Decibel & dBm Calculator
Convert power levels (dBm/dBW ⇄ W), compute dB ratios for power and voltage, and cascade gain in decibels
Enter a power level and its unit. The equivalent dBm, dBW, milliwatts and watts are shown.
Common dBm / dBW / power levels
| dBm | dBW | Power |
|---|---|---|
| -30 dBm | -60 dBW | 1 µW |
| -10 dBm | -40 dBW | 100 µW |
| 0 dBm | -30 dBW | 1 mW |
| 10 dBm | -20 dBW | 10 mW |
| 20 dBm | -10 dBW | 100 mW |
| 30 dBm | 0 dBW | 1 W |
| 40 dBm | 10 dBW | 10 W |
| 50 dBm | 20 dBW | 100 W |
| 60 dBm | 30 dBW | 1 kW |
Common dB ratios
| dB | Power ratio | Amplitude ratio |
|---|---|---|
| 0 dB | 1× | 1× |
| 3 dB | 1.9953× | 1.4125× |
| 6 dB | 3.9811× | 1.9953× |
| 10 dB | 10× | 3.1623× |
| 20 dB | 100× | 10× |
| 30 dB | 1000× | 31.623× |
About this tool
This decibel calculator covers the three things engineers reach for most: converting between absolute power levels (dBm, dBW, milliwatts and watts), turning a power or amplitude ratio into decibels and back, and adding gain to a signal in a cascade. It uses the standard definitions dBm = 10·log10(P/1 mW) and dBW = 10·log10(P/1 W), the power-ratio rule dB = 10·log10(P2/P1), and the amplitude-ratio rule dB = 20·log10(V2/V1). Because the math is logarithmic, gains and losses simply add: a +20 dB amplifier raises a −10 dBm signal to +10 dBm. It is built for RF, audio and electrical work, with reference tables so you can sanity-check results at a glance.
How to use
- 1 Pick a mode: power level, ratio (dB), or gain.
- 2 In power level mode, type a value and choose dBm, dBW, mW or W; the other units are shown instantly.
- 3 In ratio mode, enter two values to get the dB ratio, or enter dB to get the power and amplitude ratios.
- 4 In gain mode, enter an input level in dBm and a gain in dB to see the output level in dBm, mW and W.
How it works
A decibel is a logarithmic ratio. For power, dB = 10·log10(P2/P1); for amplitude such as voltage, dB = 20·log10(V2/V1), because power is proportional to voltage squared. The dBm scale references power to one milliwatt, so 0 dBm = 1 mW, 30 dBm = 1 W, and every 10 dB is a factor of ten in power. Two useful rules of thumb fall out of this: +3 dB is roughly double the power (10^0.3 ≈ 1.995), and +6 dB is roughly double the voltage (10^(6/20) ≈ 2.0). Because logarithms turn multiplication into addition, the gain of a chain is just the sum of its stages: a −10 dBm signal through a +20 dB amplifier leaves at +10 dBm.
Frequently asked questions
What is the difference between dBm and dBW?
Both are absolute power levels on a logarithmic scale, but they use different references. dBm is referenced to 1 milliwatt, while dBW is referenced to 1 watt. Because 1 W = 1000 mW, the two differ by exactly 30 dB: dBW = dBm − 30. So 30 dBm and 0 dBW both equal 1 watt.
Why is power 10·log but voltage 20·log?
Power is proportional to the square of voltage (P = V²/R), and log of a square brings the exponent out front: log10(V²) = 2·log10(V). So a voltage ratio expressed in dB uses 20·log10, while a power ratio uses 10·log10. That is why +6 dB doubles voltage but +3 dB doubles power.
How much is 3 dB, 6 dB and 10 dB?
In power terms, +3 dB is about 2× (10^0.3 ≈ 1.995), +6 dB is about 4×, and +10 dB is exactly 10×. In amplitude (voltage) terms, +6 dB is about 2× and +20 dB is 10×. These ratios are independent of the absolute level, which is why decibels are so handy.
How do I add gain or loss in decibels?
Because decibels are logarithmic, you simply add them. Output level in dBm equals the input level in dBm plus the total gain in dB (use a negative number for loss or attenuation). For example, a −10 dBm signal through a +20 dB amplifier comes out at +10 dBm, which is 10 mW.
Related tools and uses
Pair this with the Ohm's law calculator for power and resistance, the scientific notation converter for very large or small wattages, and the resistor color code calculator when building attenuators and matching networks.