About the Decibel Calculator
Decibels compare two quantities on a logarithmic scale, which is why ordinary arithmetic does not work on them: two machines that each make 60 dB together make 63 dB, not 120. This decibel calculator handles the common jobs. Convert a power ratio (using 10 log) or an amplitude ratio such as voltage or sound pressure (using 20 log) into decibels, and convert decibels back to a ratio. Add several sound levels together, take away background noise to find the level of a source alone, and work out how sound level changes when you move closer to or further from a source. Example sounds from a quiet library to a jet help you picture the numbers.
Updated October 2026
How to use the Decibel Calculator
- 1Choose what you want to do from the list.
- 2For a ratio, say whether it is a power ratio or an amplitude ratio.
- 3For adding levels, type the levels in dB separated by commas.
- 4For background noise, enter the measured total and the background alone.
- 5For distance, enter both distances and an optional starting level.
Rules of thumb
- +3 dB is double the power; +10 dB is ten times the power
- +6 dB is double the amplitude; +20 dB is ten times the amplitude
- Two equal sound sources add 3 dB
- Doubling the distance from a small source drops the level by 6 dB
- A 10 dB rise sounds roughly twice as loud
Examples of everyday sounds
Rustling leaves are around 20 dB, a quiet library 40 dB, normal conversation 60 dB, a busy street 80 dB, a lawn mower 90 dB and a rock concert 110 dB. Because the scale is logarithmic, the concert is a million times more intense than the library, even though the number is only about three times bigger. The calculator names the nearest example when you add levels.
Related tools
The dBm Converter handles radio and electrical power levels, the Frequency Converter changes hertz units, the Log Calculator works out logarithms, the Ratio Calculator scales ratios and the Tone Generator makes test tones. For noise at work, follow the official limits and guidance in your country.
Features
- Power and amplitude ratios to dB and back
- Add any number of sound levels
- Subtract background noise
- Level change with distance
- Typical sounds for comparison
- Exact logarithm formulas
- Runs in your browser
Frequently asked questions
How do I add decibels?
Decibels can't be added directly. Convert each level to power by raising 10 to the power of level over 10, add the powers, and convert the total back with 10 times the logarithm. In practice, two equal sources add 3 dB, so two 60 dB machines make 63 dB, and ten of them make 70 dB.
What is the difference between 10 log and 20 log?
Power quantities, such as watts or sound intensity, use 10 times the logarithm of the ratio. Amplitude quantities, such as volts or sound pressure, use 20 times because power goes with the square of amplitude. Doubling power is +3 dB and doubling amplitude is +6 dB.
How does sound level change with distance?
For a small source in open air, each time you double your distance the level falls by about 6 dB, from the formula minus 20 times the log of the distance ratio. A level of 85 dB at 1 metre is about 73 dB at 4 metres. Rooms, walls and large sources behave differently.
How do I remove background noise from a measurement?
Subtract the background power from the total power and convert back. If the total is 65 dB with a background of 60 dB, the source alone is about 63.3 dB. If the difference is under 3 dB, the result is uncertain, and if the total is not louder than the background, the source cannot be separated.
How loud is 85 dB?
About as loud as heavy city traffic or a food blender. Hearing damage can result from long exposure to levels around 85 dB and above, and the safe time falls fast as the level rises. At 100 dB, minutes of exposure can be too much.
Is a dB a unit with a fixed zero?
A decibel is a ratio, so it needs a reference. Sound pressure level in dB SPL uses 20 micropascals, dBm uses 1 milliwatt and dBV uses 1 volt. The calculator does the arithmetic on ratios and levels without choosing a reference for you.