dBm to Watts Converter
Convert between dBm, watts, and dBW, and find the RMS voltage into a load impedance. Leave power or dBm blank to solve for it.
dBm, dBW, and watts
dBm expresses power on a logarithmic scale referenced to 1 milliwatt, which makes the huge range of RF and audio powers easy to handle and lets gains and losses simply add up along a signal chain.
dBm = 10 · log₁₀(P / 1 mW)
P = 1 mW × 10^(dBm / 10)
dBW is the same idea referenced to 1 watt, so dBW = dBm − 30. Into a resistive load R, the RMS voltage is √(P · R) — for example 0 dBm (1 mW) into 50 Ω is about 224 mV.
Worked example: a LoRa module at 14 dBm
A LoRa radio rated 14 dBm puts out 1 mW × 10^(14/10) = 25.12 mW — about a fortieth of a watt, or −16 dBW. Driving a 50 Ω antenna, that power is √(0.02512 × 50) = 1.121 V RMS.
Related
For voltage or power ratios in decibels (gain), see thedecibel converter; for amplifier gain, the op-amp gain calculator.
Common questions
- What is 0 dBm in watts?
- Exactly 1 milliwatt. That is the definition — dBm is decibels relative to one milliwatt, so 0 dBm is the reference point itself. From there, +10 dBm is 10 mW, +20 dBm is 100 mW, +30 dBm is 1 W, and −30 dBm is 1 µW. Every 10 dB is a factor of ten in power, and every 3 dB is roughly a factor of two.
- Is dBm the same as dB?
- No, and the difference matters. A dB figure is a ratio with no units — it describes a change, such as an amplifier having 20 dB of gain. A dBm figure is an absolute power, because it is measured against a fixed 1 mW reference. So "the amplifier adds 20 dB" is meaningful on its own; "the signal is 20 dB" is not, unless the reference is stated.
- How do I add dBm values together?
- You do not add two dBm figures — that has no physical meaning, because you would be adding two absolute powers on a logarithmic scale. What you can do is add a dB gain or loss to a dBm level: −80 dBm into an amplifier with 20 dB of gain gives −60 dBm. That is the whole basis of a link budget, where powers in dBm and gains in dB are summed along a chain.
- Does dBm depend on the impedance?
- Not for the power itself — dBm is a measure of power and 0 dBm is 1 mW into any load. Impedance only enters when you convert to a voltage, because the same power produces a different voltage across a different resistance. In RF work 50 Ω is assumed almost universally, which makes 0 dBm about 223 mV RMS; in audio and video 600 Ω and 75 Ω appear instead, and the voltage figure changes accordingly.