Decibels and ratio
Decibels turn multiplication into addition: two stages of 20 dB make 40 dB, which is ×10 then ×10, or ×100.
dB = 20 × log10(Vout / Vin) power: 10 × log10(Pout / Pin)
Voltage convention, 20 × log10. Power ratios use 10 × log10 and are shown alongside. Enter ratios above 1; losses are the same figures with a minus sign. dBm, dBu and dBV are absolute levels referenced to a fixed power or voltage.