dBm is a unit of power expressed as a logarithmic ratio referenced to 1 milliwatt (mW). It is calculated as dBm = 10 x log10(power in mW / 1 mW). Because it is logarithmic, dBm is convenient for describing very large ranges of power, such as signal strength in wifi networks, fibre optic links and radio frequency (RF) systems, where power can range from nanowatts to hundreds of watts.

How do you convert dBm to watts?

To convert dBm to watts, first convert to milliwatts using mW = 10^(dBm / 10), then divide by 1000 to get watts. For example, 30 dBm equals 10^(30/10) = 1000 mW, which is 1 watt. A common reference point is 0 dBm, which always equals exactly 1 mW or 0.001 W.

Why is 3 dB roughly double the power?

Because dBm is a base-10 logarithmic scale, adding 3 dB multiplies the power by 10^(3/10), which is approximately 2.0. Adding 10 dB multiplies power by exactly 10x. This is why a small change in dBm (such as going from -70 dBm to -67 dBm wifi signal strength) represents a doubling of actual signal power, even though the number only changed by 3.

dBm to Watts Calculator NZ

Use this calculator to convert between dBm and linear power units, watts, milliwatts and microwatts, the two ways signal power gets described in radio frequency, wifi, fibre optic and telecommunications work. dBm is a logarithmic unit referenced to 1 milliwatt, and converting it into a plain wattage figure, or the other way round, matters whenever you are reading a router's transmit power spec, checking wifi signal strength, sizing an RF amplifier, or working through a fibre optic link budget. Start by choosing a direction, either dBm to watts and milliwatts, or watts and milliwatts back to dBm. Converting from dBm, type in the dBm figure, or pick a common reference point from the preset dropdown, such as a typical wifi router at +20 dBm or a weak fibre receive signal at -30 dBm. Converting the other way, enter your power value and choose whether it is in watts or milliwatts. The calculator instantly returns the result across four units, dBm, watts, milliwatts and microwatts, along with the exact formula and calculation steps used, plus a reference table showing common dBm values and their typical real-world use. Keep in mind that every 3 dB change roughly doubles or halves the actual power, while every 10 dB change multiplies or divides it by exactly 10, so small-looking dBm differences can mean large swings in real power.

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Standard formula dBm = 10 x log10(power in mW / 1 mW). This is a fixed mathematical definition, not subject to change.

1. Choose Conversion Direction

Convert

Power in dBm

dBm

Power value

Unit

2. Common Reference Presets

Load a common value (optional)

Every 3 dB change roughly doubles or halves the actual power. Every 10 dB change multiplies or divides the power by exactly 10x.

Conversion Result

dBm

30.00 dBm

Logarithmic power ratio

Watts
1 W
Linear power (W)

Milliwatts

1,000 mW

Linear power (mW)

Microwatts

1,000,000 μW

Linear power (µW)

Reference Table: dBm to Watts

dBm	Milliwatts	Watts	Typical use

Calculation Steps

Input value30 dBm

Formula usedmW = 10^(dBm / 10)

Power in mW10^(30 / 10) = 1,000 mW

Result30.00 dBm = 1,000 mW = 1 W

Quick Reference

0 dBm1 mW (0.001 W)

+3 dBapprox x2 power

+10 dBexactly x10 power

-3 dBapprox /2 power

-10 dBexactly /10 power

Result: Enter a value above.

What Is dBm?

dBm (decibel-milliwatts) is a unit of power expressed on a logarithmic scale, always referenced to 1 milliwatt. Unlike a plain decibel (dB), which expresses a ratio between two power levels, dBm expresses an absolute power level because it is always compared to the fixed reference of 1 mW. This makes dBm extremely useful in telecommunications, radio frequency (RF) engineering, wifi networking and fibre optics, where signal power can range from billionths of a watt up to hundreds of watts.

The dBm Formula

The standard formula for converting a power level in milliwatts to dBm is:

dBm = 10 x log10(P_mW / 1 mW)

To reverse the conversion and go from dBm back to milliwatts, rearrange the formula:

P_mW = 10^(dBm / 10)

To convert milliwatts to watts, divide by 1000. To convert watts to milliwatts, multiply by 1000.

Why the Logarithmic Scale?

Radio and optical signals attenuate over distance and through cables in a way that is naturally multiplicative rather than additive. Using a logarithmic scale turns these multiplicative losses and gains into simple addition and subtraction, which makes link budget calculations (adding up cable loss, connector loss, amplifier gain, and so on) far easier. It also compresses a very wide range of power values into a manageable set of numbers: a signal ranging from 1 nanowatt to 100 watts spans dBm values from about -60 dBm to +50 dBm.

Common dBm Reference Points

dBm value	Equivalent power	Typical context
+30 dBm	1 W	Small RF power amplifier
+20 dBm	100 mW	Typical wifi router transmit power
0 dBm	1 mW	Reference point (always 1 mW)
-30 dBm	1 microwatt	Weak fibre optic receive signal
-50 dBm	10 nanowatts	Strong, reliable wifi signal
-70 dBm	0.1 nanowatts	Usable but weaker wifi signal
-90 dBm	1 picowatt	Very weak, near the noise floor

Where dBm Is Used

Wifi networking: router and device transmit power, and received signal strength indicator (RSSI) readings.
Fibre optics: optical transmitter output power and receiver sensitivity in fibre broadband and telecommunications links.
Mobile and cellular networks: base station transmit power and handset signal strength.
RF and microwave engineering: amplifier gain, cable and connector loss, and antenna link budgets.
Bluetooth and IoT devices: low-power transmitter ratings, often in the -20 dBm to +20 dBm range.

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Sources: dBm is a standard unit defined in IEEE and ITU-T telecommunications standards, referenced to 1 milliwatt. The formula dBm = 10 x log10(P / 1mW) is a fixed mathematical relationship used throughout RF, wifi and fibre optic engineering.

This calculator performs a standard mathematical conversion. Results are exact given the formula above. Real-world signal readings (such as wifi RSSI) depend on your specific hardware and environment and may be reported with rounding by the manufacturer.