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Wireless & Mobility · Foundational

Rule of 10s and 3s

Mental math shortcut: +3 dB doubles power, -3 dB halves it, +10 dB multiplies by 10, -10 dB divides by 10.

Definition

The rule of 10s and 3s lets you convert between mW and dBm without logarithms by building the dB change from steps of 3 and 10. Starting from 0 dBm = 1 mW, you add or subtract 10s and 3s and apply the matching multiply or divide operations. It is a staple of CWNA RF math questions.

Example

1 mW +10 dB = 10 mW, +10 dB = 100 mW, -3 dB = 50 mW, so 17 dBm is about 50 mW.

Exam tip

Exam math questions are designed to be solved with 10s and 3s, so decompose the dB change into those steps.

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