How to convert from **dBm** (deciBel-milliwatts) to **dBm** (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