Decibel Addition Calculator
Decibels do not add like ordinary numbers. Two sources at the same level do not make twice the decibels, because the decibel scale is logarithmic.
This page adds two levels the correct way, so you can see the combined reading.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Each level is turned back into a power, the two powers are added, and the sum is turned back into decibels. In formula form, the total is ten times the base-ten logarithm of ten to the first level over ten, plus ten to the second level over ten.
Two equal levels add about 3 decibels. A much quieter source adds almost nothing to a louder one.
Worked example
A source at 85 decibels and another at 88 decibels together make about 89.8 decibels, which is 1.8 above the louder one.
Questions
Why do two equal sounds add only about 3 dB?
Doubling the sound power adds 3 decibels, because the scale is logarithmic. Doubling the number of identical sources doubles the power, not the decibel reading.
Can I add more than two levels?
This page adds two at a time. Combine the first two, then add the result to the third.
Does this show how loud a sound feels?
No. It adds decibel numbers. How loud a sound feels depends on its frequency and on the listener, and that is not measured here.
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