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

dB is **relative** number while dBm is an **absolute** number. It’s not possible to convert dB to dBm or vice versa.

You can however subtract two dBm numbers and the result is in dB. This dB value does not represent the sum or difference between the two power levels. Instead **it represents the ratio between two powers**.

Let’s understand why.

Use the following tool to calculate the difference between two dBm values.

## Formula

**P _{dB} = P_{dBm1} – P_{dBm2}**

## Calculation **Example**

If

**P**_{dBm1}= 30 dBm**P**_{dBm2}= 10 dBm

then,

**P _{dBm1} – P_{dBm2}**

**= 20 dB**

Note that this is not the difference in power level between the two. Instead it is the ratio of powers which has no units. *Use this calculator to compute the sum of two power levels.*

First convert dBm to Watt

- P
_{dBm1}= 30 dBm = 1 Watt - P
_{dBm2}= 10 dBm = 0.01 Watt

The ratio between the two is 1/0.01 = 100. On the log scale it works out to:

**10*Log _{10}(100) = 20 dB**

Let’s see if this works for negative values of dBm.

- P
_{dBm1}= 30 dBm = 1 Watt - P
_{dBm2}= -10 dBm = 0.0001 Watt

The ratio between the two is 10,000 and on the log scale this works out to:

**10*Log _{10}(10000) = 40 dB**

and

**30 dBm -(-10 dBm) = 40 dB**

So that works out as expected.

## Practical example

Let’s say the signal power at a receiver is -110 dBm and the noise power is -150 dBm, the Signal to noise ratio (calculated using voltage here) is the difference between the two:

**SNR = -110-(-150) = 40 dB**

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

dBm is an absolute number. For instance, this calculator lets you convert from Watt to dBm.

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