dBm Calculator
dBm is the ratio of a power level to 1 milliwatt, expressed logarithmically in decibels. RF engineers, radio hams, and audio techs all use dBm because logarithmic units compress the huge dynamic range between microwatt receivers (-60 dBm) and kilowatt transmitters (+60 dBm) into a readable scale. This dBm calculator handles every common conversion: watt to dBm, dBm to watt, milliwatt to dBm, dBm to Vrms, dBV to dBm, and applying dB gain or loss offsets to a dBm reading.
The dBm formula
The definition of dBm is logarithmic:
- dBm = 10 x log10(P_mW) where P_mW is power in milliwatts.
- Equivalently: dBm = 10 x log10(P_W x 1000) = 10 x log10(P_W) + 30.
- Solved for power: P_W = 10^(dBm / 10) / 1000.
That means 1 mW = 0 dBm, 1 W = 30 dBm, 1 kW = 60 dBm, and -30 dBm = 1 microwatt. Every +10 dBm means 10x the power; every +3 dBm means 2x the power.
dBm to watt table
| dBm | Watts | Common use |
|---|---|---|
| -90 dBm | 1 pW | Faint radio signal at receiver input |
| -60 dBm | 1 nW | Typical weak cellular signal |
| -30 dBm | 1 microwatt | Strong indoor WiFi at client |
| 0 dBm | 1 mW | Bluetooth Class 2 (reference) |
| 20 dBm | 100 mW | Low-power WiFi AP |
| 30 dBm | 1 W | FCC Part 15 2.4 GHz limit, HT DMR radio |
| 33 dBm | 2 W | 2 watt to dBm - handheld HT limit |
| 37 dBm | 5 W | 5 watts to dBm - GMRS and FRS portable |
| 40 dBm | 10 W | 10 watt to dBm - mobile VHF/UHF radio |
| 47 dBm | 50 W | Vehicle mobile radio |
| 60 dBm | 1 kW | Amateur radio HF linear amplifier limit |
Watt to dBm conversion examples
A watt to dBm calculator runs dBm = 10 x log10(W x 1000). Quick reference: 0.5 W = 26.99 dBm, 1 W = 30 dBm, 2 W = 33.01 dBm, 3 W = 34.77 dBm, 5 W = 36.99 dBm, 10 W = 40 dBm, 25 W = 43.98 dBm, 100 W = 50 dBm. Notice that doubling the watts adds about 3 dB: 1 W -> 2 W = +3.01 dB, 5 W -> 10 W = +3.01 dB.
dBm to Vrms conversion
dBm is a power unit; Vrms is a voltage unit. To go between them you need the system impedance. The equation is Vrms = sqrt(P_W x Z) = sqrt(10^(dBm/10) / 1000 x Z). In 50 ohm (standard RF), 0 dBm = 0.2236 Vrms = 223.6 mVrms. In 75 ohm video, 0 dBm = 0.2739 Vrms. This calculator accepts any impedance via the impedance field.
dBV to dBm conversion
dBV is voltage relative to 1 Vrms; dBm is power relative to 1 mW. Converting dBV to dBm requires impedance because voltage to power depends on Z: dBm = dBV + 10 x log10(1000 / Z). In 50 ohm that offset is +13.01 dB (so 0 dBV = 13.01 dBm). In 600 ohm (legacy audio) the offset is +2.218 dB.
dB to dBm (cascaded gain and loss)
dB is a dimensionless ratio; dBm is an absolute power level. You can add dB to dBm because log-power + log-ratio = log-power. A +20 dB antenna amplifier on a -30 dBm signal gives -30 + 20 = -10 dBm. A -3 dB splitter on 40 dBm gives 37 dBm out each leg. This db to dbm calculator mode applies any dB offset to a dBm starting value.