dBm to Watts, Volts and dBµV Converter

How to use the calculator

Choose the unit you have, type the value and set the resistance (50 Ω for most RF work). The results show the same signal as power in dBm, dBW, milliwatts and watts, and as voltage in rms, peak and peak-to-peak, plus its level in dBµV. The voltage figures assume a sine wave across the resistance you entered.

The definitions

dBm is power in decibels relative to one milliwatt:

dBW uses a one-watt reference, so dBW = dBm − 30. For voltage, the rms value that delivers power P into a resistance R is:

dBµV is voltage relative to one microvolt rms: dBµV = 20 · log10(V / 1 µV). Into 50 Ω it equals dBm + 107 (to within 0.01 dB).

Reference table at 50 Ω

Level	Power	Voltage (rms)	Voltage (peak-to-peak)	dBµV
−174 dBm	3.98e−21 W	0.446 nV	1.26 nV	−67.0
−100 dBm	0.1 pW	2.24 µV	6.32 µV	7.0
−50 dBm	10 nW	0.707 mV	2 mV	57.0
−30 dBm	1 µW	7.07 mV	20 mV	77.0
−20 dBm	10 µW	22.4 mV	63.2 mV	87.0
−10 dBm	100 µW	70.7 mV	200 mV	97.0
0 dBm	1 mW	224 mV	632 mV	107
3 dBm	2 mW	316 mV	893 mV	110
10 dBm	10 mW	707 mV	2 V	117
20 dBm	100 mW	2.24 V	6.32 V	127
23 dBm	200 mW	3.16 V	8.93 V	130
27 dBm	501 mW	5.01 V	14.2 V	134
30 dBm	1 W	7.07 V	20 V	137
33 dBm	2 W	9.99 V	28.3 V	140
37 dBm	5.01 W	15.8 V	44.8 V	144
40 dBm	10 W	22.4 V	63.2 V	147
43 dBm	20 W	31.6 V	89.3 V	150
47 dBm	50.1 W	50.1 V	142 V	154

Two reference points are worth remembering. The thermal noise floor of a 1 Hz bandwidth at 290 K is about −174 dBm, which sets the limit for receiver sensitivity. And 23 dBm (200 mW) is the maximum output power of a power class 3 LTE handset, a level that comes up often in mobile RF work.

Rules of thumb

You can estimate most conversions without a calculator:

• +3 dB is about ×2 in power, and −3 dB is about ÷2. (Exactly, 3.01 dB.)
• +10 dB is ×10, and −10 dB is ÷10.
• Combine them: 13 dBm is 10 dBm + 3 dB, so 10 mW × 2 = 20 mW. 27 dBm is 30 dBm − 3 dB, so 1 W ÷ 2 = 0.5 W.
• +6 dB doubles the voltage (and quadruples the power).
• Gain and loss in dB add to a level in dBm (this is the basis of a power budget): a −10 dBm signal through a 20 dB amplifier and a 6 dB attenuator is −10 + 20 − 6 = +4 dBm.

Common mistakes

Adding dBm values. Two 0 dBm signals do not make 0 dBm + 0 dBm = 0 dBm, and they do not make 6 dBm either. If they are uncorrelated their powers add (1 mW + 1 mW = 2 mW), which is +3 dBm. If they are coherent and in phase their voltages add, giving four times the power and +6 dBm. Know which case you are in.

Assuming 50 Ω everywhere. dBm is independent of impedance, but the voltage is not. A scope or probe with a high input impedance does not show the voltage computed into 50 Ω unless it is terminated.

Mixing rms, peak and peak-to-peak. Instruments and datasheets quote any of the three. A 0 dBm sine wave is 224 mV rms, 316 mV peak and 632 mV peak-to-peak into 50 Ω.

Ignoring the signal type. For modulated signals such as LTE or Wi-Fi the average power and the peak envelope power differ by the crest factor (peak-to-average ratio). A power amplifier may be rated by average output power (and its linearity limit is described by the P1dB compression point) while the peak voltage is several dB higher, which matters for compression and for the voltage rating of components.

Confusing dBm with dBc, dBi and dBFS. Each decibel unit has its own reference: dBc is relative to a carrier, dBi is antenna gain relative to an isotropic radiator, and dBFS is relative to the full scale of a converter. Only dBm and dBW are absolute power.

In plain words · for students and beginners

Decibels and dBm in plain words

Why decibels exist

RF signals cover a huge range: a transmitter may put out a watt while a receiver has to detect a trillionth of a watt. A normal ruler cannot show both. Decibels use a logarithmic ruler, like the Richter scale for earthquakes, where each step is a multiplication instead of an addition. On this ruler, every +10 dB is ten times the power and every +3 dB is about double.

What dBm adds

A decibel is only a ratio. To describe an actual power level you need a starting point. dBm is decibels compared with one milliwatt: 0 dBm is 1 mW, 10 dBm is 10 mW, 20 dBm is 100 mW and 30 dBm is 1 W. An LTE phone transmitting at its maximum power of 23 dBm sends about 0.2 W.

Adding and subtracting

Gains and losses in dB simply add. A signal of -10 dBm that goes through an amplifier with 20 dB of gain and then a cable with 3 dB of loss ends up at -10 + 20 - 3 = +7 dBm, which is about 5 mW. This is why RF engineers love decibels: a whole signal chain becomes a column of additions.

Where voltage comes in

Power does not depend on the impedance, but voltage does. At 50 ohm, 0 dBm (1 mW) is about 224 mV rms. If you measure with a probe that is not terminated in 50 ohm, you will not read that voltage.

Mistakes beginners make

• Adding two dBm numbers. Two equal signals of 0 dBm do not make 0 dBm + 0 dBm. If their powers add, you get about +3 dBm.
• Mixing up dB (a ratio) and dBm (a level). You can add a dB value to a dBm value, but not two dBm values.
• Mixing dBm and dBW. They are 30 dB apart.

Remember

• +10 dB is 10 times the power. +3 dB is about 2 times.
• 0 dBm is 1 mW, 30 dBm is 1 W.
• Gains and losses add. Levels do not.

Related pages on RF View

Levels and budgets

• RF power budget
• RF link budget calculator
• P1dB compression point
• Dynamic range

Measurement

• RF power measurement methods
• RF power measurement

Calculators

Use the attenuator calculator to set a level with a resistive pad, and the VSWR and return loss calculator to see how much power a mismatch removes from a level you have computed.