dBm to Watts Calculator
The dBm is a logarithmic unit of power referenced to one milliwatt, used everywhere in radio, fiber optics and audio because it turns the enormous range of signal powers into a manageable scale. Zero dBm equals exactly one milliwatt, every 10 dBm step is a tenfold change in power, and every 3 dBm is roughly a doubling. To convert a level in dBm to power in watts you divide the dBm value by ten, raise ten to that power to get milliwatts, then divide by one thousand to reach watts; equivalently, watts equal ten raised to the power of the dBm minus thirty, all over ten. This calculator takes a power level in dBm and returns the power in watts, switching to scientific notation for very small values so a tiny but real power never displays as zero. The relationship is the standard definition of the dBm and is exact, not an approximation. The definitions of the watt and the decibel-referenced power scales are maintained by the National Institute of Standards and Technology. Every figure here is computed deterministically from the conversion formula, shown in full below, with a worked example that reconciles exactly to the calculator so you can verify each step yourself.
Power in watts is 10^((dBm - 30) / 10). A level of 20 dBm equals 0.10 W (100 milliwatts), since 0 dBm is 1 mW and each 10 dB is a tenfold change.
dBm to watts formula
P(W) = 10^((dBm - 30) / 10)
equivalently P(mW) = 10^(dBm / 10)
and P(W) = P(mW) / 1000
0 dBm = 1 mW reference
Subtract thirty from the dBm value and divide by ten, then raise ten to that result to get watts. The minus thirty converts the milliwatt reference of dBm into watts. Each 10 dBm represents a factor of ten in power, and each 3 dBm is approximately a doubling.
Worked example
A transmitter outputs a power level of 20 dBm.
- Subtract 30: 20 - 30 = -10
- Divide by 10: -10 / 10 = -1
- Raise 10 to that power: 10^(-1) = 0.1
- The power is 0.10 W, which is 100 milliwatts
So 20 dBm corresponds to 0.10 W (100 mW). This is the calculator's default input, so the result above matches the widget exactly.
dBm to watts reference
Each 10 dBm is a tenfold change in power; 0 dBm is one milliwatt.
| dBm | Power |
|---|---|
| 0 | 1 mW |
| 10 | 10 mW |
| 20 | 100 mW (0.10 W) |
| 30 | 1 W |
| 40 | 10 W |
Power unit reference: US National Institute of Standards and Technology (NIST).
dBm to Watts Calculator: frequently asked questions
How do you convert dBm to watts?
Use P(W) = 10^((dBm - 30) / 10). For 20 dBm, that is 10^((20 - 30)/10) = 10^(-1) = 0.10 W, which is 100 milliwatts. The minus 30 accounts for dBm being referenced to one milliwatt rather than one watt.
What is 0 dBm in watts?
Zero dBm equals exactly one milliwatt, which is 0.001 W. The dBm scale is defined relative to one milliwatt, so a level of 0 dBm is the reference point of the scale.
How much is a 3 dB change?
A 3 dB increase roughly doubles the power and a 3 dB decrease roughly halves it, because 10^(3/10) is about 2. A 10 dB step is an exact tenfold change. These shortcuts let you estimate power changes without a calculator.
Why does the calculator use scientific notation for small powers?
Very low signal levels, common in radio receivers, correspond to tiny fractions of a watt that would round to 0.00 at two decimals. Scientific notation shows the true nonzero value, for example 2.50e-7 W, instead of a misleading zero.
What is the dBm to watts formula?
Power in watts equals 10^((dBm - 30) / 10), or equivalently 10^(dBm/10) milliwatts divided by 1,000.
Official sources
- Electrical units and measurement standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.