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dB or dBm?
The actual equation used to calculate dB when the power is measured in watts is:
dB = 10 log (measured power / reference power).
It can be expressed more simply:
dB = measured power(dB) – reference power (dB)
the table below shows the ratio of power for dB differences in power:
| dB (gain) | Power ratio | dB (loss) | Power ratio |
| 0 | 1.000 | 0 | 1.000 |
| 0.1 | 1.023 | -0.1 | 0.977 |
| 0.2 | 1.047 | -0.2 | 0.955 |
| 0.3 | 1.072 | -0.3 | 0.933 |
| 0.4 | 1.096 | -0.4 | 0.912 |
| 0.5 | 1.122 | -0.5 | 0.891 |
| 0.6 | 1.148 | -0.6 | 0.871 |
| 0.7 | 1.175 | -0.7 | 0.851 |
| 0.8 | 1.202 | -0.8 | 0.832 |
| 0.9 | 1.230 | -0.9 | 0.813 |
| 1 | 1.259 | -1 | 0.794 |
| 2 | 1.585 | -2 | 0.631 |
| 3 | 1.995 | -3 | 0.501 |
| 4 | 2.512 | -4 | 0.398 |
| 5 | 3.162 | -5 | 0.316 |
| 6 | 3.981 | -6 | 0.251 |
| 7 | 5.012 | -7 | 0.200 |
| 8 | 6.310 | -8 | 0.158 |
| 9 | 7.943 | -9 | 0.126 |
| 10 | 10 | -10 | 0.1 |
| 20 | 100 | -20 | 0.01 |
| 30 | 1000 | -30 | 0.001 |
| 40 | 10000 | -40 | 0.0001 |
| 50 | 100000 | -50 | 0.00001 |
| 60 | 1000000 | -60 | 0.000001 |
The “m” in dBm refers to the reference power which is 1 milliwatt.
dBm = 10 log (measured power / 1mw)
dB is for measuring loss, dBm is for measuring power.
