dB to Ratio Converter
Convert decibels to a power ratio and a voltage ratio, and back, with a table of common values and the 10 log and 20 log formulas.
Common values
| dB | Power ratio | Voltage ratio |
|---|---|---|
| -20 | 0.01 | 0.1 |
| -10 | 0.1 | 0.3162278 |
| -6 | 0.2511886 | 0.5011872 |
| -3 | 0.5011872 | 0.7079458 |
| 0 | 1 | 1 |
| 3 | 1.995262 | 1.412538 |
| 6 | 3.981072 | 1.995262 |
| 10 | 10 | 3.162278 |
| 20 | 100 | 10 |
| 30 | 1000 | 31.62278 |
| 40 | 10000 | 100 |
Formulas
dB = 10 × log10(power ratio)
dB = 20 × log10(voltage ratio)
power ratio = 10^(dB / 10)
voltage ratio = 10^(dB / 20)
When to use 10 and when to use 20
Use 10 × log10 for power. Use 20 × log10 for voltage, current or sound pressure when both values are across the same impedance, because power goes with the square. A level in dB always compares two quantities of the same kind. A gain of 20 dB is a power ratio of 100 and a voltage ratio of 10.
Related
dBm to WattsPercent and PPM ConverterPower ConverterConversion Centre
Frequently asked questions
How do I convert dB to a power ratio?
Power ratio = 10^(dB / 10). 3 dB is a ratio of about 2, 10 dB is 10 and 20 dB is 100. A negative dB is a ratio below 1, so -3 dB is about one half.
How do I convert dB to a voltage ratio?
Voltage ratio = 10^(dB / 20). It uses 20 instead of 10 because power goes with the square of voltage or current across the same impedance. 6 dB is a voltage ratio of about 2 and 20 dB is 10.
Why is 3 dB double the power but 6 dB double the voltage?
Doubling the power is 10 × log10(2) = 3.01 dB. Doubling the voltage quadruples the power across the same impedance, which is 10 × log10(4) = 6.02 dB, the same as 20 × log10(2).
Is dB the same as dBm?
No. dB is a ratio between two powers, so it has no reference. dBm is a power level compared with 1 mW. Use the dBm to watts converter to turn a dBm level into watts.