Decibels don’t add up like normal numbers: 60 dB plus 60 dB is 63 dB, not 120. This free decibel calculator does the maths for you. Add several sound levels, subtract background noise, average dB readings, work out how loud a sound is at a new distance, convert dB to power or amplitude ratios and dBm to watts, or turn a fan’s sone rating into phons. Every result shows its formula.
Decibel Calculator
Choose a calculation, enter your numbers and press Calculate. Each result shows the formula used.
60 dB + 60 dB = 63.0 dB
L = 10·log10(10^(60/10) + 10^(60/10)) = 63.0 dB. Two equal sources add 3 dB, not double the decibels.
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How to Use the Decibel Calculator
- Choose a calculation: add levels, subtract background, average, distance, dB ↔ ratio, or sones ↔ dB.
- Enter your values. For adding, use + Add source for up to 10 sources.
- Press Calculate. The result appears with the formula and a short explanation; use Copy result to paste it elsewhere.
Adding Decibels
Sound levels add as energy, not as numbers: L = 10·log10(10^(L1/10) + 10^(L2/10) + …). Two equal sources are 3 dB louder than one, ten equal sources are 10 dB louder, and a quieter source adds very little. As a shortcut, find the difference between two levels and add this to the higher one (why the scale is logarithmic):
| Difference between the two levels | Add to the higher level |
|---|---|
| 0 dB | +3.0 dB |
| 1 dB | +2.5 dB |
| 2 dB | +2.1 dB |
| 3 dB | +1.8 dB |
| 4 dB | +1.5 dB |
| 5 dB | +1.2 dB |
| 6 dB | +1.0 dB |
| 8 dB | +0.6 dB |
| 10 dB | +0.4 dB |
Subtracting Background Noise
When you measure a machine or speaker with other noise around, the reading includes both. Measure once with the source on and once with it off, then subtract: Ls = 10·log10(10^(Ltotal/10) − 10^(Lbackground/10)). If the difference is 10 dB or more, the background hardly matters; under 3 dB the result is unreliable.
| Total minus background | Correction to the total |
|---|---|
| 3 dB | −3.0 dB |
| 4 dB | −2.2 dB |
| 5 dB | −1.7 dB |
| 6 dB | −1.3 dB |
| 7 dB | −1.0 dB |
| 8 dB | −0.7 dB |
| 9 dB | −0.6 dB |
| 10 dB | −0.5 dB |
Averaging Decibel Readings
To average several readings, convert them to energy, average, and convert back, which is exactly how the equivalent continuous level (Leq) works. A plain average understates loud moments: 60 dB and 70 dB average to 67.4 dB, not 65 dB. See how to measure decibels for measuring an average level.
Sound Level and Distance
In open space, sound spreads out as it travels. From a point source such as a machine or loudspeaker, the level drops by about 6 dB each time the distance doubles (L2 = L1 − 20·log10(r2/r1)); from a line source such as a busy road it drops by about 3 dB per doubling. Indoors, reflections from walls reduce the drop. Sound pressure level explains what is being measured.
dB, Ratios, Gain and dBm
Use 10·log10 for power quantities (watts, sound intensity) and 20·log10 for amplitude quantities (voltage, current, sound pressure, gain). dBm is power relative to 1 milliwatt: 0 dBm = 1 mW and 30 dBm = 1 W. See dB vs dBm vs dBW and, for digital audio levels, dB vs dBFS.
| dB | Power ratio | Amplitude ratio |
|---|---|---|
| +3 dB | 2× | 1.41× |
| +6 dB | 4× | 2× |
| +10 dB | 10× | 3.16× |
| +20 dB | 100× | 10× |
| −3 dB | 0.5× | 0.71× |
| −6 dB | 0.25× | 0.5× |
Sones, Phons and Decibels
Bathroom fans and range hoods are often rated in sones, a scale of perceived loudness: 1 sone equals 40 phons, and every doubling of sones adds 10 phons. Phons match decibels only for a 1 kHz tone, so treat a sone-to-dB conversion as an estimate. The decibel units guide covers these and other units.
Frequently Asked Questions
Why is 60 dB + 60 dB only 63 dB?
Decibels are logarithmic. Two equal sounds carry twice the energy, and doubling the energy adds 10·log10(2) ≈ 3 dB. See what a decibel is.
How do I convert dB to dBA?
You can’t convert a single dB number to dB(A): the A-weighting depends on how much of the sound is at each frequency. Measure in dB(A) directly, or apply A-weighting to each frequency band (dB vs dBA).
Can I convert Hz to dB?
No. Hertz measure frequency (pitch) and decibels measure level (loudness), so there is no conversion between them (dB vs Hz).