How to use it
- Choose Calculation: a ratio, dBm and watts, volts and dBV or dBu, or the sum of levels.
- For a ratio, type two of First value, Second value and Gain or loss (dB). The third is worked out.
- For dBm or volts, type one box only and read the converted value in the result.
- For Add decibel levels, type two or more levels. The Total level is their sum as powers.
The formulas
A decibel is ten (or twenty) times the logarithm of a ratio, so decibels add where the ratios multiply. Power ratios use ten times the logarithm. Voltage and current ratios use twenty times, because power goes with the square of the voltage.
Power: dB = 10 × log10(P2 ÷ P1)
Voltage: dB = 20 × log10(V2 ÷ V1)
dBm: P(dBm) = 10 × log10(P ÷ 1 mW)
dBV: V(dBV) = 20 × log10(V ÷ 1 V)
dBu: V(dBu) = 20 × log10(V ÷ 0.7746 V)
Sum: L = 10 × log10(Σ 10^(Li ÷ 10))- 0.7746 V RMS is the voltage that gives 1 mW in 600 Ω, so 0 dBu is that voltage. In a 600 Ω circuit dBu and dBm are the same number.
- Adding decibels is right only for incoherent sources, which do not interfere with each other. Two equal levels give 3 dB more, not double the level in decibels.
What the calculator does not cover
- Coherent sources, which can add or cancel depending on their phase. Their levels do not add by this rule.
- Sound pressure levels (dB SPL) and frequency-weighted levels (dBA), which use their own reference values.
- The difference between peak and RMS values of a signal. Every voltage here is an RMS value.
Worked example: 1 W to 2 W, and two sources at 70 dB and 65 dB
Doubling a power is a ratio of 2, so dB = 10 × log10(2 ÷ 1) = 3.01 dB. The same ratio in voltage would be 20 × log10(2) = 6.02 dB.
Going the other way, 30 dBm is P = 1 mW × 10^(30 ÷ 10) = 1 W.
Two incoherent sources at 70 dB and 65 dB give L = 10 × log10(10^7 + 10^6.5) = 71.19 dB.
Frequently asked questions
What is the difference between dBm and dBW?
Both are absolute power levels. dBm uses 1 mW as the reference and dBW uses 1 W, so 0 dBW is 30 dBm. The calculator gives dBm, and its watts box accepts any power with a prefix, such as 1m or 500m.
Why are dBu and dBV different?
dBV is relative to 1 V RMS, while dBu is relative to 0.7746 V RMS, the voltage that gives 1 mW in 600 Ω. So 0 dBV is about 2.22 dBu.
Why does 3 dB mean a doubling of power?
Because 10 × log10(2) = 3.01, a power ratio of 2 is 3 dB. A doubling of voltage is 6 dB by contrast, and a gain of 10 dB multiplies the power by ten.
Can I add decibel levels by adding the numbers?
No. Decibels are logarithms, so two levels of 70 dB make about 73 dB, not 140 dB. The calculator adds the powers and converts back. This works only for incoherent sources.
Which reference is dBu measured against?
dBu is measured against 0.7746 V RMS, the voltage that gives 1 mW into 600 Ω. Professional audio equipment often quotes levels in dBu, and telephone circuits historically used the 600 Ω reference. Check the specification to see which one applies.
Sources
Figures and text last checked on Tuesday, November 10, 2026.