dB to Watts Calculator: Convert dBm to Watts Instantly

Your RF Equipment Spec Shows −20 dBm Output, But How Much Actual Power Is That?

In telecommunications and RF (radio frequency) work, power is often expressed in dBm (decibels relative to 1 milliwatt). A WiFi router might transmit at +20 dBm (100 mW). A cellular phone at maximum might be +23 dBm (200 mW). A weak signal might be −80 dBm. The decibel scale compresses enormous power ranges into manageable numbers, but converting between dBm and actual watts isn't intuitive. This calculator converts instantly between dBm and power in watts or milliwatts.

What This Calculator Does

This calculator converts between dBm and milliwatts (mW) or watts (W). Enter a power in dBm and get the equivalent power in watts. Enter a power in watts and get the dBm value. It also shows comparisons to familiar power levels (WiFi, cellular, Bluetooth, microwave ovens) and calculates equivalent dBW (decibels relative to 1 watt) for reference.

How to Use This Calculator

Power in dBm: Enter a value in dBm (which can be negative, zero, or positive). Typical RF values range from −100 dBm (very weak) to +50 dBm (high power equipment).

Power in Watts: Enter a value in watts (W) or milliwatts (mW). The calculator converts automatically.

Power in dBW: Select this if you prefer the alternative reference (relative to 1 watt instead of 1 milliwatt).

Enter either dBm or watts, and the calculator shows the equivalent in all three formats, plus real-world comparisons.

The Formula Behind the Math

Power in dBm is defined as:

dBm = 10 × log₁₀(P_mW / 1 mW) = 10 × log₁₀(P_mW)

Where P_mW is power in milliwatts.

Rearranging to find power in milliwatts:

P_mW = 10^(dBm / 10)

To convert to watts, divide by 1000:

P_W = 10^(dBm / 10) / 1000 = 10^((dBm − 30) / 10)

The relationship between dBm and dBW is:

dBW = dBm − 30

Or:

dBm = dBW + 30

This is because 1 watt = 1000 milliwatts, and 10 × log₁₀(1000) = 30.

Worked Example:

Convert −20 dBm to watts.

So −20 dBm is 10 microwatts. This is a weak signal, like from a distant cell phone or a Bluetooth headset.

Now convert 100 mW to dBm:

100 mW is +20 dBm. This is typical WiFi power.

Our calculator does all of this instantly, but now you understand exactly what it's computing.

Wireless Communications and Signal Strength

Cellular networks specify coverage in terms of received signal strength (RSSI) in dBm. A strong signal is around −50 dBm. A weak signal is −100 dBm. The difference, 50 dB, represents a 100,000-fold difference in power. Each 3 dB increase represents doubling power. WiFi routers transmit at +20 to +23 dBm. Bluetooth devices transmit at −20 to +10 dBm depending on class (long-range or short-range).

RF Measurement and Compliance

FCC regulations limit transmit power in various frequency bands. WiFi is limited to +36 dBm (4 watts) EIRP (effective isotropic radiated power). Cellular base stations transmit at +40–50 dBm. Measurement of actual output power is critical for compliance testing and performance verification. Power meters measure in watts, which are then converted to dBm for reporting.

Link Budget Calculation

In RF system design, engineers calculate the "link budget"-the power available at the receiver. Path loss (attenuation over distance) is expressed in dB. Transmitted power minus path loss equals received power. If a transmitter outputs +20 dBm and path loss is 50 dB, received power is +20 − 50 = −30 dBm. Sensitivity requirements determine if the receiver can detect the signal.

Power Amplifiers and Efficiency

RF power amplifiers boost weak signals to higher power. A 20 dBm input amplified by a 30 dB gain amplifier yields +50 dBm output (7 watts). Typical RF amps are 30–50% efficient; the rest is heat. A 50 W RF amplifier dissipates 50–117 watts as heat and requires cooling. Power is often specified in dBm to compress the wide range of values.

Tips and Things to Watch Out For

dBm is always relative to 1 milliwatt. −3 dBm means 0.5 milliwatts (half a milliwatt). 0 dBm means exactly 1 milliwatt. +3 dBm means 2 milliwatts. If the reference changes (dBW, dBµV, etc.), the numbers change.

Negative dBm represents power less than 1 mW. −10 dBm = 0.1 mW, −20 dBm = 0.01 mW, −30 dBm = 0.001 mW. The more negative, the weaker the power.

EIRP vs. conducted power. Transmitters output "conducted power" at the antenna port (in dBm). With antenna gain, the "radiated power" (EIRP) can be higher. FCC regulations often limit EIRP, not conducted power. A +20 dBm transmitter with a 10 dBi antenna radiates +30 dBm EIRP.

Don't confuse dBm with dB. dB is a ratio (unitless). dBm is an absolute power level (relative to 1 mW). Adding dB values (ratios) to dBm (absolute power) requires care. Power (dBm) = Reference Power (dBm) + Gain (dB).

Impedance matters for RF power. RF power formulas assume 50 ohms (standard impedance). At other impedances, the relationship between voltage and power changes. RF components are always 50 ohms unless specified otherwise.

Frequently Asked Questions

What's the difference between dBm and dBW?

dBm is referenced to 1 milliwatt. dBW is referenced to 1 watt. Since 1 watt = 1000 mW, dBW = dBm − 30. A +10 dBm signal is 10 mW = 0.01 W = −20 dBW.

How sensitive is my receiver?

Receiver sensitivity is specified in dBm. A typical WiFi receiver is −90 dBm (very weak signals). Cellular receivers are −120 dBm or better. Weaker numbers (more negative) mean better sensitivity. A difference of 10 dB means the second receiver can detect signals 10 times weaker.

What's the maximum WiFi power?

Most regulations limit WiFi to +20 to +36 dBm (100 mW to 4 W) depending on country and frequency band. US allows higher power in some bands. Most routers use +18 to +24 dBm to balance range and interference.

How far can a +20 dBm signal travel?

Depends on antenna, frequency, and environment. A direct line-of-sight +20 dBm signal can reach miles. Indoors, walls attenuate signals, limiting range to 50–100 meters. Frequency matters: lower frequencies penetrate better; higher frequencies don't.

Why use dBm instead of just milliwatts?

The logarithmic scale is convenient. A 10 dB increase anywhere on the scale (in signal, gain, or power) always represents the same power ratio (10×). dB makes multiplication/division of power ratios simple addition/subtraction. It also compresses the huge range of RF powers (femtowatts to kilowatts) into a manageable scale (−100 to +100 dBm).

What's the relationship between power and signal strength?

Signal strength (RSSI) at the receiver is proportional to transmitted power but decreases with distance. Path loss (attenuation) depends on distance, frequency, and environment. Higher frequency or greater distance means larger path loss (more negative RSSI).

Related Calculators

Use our Decibel Calculator for general dB conversions. The Wavelength Calculator helps with RF frequency and wavelength relationships. The Ohm's Law Calculator relates voltage, current, and impedance to power. The Speed of Sound Calculator helps with propagation characteristics. For more RF concepts, explore our Capacitor and Inductor Calculators.