dB Calculator: Decibels, Sound Pressure and Intensity
Choose a calculation mode, enter your values, and get the decibel level instantly. Switch between sound pressure level (SPL), sound intensity level (SIL), power ratio, or voltage ratio. The calculator also converts any dB value back to its linear equivalent so you always know what the number means in physical terms.
What is a decibel?
The decibel (dB) is a logarithmic unit used to express the ratio of a quantity (power, pressure, voltage, or intensity) to a reference value. Because the human ear responds to sound over an enormous range - roughly a factor of one trillion in intensity between the quietest audible sound and the threshold of pain - a logarithmic scale is far more practical than a linear one. On the decibel scale, each 10 dB increase represents a tenfold increase in acoustic power, and humans perceive it as roughly twice as loud. Every 3 dB change in power (or 6 dB change in amplitude/pressure) is the smallest difference most listeners can reliably detect.
The two core formulas: power vs amplitude
There are two decibel formulas, and using the wrong one is the most common mistake. For power quantities (acoustic intensity, electrical power): dB = 10 x log10(P / P0), where P0 is the reference power. For amplitude quantities (sound pressure, voltage, current): dB = 20 x log10(V / V0), where V0 is the reference amplitude. The factor of 20 in the amplitude formula comes from the fact that power is proportional to the square of the amplitude, and log10(x^2) = 2 x log10(x). The standard reference for sound pressure level (SPL) is 20 micropascals (0.00002 Pa), which is approximately the threshold of human hearing at 1 kHz. The standard reference for sound intensity level (SIL) is 1 picowatt per square metre (1 x 10^-12 W/m2). At these references, a normal conversation at 1 metre reads about 60 dB SPL and 60 dB SIL.
How to interpret dB values: key benchmarks
A few anchor values are worth memorising: +3 dB doubles acoustic power (barely noticeable); +6 dB doubles sound pressure amplitude (clearly louder); +10 dB represents a tenfold power increase and sounds roughly twice as loud; +20 dB is a hundredfold power increase and sounds four times as loud. Occupational health guidelines (NIOSH, OSHA) set 85 dB as the threshold above which daily noise exposure should be limited. Above 85 dB, the permissible exposure time halves for every 3 dB increase under the NIOSH criterion, or every 5 dB under the OSHA standard. Prolonged exposure above 85 dB causes permanent noise-induced hearing loss (NIHL), the most preventable form of hearing loss worldwide.
Combining and subtracting decibel levels
You cannot add decibel values directly because they are logarithmic. To combine two independent sound sources, convert each to a linear power scale, add them, then convert back: dB_total = 10 x log10(10^(dB1/10) + 10^(dB2/10)). As a rule of thumb, combining two identical sources (e.g. two 80 dB machines) gives roughly 3 dB more (83 dB), not 160 dB. Combining a 90 dB source with a 70 dB source gives approximately 90.4 dB, because the weaker source contributes only a tiny fraction of the total power. To subtract background noise from a measurement, reverse the same algebra: dB_signal = 10 x log10(10^(dB_total/10) - 10^(dB_background/10)). This subtraction only works if both readings were made with a calibrated meter and the background is more than about 3 dB below the combined level.
Common sound levels reference
| Sound source | Approx. level (dB SPL) | Perceived loudness |
|---|---|---|
| Threshold of hearing | 0 | Inaudible |
| Anechoic chamber (quietest rooms) | 10-15 | Barely perceptible |
| Quiet rural night, rustling leaves | 20-30 | Very quiet |
| Whisper at 1 metre | 30 | Quiet |
| Library, quiet bedroom | 35-40 | Quiet |
| Normal conversation at 1 metre | 60-65 | Moderate |
| Busy office or restaurant | 65-70 | Moderate-loud |
| Heavy city traffic | 80-85 | Loud |
| Lawn mower at 1 metre | 90 | Very loud |
| Motorcycle at 10 metres | 95 | Very loud |
| Chainsaw at 1 metre | 100-110 | Extremely loud |
| Rock concert near speakers | 110-120 | Extremely loud |
| Threshold of pain | 120-130 | Painful |
| Jet engine at 30 metres | 140 | Immediate damage |
Approximate SPL values for everyday sound sources, measured at typical distances. Values rounded to the nearest 5 dB.
Frequently asked questions
Why do I multiply by 20 for sound pressure but 10 for power?
The factor of 20 applies whenever you are working with an amplitude quantity (pressure, voltage, current, electric field). Acoustic power is proportional to the square of pressure, so squaring the ratio before taking the log is equivalent to multiplying the log by 2: 10 x log10((p/p0)^2) = 20 x log10(p/p0). For quantities that are already proportional to power (intensity, watts), the factor is simply 10.
What does 0 dB mean?
Zero decibels does not mean no sound. It means the measured quantity equals the reference quantity exactly (ratio = 1, log10(1) = 0). For SPL, 0 dB SPL is 20 micropascals, the approximate threshold of human hearing. Negative dB values are possible and mean the quantity is below the reference level, which can occur in very quiet anechoic chambers or when a signal is attenuated below its reference.
How much louder is 10 dB than 0 dB?
A 10 dB increase corresponds to a 10-fold increase in acoustic power and roughly double the perceived loudness. A 20 dB increase is 100 times more power and sounds about four times louder. A 30 dB increase is 1,000 times more power. The logarithmic scale compresses this enormous range into manageable numbers.
What dB level is safe for my hearing?
NIOSH recommends a maximum daily exposure of 85 dB for 8 hours. For every 3 dB increase above 85 dB, the safe exposure time halves: 88 dB for 4 hours, 91 dB for 2 hours, and so on. A single brief exposure above 140 dB can cause immediate, irreversible damage. If you cannot hold a conversation at arm's length without raising your voice, the environment is likely above 85 dB.
What is the difference between dB SPL and dBA?
dB SPL measures the actual physical sound pressure level relative to 20 micropascals across all frequencies equally. dBA applies an A-weighting filter that emphasises frequencies in the 1-4 kHz range where the human ear is most sensitive, and de-emphasises very low and very high frequencies. dBA readings better predict how loud a sound is perceived and how much hearing damage it may cause, which is why occupational health regulations typically cite dBA values.