dBm to Watts Calculator

Q: How do you convert dBm to volts in a 50 ohm system?

First change dBm to watts, then use V = √(P × R) with R = 50 ohms. Example: 0 dBm is 0.001 W. √(0.001 × 50) = 0.2236 volts RMS, about 224 mV. This only works if you know the impedance. dBm is power, so you need the resistance to get voltage.

Introduction

This dBm to Watts calculator changes radio power from decibel-milliwatts (dBm) into watts, and back again. Type a number, pick your unit, and read the answer.

dBm is a way to show power using a log scale. It is used for Wi-Fi, cell phones, radios, and fiber optic links. Watts show the same power in plain numbers. A small change in dBm means a big change in watts, which is why the math is hard to do in your head.

You can switch between dBm → power and power → dBm with one click. Results show up in kilowatts, watts, milliwatts, microwatts, nanowatts, and dBW at the same time. You also get step-by-step math, a chart of the power curve, and a table of common values so you can check your work.

How to use our dBm to Watts Calculator

Enter a power level in dBm or a power value in watts, and the calculator shows the matching result in dBm, dBW, kilowatts, watts, milliwatts, microwatts, and nanowatts, plus a step-by-step solution, a power curve chart, and a reference table.

Conversion Direction: Pick "dBm → Power" to turn a dBm value into watts, or "Power → dBm" to turn watts into dBm. The field you are not typing in becomes read-only and fills in for you.

Power (dBm): Type the power level in dBm. Negative numbers work fine, like -30 dBm for a weak Wi-Fi signal.

Power (linear): Type the power value, then choose its unit from the drop-down: pW, nW, µW, mW, W, kW, or MW. You can use scientific notation like 1e-6. The value must be greater than zero.

Primary Output Unit: Choose the unit for the big headline answer and the chart's y-axis. Milliwatts is the default.

Significant Figures: Pick how many digits to show in every result, from 2 to 10. Six is the default.

Click Convert to see your answer, or Reset to return to the starting values. Use the copy buttons to copy any result to your clipboard.

What Is dBm?

dBm means "decibels relative to one milliwatt." It is a way to measure radio, Wi-Fi, and fiber-optic signal power. The number tells you how strong a signal is compared to 1 milliwatt (0.001 watt). So 0 dBm is exactly 1 mW. Numbers above 0 are stronger than 1 mW. Numbers below 0, like −70 dBm, are weaker.

Why Engineers Use dBm Instead of Watts

Radio power covers a huge range. A phone may send out 0.2 watts but receive only 0.000000001 watts. Writing all those zeros is messy. The dBm scale is a log scale, so it turns those tiny and big numbers into simple ones like −90 or +23. It also makes math easy: when a signal passes through a cable or an amplifier, you just add or subtract dB instead of multiplying.

The dBm to Watts Formulas

To go from dBm to watts:

P(W) = 10^{(dBm − 30) / 10}

To go from watts back to dBm:

P(dBm) = 10 × log₁₀(P in watts) + 30

The "30" in each formula changes between watts and milliwatts, since 1 watt equals 1,000 mW and 10 × log₁₀(1000) = 30.

Quick Rules You Can Do In Your Head

+3 dB means about double the power.
−3 dB means about half the power.
+10 dB means 10 times the power.
−10 dB means one tenth the power.

Example: 30 dBm is 1 W. Add 3 dB and 33 dBm is about 2 W. Add 10 dB and 40 dBm is 10 W.

Common dBm Values

−90 dBm (1 pW) is the weakest signal most Wi-Fi or cell radios can still hear.
−67 dBm is a good Wi-Fi signal for calls and video.
0 dBm (1 mW) is a typical fiber-optic or Bluetooth transmit level.
20 dBm (100 mW) is what a laptop or phone sends over Wi-Fi.
30 dBm (1 W) is a Wi-Fi access point or a handheld two-way radio.
43 dBm (20 W) is a cell tower carrier.

dBm vs. dBW

dBW uses 1 watt as its starting point instead of 1 milliwatt. The two scales are always 30 apart. To change dBm to dBW, subtract 30. To change dBW to dBm, add 30. So 30 dBm is the same as 0 dBW, and both equal 1 watt.

Where This Is Used

You will see dBm on Wi-Fi signal meters, cell phone field test screens, radio transmitter specs, fiber-optic power meters, antenna and amplifier data sheets, and satellite link budgets. Anyone working with signal strength, cable loss, or antenna gain uses these numbers every day.

Formulas used

dBm to watts

P_{W} = 10^{\frac{P_{\mathrm{dBm}} - 30}{10}}

Watts to dBm

P_{\mathrm{dBm}} = 10\log_{10}\left(P_{W}\right) + 30

dBm to dBW

P_{\mathrm{dBW}} = P_{\mathrm{dBm}} - 30

Unit conversion to SI watts

P_{W} = P_{\text{unit}} \times k_{\text{unit}}, \quad k \in \{10^{-12}, 10^{-9}, 10^{-6}, 10^{-3}, 1, 10^{3}, 10^{6}\}

SI watts to selected output unit

P_{\text{unit}} = \frac{P_{W}}{k_{\text{unit}}}

Frequently asked questions

Why are dBm values negative?

A negative dBm means the power is less than 1 milliwatt. Since 0 dBm equals 1 mW, anything weaker gets a minus sign.

Received radio signals are tiny, so they are almost always negative. For example, −70 dBm is only 0.0000001 watt (100 pW). The more negative the number, the weaker the signal.

What is the difference between dB and dBm?

dBm is a real power level. dB is only a ratio, or a change.

dBm answers "how much power?" 10 dBm is 10 mW.
dB answers "how much bigger or smaller?" A 10 dB gain means 10 times more power.

So an amplifier has gain in dB, but its output has power in dBm. dBm + dB = dBm.

Is -50 dBm better than -70 dBm?

Yes. −50 dBm is much stronger. Closer to zero always means more power.

The gap is 20 dB, which is 100 times more power. −50 dBm is 10 nW, while −70 dBm is only 0.1 nW.

Can you add two dBm values together?

No. You cannot just add dBm to dBm, because it is a log scale.

To combine two power sources, change both to watts, add the watts, then convert back. Two equal signals give you 3 dB more, not double the dBm. So 0 dBm plus 0 dBm equals 3 dBm (2 mW), not 0 dBm.

You can add dB gains and losses to a dBm value.

How do you work out transmit power after cable loss and antenna gain?

Add the gains and subtract the losses, all in dB, from the radio's output in dBm.

EIRP (dBm) = radio power (dBm) − cable loss (dB) + antenna gain (dBi)

Example: 30 dBm radio, 3 dB of cable loss, 9 dBi antenna. 30 − 3 + 9 = 36 dBm, which is 4 watts.

What is a good cell phone signal in dBm?

Most phones show signal in dBm in their field test or network settings screen.

−50 to −79 dBm: excellent
−80 to −89 dBm: good
−90 to −99 dBm: fair, slower data
−100 dBm or lower: poor, calls may drop
−110 dBm: almost no service

How do you convert dBm to volts in a 50 ohm system?

First change dBm to watts, then use V = √(P × R) with R = 50 ohms.

Example: 0 dBm is 0.001 W. √(0.001 × 50) = 0.2236 volts RMS, about 224 mV.

This only works if you know the impedance. dBm is power, so you need the resistance to get voltage.

How many watts does a cell phone transmit?

Most phones top out near 23 dBm, which is 200 milliwatts (0.2 watt). Many transmit far less when the tower is close, sometimes under 1 mW, to save battery and cut interference.

Older 2G phones could reach 33 dBm (2 watts).

What is RSSI and how does it relate to dBm?

RSSI stands for Received Signal Strength Indicator. It tells you how strong a signal is at the receiver.

Most Wi-Fi and cell gear reports RSSI directly in dBm, like −65 dBm. Some chipsets use their own scale, such as 0 to 100, so the same RSSI number can mean different power on different hardware. dBm is the exact unit; raw RSSI is not always.

Does doubling the transmit power double the range?

No. Doubling power is only +3 dB, and that adds about 40% more distance in open space.

To double the range you need 4 times the power, or +6 dB. Walls, trees, and noise cut this down even more, so big power jumps give small distance gains.

What is a normal fiber optic power reading in dBm?

Transmitters usually put out near 0 dBm (1 mW), sometimes a few dB above or below.

At the far end, receivers often read between −3 dBm and −25 dBm, depending on the link. If your reading is lower than the receiver's rated sensitivity, the link will drop errors. A dirty or bent fiber shows up as a sudden drop of several dB.

How much Wi-Fi transmit power is allowed?

In the United States, the FCC caps 2.4 GHz point-to-multipoint gear at 30 dBm (1 watt) at the radio and 36 dBm (4 watts) EIRP after antenna gain.

Home routers run well under that, usually 20 to 23 dBm (100 to 200 mW). Limits change by band and country, so check your local rules.

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