How to convert from **dB** (deciBels) to **dBm** (deciBel-milliwatts)?

dB is **relative** number while dBm is an **absolute** number. There is no way to convert dB to dBm.

You can however subtract two dBm numbers and the result is a dB value.

## Formula

**dB = dBm _{1} – dBm_{2}**

**Example**

If

**dBm**_{1}= 40 dBm**dBm**_{2}= 30 dBm

then,

**dBm _{1} – dBm_{2} = 10 dB**

Note that this is not the difference in power level between the two. For that use this calculator.

Instead it is the ratio of powers which has no units.

- dBm1 = 40 dBm = 10 Watt
- dBm2 = 30 dBm = 1 Watt

The ratio between the two is 10 and on the log scale it works out to:

**10*Log _{10}(10) = 10 dB**

**Why can you not convert between dBm and dB?**

dBm is an absolute number. For instance, this calculator lets you convert between Volt and dBm.

dB is a relative number and represents a ratio between two quantities. In communications engineering, dB is used to represent the signal-to-noise ratio, gain of an amplifier and more.

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