Decibel Calculator
Add, subtract, and convert decibels for sound, RF, and electronics applications.
Need to convert between other units? Try our Unit Conversion Calculator.
Input
Result
Formulas
dB Addition: Result = 10 Γ logββ(Ξ£10^(dBα΅’/10))
Understanding Decibel Calculations
The decibel (dB) is a logarithmic unit used to express ratios, most commonly of power or amplitude. Because human perception of sound and signal strength is logarithmic, dB is the natural unit in acoustics, RF engineering, and electronics. Decibels cannot be added or subtracted directly β they must first be converted to linear values, combined, then converted back.
Decibel Formulas by Mode
Two equal-power sources that are each 80 dB combine to approximately 83 dB, not 160 dB β because dB values are logarithmic. A 3 dB increase roughly doubles the power; a 10 dB increase is perceived as twice as loud.
How to Use the Decibel Calculator
Frequently Asked Questions
Can I add decibel values directly?
No. Decibels are logarithmic and cannot be simply added. Two sources of 80 dB each do not equal 160 dB. The correct method is: Combined = 10 Γ logββ(10^(80/10) + 10^(80/10)) = 10 Γ logββ(2 Γ 10βΈ) = 83 dB. Two equal sources add approximately 3 dB.
What is the difference between dB and dBm?
dB is a dimensionless ratio expressing the relative difference between two power levels. dBm is an absolute power measurement referenced to 1 milliwatt: 0 dBm = 1 mW. So "3 dB" means "twice the power," while "3 dBm" means "2 mW."
What is the 3 dB rule?
A 3 dB increase roughly doubles the power, while a 3 dB decrease halves it. For voltage or amplitude, a 6 dB change represents doubling or halving. For perceived loudness, a 10 dB increase is perceived as approximately twice as loud by the human ear.
How do I convert dBm to watts?
P(watts) = 10^((dBm β 30) / 10). For example, 30 dBm: P = 10^((30β30)/10) = 10β° = 1 watt. 0 dBm = 1 mW = 0.001 watts. 40 dBm = 10 watts.
What does 0 dB mean?
0 dB means the measured level equals the reference level. For power ratios, 0 dB = ratio of 1 (no change). For dBm, 0 dBm = 1 milliwatt. For dBV, 0 dBV = 1 volt. For dB SPL, 0 dB SPL is the threshold of human hearing (20 micropascals). Always check the reference to interpret 0 dB correctly.