What this calculator does
dBm is a logarithmic unit used throughout radio, telecom and wireless engineering to express power relative to one milliwatt, because signal power in these fields spans an enormous range that is awkward to write out in plain watts. Converting a dbm to watt figure, or the reverse, is a routine step when reading equipment specifications, antenna gain figures or signal strength readings.
The relationship is P(mW) = 10 raised to the power of dBm divided by 10, and going the other way, dBm = 10 × log base 10 of the power in milliwatts. This calculator handles both directions, converting a dBm reading to watts and milliwatts, or a watt figure back to dBm.
The formula
For dBm to watts, milliwatts equal 10^(dBm ÷ 10), and watts equal that milliwatt figure divided by 1,000. For watts to dBm, milliwatts equal watts × 1,000, and dBm equals 10 × log10 of that milliwatt figure.
| Term | Meaning |
|---|---|
| dBm | Power expressed in decibels relative to one milliwatt. |
| mW | Milliwatts, one thousandth of a watt, the reference unit dBm is measured against. |
| W | Watts, the standard unit of power. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert | Choose which direction to convert. |
| Power in dBm (dBm) | The power reading in dBm, used when converting dBm to watts. |
| Power in watts (W) | The power in watts, used when converting watts to dBm. |
When to use it
Reading a radio or Wi-Fi transmitter's output
Transmitter power is almost always specified in dBm. Converting to watts gives a more intuitive sense of the actual power being radiated.
Comparing a watt to dbm spec across two devices
One datasheet in watts and another in dBm cannot be compared directly until both are converted to the same unit.
Working out a link budget
RF link budgets are typically calculated in dBm because gains and losses along the path add and subtract in decibels rather than multiplying, so converting a starting watt figure to dBm is often the first step.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do common dBm readings convert to in watts?
A range of dBm values commonly seen on transmitter and signal specifications.
| Power | Power | Power in milliwatts |
|---|---|---|
| 0 dBm | 0.001000 W | 1.000 mW |
| 10 dBm | 0.010000 W | 10.000 mW |
| 20 dBm | 0.100000 W | 100.000 mW |
| 23 dBm | 0.199526 W | 199.526 mW |
| 30 dBm | 1.0000 W | 1,000.00 mW |
| 40 dBm | 10.0000 W | 10,000.00 mW |
Questions
What is 0 dBm in watts?
0 dBm equals exactly 1 milliwatt, or 0.001 watts, since dBm is defined relative to that reference point.
Why use dBm instead of watts for RF power?
Radio power levels span many orders of magnitude, from microwatts to hundreds of watts. A logarithmic scale like dBm keeps these figures in a manageable range and lets gains and losses along a signal path be added and subtracted rather than multiplied and divided.
How much does adding 3 dBm change the power?
Adding 3 dB roughly doubles the power, and subtracting 3 dB roughly halves it, which is a useful rule of thumb when reading link budgets or antenna gain figures.
Is dBm the same as dB?
No. dB on its own is a ratio between two power levels with no fixed reference, while dBm always measures power relative to a fixed reference of 1 milliwatt, making it an absolute unit rather than a relative one.
For other electrical calculations, see the Ohm's law calculator.