A decibel calculator for sound levels, signals and gain
Decibels are logarithmic, so they do not add like ordinary numbers: two 85 dB machines together make 88 dB, not 170. This decibel calculator does the five sums people most often need: adding or averaging levels, removing background noise from a reading, converting dB to a linear power or voltage ratio (and back), working out how a sound level drops with distance, and converting between dBu, dBV, volts and dBFS for audio gear.
Every result shows the formula used, so you can check it or copy it into a report. It runs in your browser; nothing is uploaded.
How to use the Decibel Calculator
- Pick what to calculate: Add levels, Subtract background, dB ↔ ratio, Distance, or dBu · dBV · dBFS.
- Type your numbers; the result updates as you type.
- For signal levels, choose what 0 dBFS equals on your equipment (+24 dBu or +18 dBu are the common studio and broadcast alignments).
- Read the breakdown under the answer, then press Copy result to paste it elsewhere.
Decibel Calculator features
Add and average dB
Energy sum of any number of levels, the energy average and each source’s share of the total.
Background correction
Takes the background out of a measured total and warns when the difference is too small to trust.
dB to linear
Power ratio 10^(dB/10) and voltage or pressure ratio 10^(dB/20), plus dB SPL to pascals.
Distance
Inverse-square drop for a point source and the 3 dB-per-doubling rule for a line source, with a table.
Audio level units
dBu, dBV, volts RMS, peak and peak-to-peak, dBFS and dBm on 600 Ω.
Formulas used
- Adding levels:
L = 10 log₁₀(10^(L₁/10) + 10^(L₂/10) + …). Two equal levels give +3.01 dB, ten give +10 dB. - Subtracting background:
L_source = 10 log₁₀(10^(L_total/10) − 10^(L_background/10)). - Ratios:
dB = 10 log₁₀(P₂/P₁) = 20 log₁₀(V₂/V₁), because power is proportional to voltage squared. - Sound pressure level:
dB SPL = 20 log₁₀(p ÷ 20 µPa); 94 dB SPL is almost exactly 1 pascal. - Distance:
L₂ = L₁ − 20 log₁₀(d₂/d₁)for a point source in free field (−6 dB per doubling); a long line source such as a busy road loses about 3 dB per doubling. - Signal levels: 0 dBu = 0.7746 V RMS (1 mW into 600 Ω), 0 dBV = 1 V RMS, so dBV = dBu − 2.21. dBFS is relative to the converter’s full scale.
Worked examples
Three sources at 85, 85 and 82 dB: 10 log₁₀(2 × 10^8.5 + 10^8.2) = 89.0 dB. A machine reads 90 dB with the background at 84 dB; the machine alone is 10 log₁₀(10^9 − 10^8.4) = 88.7 dB. A speaker giving 100 dB at 1 m gives 100 − 20 log₁₀ 8 = 81.9 dB at 8 m in the open.
A professional line level of +4 dBu is 1.228 V RMS, which is 1.78 dBV and, on a desk where 0 dBFS = +24 dBu, −20 dBFS. To size the amplifier that drives a speaker to a target level, use the speaker wattage calculator; to measure a real level with your microphone, try the decibel meter.
Which 0 dBFS reference?
Digital full scale is not tied to a voltage, so equipment is aligned to a reference. SMPTE RP 155 places a +4 dBu line-up tone at −20 dBFS (so 0 dBFS = +24 dBu); EBU R68 places 0 dBu at −18 dBFS (0 dBFS = +18 dBu). ITU-R BS.1726 notes both practices. Consumer and some converters use other values, so choose Other… and type your converter’s maximum input level in dBu if you know it.
Limits
- Adding assumes uncorrelated sources (different machines, noise). Two identical, in-phase signals add up to +6 dB instead, and out-of-phase signals cancel.
- The distance rule is for free field. Indoors the reverberant sound stops the level falling much beyond a few metres; see the RT60 calculator.
- A-, C- and Z-weighting are not applied here; add weighted levels only to other weighted levels.
Decibel Calculator FAQ
How do I add decibels?
Convert each level to energy (10^(L/10)), add them, then take 10 log₁₀ of the sum. This decibel calculator does it for any number of levels. Two equal sources add 3 dB.
How do I convert dB to a linear ratio?
For power use 10^(dB/10); for voltage, current or sound pressure use 10^(dB/20). +6 dB is about 4× the power and 2× the voltage.
How much does sound drop with distance?
About 6 dB each time the distance doubles from a small source outdoors, and about 3 dB per doubling from a long line source such as a road.
What is the difference between dBu and dBV?
Both are voltage levels. 0 dBu is 0.7746 V and 0 dBV is 1 V, so a level in dBV is 2.21 dB lower than the same level in dBu.
What is −20 dBFS in dBu?
It depends on the alignment. With the common studio alignment (0 dBFS = +24 dBu) it is +4 dBu; with the EBU alignment (0 dBFS = +18 dBu) it is −2 dBu.
Is 3 dB double the loudness?
No. 3 dB is double the power, 6 dB is double the sound pressure, and about 10 dB is heard as roughly twice as loud.
Can I subtract background noise from a reading?
Yes, if the total is at least 3 dB above the background. Below that the result is unreliable; above 10 dB the background makes almost no difference.