Unit Conversions

The Problem

You calculated a received power of −72 dBm.

But sometimes you need the actual power in Watts. How do you convert?

What dBm Actually Means

• dBm measures power relative to 1 milliwatt.

0 dBm = 1 mW (this is the reference point)

From there, the pattern is simple:

Every +10 dB = 10× more power. Every −10 dB = 10× less power.

From dBm to Watts

To undo the logarithm and get back to Watts:

$P_{\text{Watts}} = 10^{(\text{dBm} - 30) / 10}$

Example: Convert 40 dBm to Watts

This can be written as 10,000 mW (milliwatts) since 1 W = 1000 mW.

From Watts to dBm

Going the other direction is simpler. If you have power in milliwatts:

$\text{dBm} = 10\log_{10}(P_{\text{mW}})$

If you have power in Watts, multiply by 1000 first (or add 30 at the end):

$\text{dBm} = 10\log_{10}(P_{\text{W}}) + 30$

Example: Convert 10,000 mW to dBm

What About dBW?

You might see dBW in some contexts. It’s the same idea, but relative to 1 Watt instead of 1 milliwatt.

Since 1 W = 1000 mW, and $10\log_{10}(1000) = 30$:

$\text{dBm} = \text{dBW} + 30$

dBm is more common in wireless communications. dBW is sometimes used for higher power systems.

Quick Reference

Common Values

Power Unit Prefixes

Each step down = divide by 1000 (or multiply exponent by $10^{-3}$)