What Is dBm

Question

What does the formula on the board mean, and how should dBm be interpreted?

Core Definition

dBm is a logarithmic power unit referenced to 1 mW.

Formula:

dBm = 10 log10(P / 1 mW)

Where:

• P is power
• the reference value is 1 mW

Interpretation

• 0 dBm = 1 mW
• 10 dBm = 10 mW
• 20 dBm = 100 mW
• 30 dBm = 1000 mW = 1 W

And:

• -10 dBm = 0.1 mW

Why 0 dBm = 1 mW

Because:

P / 1 mW = 1
log10(1) = 0

So:

dBm = 10 * 0 = 0

Why -10 dBm = 0.1 mW

Starting from:

-10 = 10 log10(P / 1 mW)

Then:

-1 = log10(P / 1 mW)
P / 1 mW = 10^-1 = 0.1
P = 0.1 mW

Useful Rules Of Thumb

• every increase of 10 dB means power multiplies by 10
• every decrease of 10 dB means power divides by 10
• every increase of about 3 dB means power roughly doubles

Counterpoints and Gaps

• dBm is an absolute power unit because it is referenced to 1 mW; plain dB by itself is only a ratio
• this note explains power scaling only and does not yet cover the difference between dB, dBm, and dBW

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