Calculator KB-40
Decibel calculator
A logarithmic scale for gain, attenuation, and signal levels.
Result
In brief: Convert decibels: voltage and power ratios to and from dB, and dBm to milliwatts and volts across a 50 Ω or other load. Example: Level: 20 dB.
Calculation example
| What to calculate | Voltage ratio → dB |
|---|---|
| Before (input) | 1 |
| After (output) | 10 |
| Level | 20 dB |
|---|---|
| Voltage ratio | 10 × |
| Power ratio | 100 × |
How it is calculated
P = 1 mW · 10dBm / 10
Decibels turn huge ratios into convenient numbers, and the gains of cascaded stages simply add up. 6 dB is twice the voltage, 3 dB is twice the power, and 20 dB is ten times the voltage.
What to look for
- dB is a ratio, while dBm is an absolute level referenced to 1 mW. They must not be confused.
- Comparing voltages using 20·lg is valid only when the resistance is the same.
Frequently asked questions
How many decibels is a 10× gain?
20 dB for voltage, 10 dB for power. 6 dB means twice the voltage, and 3 dB means twice the power.
How do you convert dBm to watts?
P = 1 mW · 10^(dBm / 10). 30 dBm equals 1 W, 20 dBm equals 100 mW, and −10 dBm equals 0.1 mW. Across a 50 Ω load, 30 dBm is 7.07 V.
What's the difference between dB and dBm?
dB is the ratio between two values; dBm is an absolute level referenced to 1 mW. You can add gain to a level: 10 dBm plus 20 dB gives 30 dBm.
