Type a ratio, get dB — or type dB, get the ratio. Toggle between power (10·log) and voltage (20·log) modes.
A decibel is a logarithmic ratio. It compresses huge dynamic ranges (a whisper to a jet engine) into a manageable scale, and it turns multiplication into addition — a 20 dB amplifier followed by a 10 dB attenuator gives 30 dB total, no math required.
The formula changes based on what you're measuring: Power: 10 · log(P2/P1), Voltage or current: 20 · log(V2/V1). The factor-of-2 difference is because power is proportional to V².