,15
-on calculator, press INV log (of 0,15) and you`ll get 1,41 times