Convert between linear ratios and decibels. Power quantities use 10·log₁₀, voltage and field quantities use 20·log₁₀ — switch with one click.
Power dB uses 10·log₁₀(P1/P2). Voltage (and other field quantities like current, sound pressure, electric field) uses 20·log₁₀(V1/V2). The factor of 2 comes from P ∝ V² in a fixed impedance.
The reason both produce "+3 dB ≈ double" depends on context: doubling power gives +3 dB, but doubling voltage (which quadruples power) gives +6 dB. Watch units, not just numbers.