Decibel Calculator
Ratios to dB and back, adding sound levels, and dBm, dBu and dBV.
About this tool
Three calculators in one. Convert a power or amplitude ratio to decibels and decibels back to a ratio; add several sound levels to find their combined level; and convert milliwatts to dBm, dBm to power, and RMS volts to dBu and dBV.
How decibels work
The decibel is a logarithmic way of expressing a ratio, which suits quantities that span huge ranges, such as sound intensity or signal power. For power, dB = 10 ร logโโ(Pโ/Pโ): doubling the power adds about 3 dB, and ten times the power adds 10 dB. For amplitude quantities such as voltage, current or sound pressure, power is proportional to the square, so dB = 20 ร logโโ(Vโ/Vโ): doubling the voltage adds about 6 dB. Because decibels are logarithmic, sound levels from separate, unrelated sources don't add arithmetically. Their powers add, so two machines at 85 dB together make about 88 dB, and ten would make 95 dB. Several reference levels turn decibels into absolute values: dBm is power relative to 1 milliwatt, so 30 dBm is 1 watt; dBu is voltage relative to 0.775 V, so professional line level, +4 dBu, is about 1.23 V; and dBV is relative to 1 V. Sound pressure level in dB SPL is relative to the threshold of hearing, and A-weighting (dBA) adjusts for the ear's sensitivity. Honest limits: adding levels assumes uncorrelated sources; identical signals in phase add 6 dB and out of phase can cancel. The sound meter, the hearing test and the RC filter calculator are related.
Frequently asked questions
How many decibels is double the power?
About 3 dB. Double the voltage or sound pressure is about 6 dB.
What do two 85 dB sounds add up to?
About 88 dB, because decibel levels add logarithmically.
What is the difference between dBu and dBV?
dBu is relative to 0.775 V and dBV to 1 V, so 0 dBV โ +2.2 dBu.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.