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

dB is **relative** number while dBm is an **absolute** number. There’s no explicit conversion between the two. As an example 3 dBm cannot be converted to an equivalent dB value.

ðŸ’¡ 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.** This ratio is used in Radio Frequency engineering – the Signal-to-Noise ratio for example.

Let’s dive into the details.

Contents

**Calculator**

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, 20 dB is the ratio of powers which has no units.

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 logarithmic scale:

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

Taking the difference between the two dBm values:

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

So the two are equal as expected.

Note that the following applies as well:

**P _{dBm2} = P_{dBm1} – P_{dB}**

dB gains and losses ** P_{dB}** can be subtracted from the input power

**P**to give the output power

_{dBm1}**P**. This is a similar amplifier gain calculation done in the linear domain.

_{dBm2}**Practical examples**

If the signal power at a receiver is **-110 dBm** and the noise power is **-150 dBm**, the Signal-to-Noise ratio is the difference between the two dBm values:

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

Another example of this is when converting from EIRP to ERP or ERP to EIRP using the numerical value of the ratio between the two.

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

dBm is an absolute number. For instance, you can convert 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. Unlike dBm, dB numbers can be added and subtracted from one another.

**Related Calculators**

- Use this calculator to compute the sum of two power levels
- dBm Average
- SNR from voltage