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dBm vs Watts: a simple RF power explanation

Understand how dBm and Watts relate, why RF engineers use logarithmic power units, and how to convert values for link budgets and signal planning.

Published Jan 17, 2026Updated Jun 30, 20268 min read

RF power guide comparing dBm and Watts for link budgets

dBm and Watts in one sentence

Watts measure power on a linear scale, while dBm expresses power on a logarithmic scale relative to 1 milliwatt.

RF engineers use dBm because radio systems often involve very large and very small values. Logarithmic units make gains and losses easier to add and subtract.

The conversion formula

To convert dBm to Watts:

P(W) = 10^{\frac{dBm-30}{10}}

To convert Watts to dBm:

dBm = 10 \log_{10}(P(W) \times 1000)

Use the dBm to Watt converter if you want an instant result.

Quick reference table

dBm	Watts
0 dBm	0.001 W
10 dBm	0.01 W
20 dBm	0.1 W
30 dBm	1 W
40 dBm	10 W

Every 10 dB increase means power is multiplied by 10. Every 3 dB increase is roughly double the power.

Why RF engineers prefer dBm

In a link budget, antenna gain, cable loss, path loss, and amplifier gain are usually expressed in dB. With dBm and dB, you can add and subtract:

Transmit power + antenna gain - cable loss - path loss = received power

That is much easier than multiplying and dividing linear Watt values at every step.

Practical example

Imagine a transmitter outputs 20 dBm. The antenna adds 5 dBi, and the cable loses 2 dB.

20 dBm + 5 dBi - 2 dB = 23 dBm EIRP before path loss

This is why dBm is useful in RF planning: the math stays readable.

Common mistakes

dB is a ratio, but dBm is an absolute power level.
dBi is antenna gain relative to an isotropic radiator.
Watts are linear, so doubling Watts is not the same as adding 10 dB.
Always check whether a value is dBm, dBW, dB, or dBi.