dB & dBm Converter
Convert dBm ↔ mW and W, dBW, and for 50, 75 or 600 Ω dBµV, dBV, dBu and volts; power or voltage ratios ↔ dB. For RSSI and cable loss.
Formulas
- Power ratio: dB = 10 × lg(P2 ÷ P1)
- Voltage or current ratio (same impedance): dB = 20 × lg(U2 ÷ U1)
- dBm = 10 × lg(P ÷ 1 mW), dBW = 10 × lg(P ÷ 1 W) = dBm − 30
- dBµV = 20 × lg(U ÷ 1 µV), dBV = 20 × lg(U ÷ 1 V) = dBµV − 120
- dBu = 20 × lg(U ÷ 0.775 V) (the voltage giving 1 mW in 600 Ω)
- Voltage ↔ power: P = U² ÷ R → dBµV = dBm + 10 × lg(R × 1 mW) + 120 (50 Ω: +107.0, 75 Ω: +108.8)
A decibel is a ratio of powers. Voltage and current enter squared, hence the 20 — twice the voltage across the same resistance is four times the power, 6.02 dB. dBm and dBW carry a reference, so they are absolute levels, and moving between dBm and dBµV needs the impedance. The 0.775 V dBu reference is √0.6 V ≈ 0.7746 V (1 mW in 600 Ω) rounded; this calculator uses √0.6 V.
Where dB shows up on site
- Radio modules and gateways report RSSI in dBm. −50 dBm is 10 nW and −90 dBm is 1 pW; 40 dB is a factor of 10 000 in power. The gap between receiver sensitivity (say −110 dBm) and RSSI is your margin.
- Cable and connector losses, splitters and amplifier gains add and subtract in dB: +20 dB amplifier − 6 dB cable − 3 dB splitter = +11 dB.
- −3 dB is half the power (0.501); −6 dB is half the voltage (0.501). A filter's «−3 dB cut-off» is where the power halves.
- EMC conducted-emission limits are written in dBµV and radiated limits in dBµV/m (with a 50 Ω receiver, 0 dBm ≈ 107 dBµV).
dBm ↔ power
| dBm | Power | Voltage in 50 Ω (rms) | dBµV (50 Ω) |
|---|---|---|---|
| +40 | 10 W | 22.36 V | 146.99 |
| +30 | 1 W | 7.071 V | 136.99 |
| +20 | 100 mW | 2.236 V | 126.99 |
| +10 | 10 mW | 707.1 mV | 116.99 |
| 0 | 1 mW | 223.6 mV | 106.99 |
| -10 | 100 µW | 70.71 mV | 96.99 |
| -30 | 1 µW | 7.071 mV | 76.99 |
| -50 | 10 nW | 707.1 µV | 56.99 |
| -70 | 100 pW | 70.71 µV | 36.99 |
| -90 | 1 pW | 7.071 µV | 16.99 |
| -110 | 0.01 pW | 0.7071 µV | -3.01 |
dB ↔ ratio
| 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 |
| -1 | 0.7943 | 0.8913 |
| +1 | 1.259 | 1.122 |
| +3 | 1.995 | 1.413 |
| +6 | 3.981 | 1.995 |
| +10 | 10 | 3.162 |
| +20 | 100 | 10 |
FAQ
How many watts is 0 dBm?
1 mW (0.001 W). 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W; −30 dBm is 1 µW.
How do I convert dBm to dBµV?
You need the impedance. In 50 Ω, dBµV = dBm + 107 (106.99 exactly); in 75 Ω, + 108.75. Example: −60 dBm in 50 Ω ≈ 47 dBµV.
Do I multiply by 10 or 20?
10 for power ratios, 20 for voltage or current ratios. The two dB figures agree only when both are across the same impedance.
What is the difference between dBV and dBu?
The references are 1 V and 0.775 V, so dBu reads about 2.2 dB higher for the same voltage (0 dBV ≈ +2.2 dBu).