dBm ↔ Watt Conversion

Convert RF power levels between dBm (logarithmic) and milliwatts or watts (linear).

What is dBm?

dBm is a logarithmic unit of power referenced to 1 milliwatt (mW). It is the standard unit for expressing RF signal levels in wireless communications because the logarithmic scale compresses the enormous dynamic range of signal power — from femtowatts at a distant receiver to kilowatts at a transmitter — into manageable numbers.

A signal power of 0 dBm equals exactly 1 mW. Positive values indicate powers above 1 mW (transmitter outputs), while negative values indicate powers below 1 mW (received signals at antennas, typically in the range of −60 to −130 dBm for practical wireless systems).

The key advantage of dBm in link budgets is that signal gains and losses can be added and subtracted rather than multiplied and divided. A 20 dBi antenna gain simply adds 20 to the dBm level rather than multiplying the power by 100.

Why Does It Matter?

Transmit power specs, receiver sensitivity, and noise floor are all expressed in dBm on datasheets
Adding gains and subtracting losses in dBm gives the received signal level directly
Regulatory EIRP limits are specified in dBm (or dBW) in FCC and ETSI documents
Spectrum analyzers, network analyzers, and power meters display power in dBm

Quick dBm ↔ Watt Converter

Power in dBm

↕

Power in Watts

Formulas Used by LinkBudgetPro

\[ P_{\text{dBm}} = 10 \cdot \log_{10}(P_{\text{W}} \times 1000) \]

Watts \(\to\) dBm: multiply by 1000 to convert to mW, then take \(10\log_{10}\)

\[ P_{\text{W}} = 10^{\left(\dfrac{P_{\text{dBm}} - 30}{10}\right)} \]

dBm \(\to\) Watts: the \(-30\) shifts reference from 1 mW to 1 W

Parameter Explanation

Parameter	Symbol	Unit	Description
Power (linear)	P_W	W	RF power in watts (absolute linear unit)
Power (milliwatts)	P_mW	mW	RF power in milliwatts: P_mW = P_W × 1000
Power (dBm)	P_dBm	dBm	Power in dB relative to 1 mW: P_dBm = 10·log₁₀(P_mW)
Power (dBW)	P_dBW	dBW	Power in dB relative to 1 W: P_dBW = P_dBm − 30

Worked Example

Convert 500 mW (0.5 W) to dBm, then convert −90 dBm to watts:

Watts to dBm:
P = 0.5 W = 500 mW
P_dBm = 10 · log₁₀(500) = 10 · 2.699
P_dBm = 26.99 dBm ≈ 27 dBm
dBm to Watts:
P = −90 dBm (typical Wi-Fi receiver sensitivity)
P_W = 10^((−90 − 30) / 10) = 10^(−12)
P_W = 1 picowatt = 0.000 000 000 001 W

Quick Reference Table

dBm	Milliwatts	Watts	Typical Use
−130 dBm	0.0000001 mW	0.1 pW	GPS receiver minimum signal
−100 dBm	0.0001 mW	0.1 nW	Weak receiver signal
−90 dBm	0.001 mW	1 nW	Typical Wi-Fi sensitivity
−70 dBm	0.1 mW	0.1 µW	Good Wi-Fi signal level
0 dBm	1 mW	0.001 W	Reference level; small signal
10 dBm	10 mW	0.01 W	Bluetooth Class 1 max
20 dBm	100 mW	0.1 W	Wi-Fi access point typical
30 dBm	1000 mW	1 W	Typical point-to-point radio Tx
37 dBm	5000 mW	5 W	High-power RF amplifier
40 dBm	10000 mW	10 W	Amateur radio HF transceiver
50 dBm	100000 mW	100 W	Commercial FM broadcast

When Should You Use It?

Reading datasheets — convert Tx power between watts (as labeled on hardware) and dBm (for link budget use)
Setting up spectrum analyzers — understand displayed dBm readings in real-world power terms
Computing received power — receiver sensitivity in dBm directly compares to received signal in dBm
Regulatory limits — convert between dBm/dBW/watts for FCC/ETSI compliance checking

Related Calculations

EIRP Calculator — compute EIRP in dBm from Tx power and antenna gain
Receiver Sensitivity — minimum detectable signal in dBm
Noise Figure Calculator — thermal noise floor in dBm/Hz
Fade Margin Calculator — link margin in dB between Rx power and sensitivity
RF Documentation Index — all RF engineering reference pages

The full RF calculator accepts power in both dBm and watts — all conversions handled automatically.

Open Full RF Link Budget Calculator