Decibel Calculator
dB from a power or amplitude ratio and back; dBm, dBW, mW, watts, dBV and dBµV at any impedance; add sound levels or subtract a background — references stated.
The dB value or ratio, all absolute levels at once (dBm, W, dBW, V rms, dBV, dBµV), the combined level of several sources or the source level with the background removed.
Example: 2 : 1 in power is 3.01 dB, 10 : 1 in voltage is 20 dB; 30 dBm is 1 W and 7.07 V rms at 50 Ω; two 80 dB machines together are 83 dB; 75 dB measured over a 70 dB background leaves a 73.3 dB source.
Ten or twenty times the log,
and always a reference.
The two dB formulas, what the absolute units mean, and how levels combine.
Ratios
A decibel is ten times the base-10 logarithm of a power ratio: 10·log₁₀(P/P₀). Voltage, current and sound pressure are amplitudes whose square is a power, so they use 20·log₁₀(A/A₀) — the same dB for the same power change. Hence 3 dB is double the power (× 1.41 in voltage) and 6 dB double the amplitude (× 4 in power); 10 dB is ten times the power, 20 dB ten times the amplitude.
Absolute levels
dBm is power relative to 1 mW and dBW relative to 1 W (dBW = dBm − 30); dBV is voltage relative to 1 V rms and dBµV relative to 1 µV (dBµV = dBV + 120). Converting between power and voltage needs the impedance, P = V²/Z — 50 Ω for RF, 75 Ω for video, 600 Ω in older audio — which is why the page asks for it. dB SPL (20 µPa) and dB(A) belong to acoustics and are not converted here.
Adding levels
Independent sources add as powers: L = 10·log₁₀ Σ 10^(Lᵢ/10). Two equal sources give +3 dB, ten give +10 dB, and a source 10 dB below the loudest adds only 0.4 dB. Subtracting a background reverses this; when the total is within about 3 dB of the background the result is unreliable and the page says so. Coherent signals in phase add as amplitudes instead. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- dB = 10·log₁₀(P/P₀) for power, 20·log₁₀(A/A₀) for amplitude; dBm re 1 mW, dBW re 1 W, dBV re 1 V rms, dBµV re 1 µV, P = V²/Z; incoherent levels sum as 10·log₁₀ Σ 10^(Lᵢ/10)
Last reviewed 22 September 2026. How results are checked: How we verify.