Decibel Calculator (dB Power and Voltage Ratios)
By Santaji GadepatilinUpdated October 27, 2026
Decibels express ratios of power, voltage or sound on a logarithmic scale, so huge ranges fit into manageable numbers and gains simply add. See decibels in electronics: dB and dBm for practical examples.
How to use the decibel calculator
Use the top section for power ratios and the bottom for voltage (or current, or sound pressure) ratios. Enter the reference and the measured value to get decibels, or enter dB and a reference to find the other value.
Decibel formulas
dB = 10 log₁₀(P₂ ÷ P₁) · dB = 20 log₁₀(V₂ ÷ V₁)
dB reference table
| dB | Power ratio | Voltage ratio |
|---|---|---|
| -20 | 0.01× | 0.1× |
| -10 | 0.1× | 0.3162× |
| -6 | 0.2512× | 0.5012× |
| -3 | 0.5012× | 0.7079× |
| +0 | 1× | 1× |
| +3 | 1.995× | 1.413× |
| +6 | 3.981× | 1.995× |
| +10 | 10× | 3.162× |
| +20 | 100× | 10× |
| +30 | 1000× | 31.62× |
| +40 | 1e+04× | 100× |
| +60 | 1e+06× | 1000× |
dB units with references
| Unit | Reference |
|---|---|
| dBm | 1 mW |
| dBW | 1 W |
| dBV | 1 V |
| dBu | 0.775 V |
| dB SPL | 20 µPa sound pressure |
Sound levels (dB SPL)
| Sound | Level |
|---|---|
| Threshold of hearing | 0 dB |
| Quiet room | 30 dB |
| Normal conversation | 60 dB |
| Busy traffic | 80 dB |
| Hearing damage risk with long exposure | 85 dB and above |
| Rock concert | 110 dB |
| Pain threshold | 120–130 dB |
Every 10 dB increase sounds roughly twice as loud and carries ten times the sound power.
Signal chains
In RF and audio systems, gains and losses in dB simply add, which is why engineers prefer them to multiplying ratios.
Frequently asked questions
Why is power 10 log but voltage 20 log?
Power is proportional to voltage squared, so 10 log(V²) = 20 log(V). A tenfold voltage increase is a hundredfold power increase — both 20 dB.
What does 3 dB mean?
A doubling of power (or halving for −3 dB). For voltage, doubling is about 6 dB.
How do I add gains in a signal chain?
Add the dB values. An amplifier of +20 dB followed by a cable loss of −3 dB gives +17 dB overall.
What is dBm?
Power relative to 1 mW. 0 dBm = 1 mW, 30 dBm = 1 W.