What is the difference between dB and dBm?

dB is a ratio between two powers and says nothing about size — 3 dB of gain doubles whatever went in. dBm is an absolute level, measured against one milliwatt. You can add a dB figure to a dBm figure, but adding two dBm figures is meaningless.

Why is 3 dB "double"?

Because 10 × log₁₀(2) is 3.0103, and the scale is rounded in conversation. Doubling power is 3.0103 dB exactly, so a chain of ten "3 dB" doublings is 30.103 dB, not 30 — small, but it is why link budgets are worked in the exact figures.

How do I convert a voltage ratio?

Voltage and other amplitude quantities use 20 × log₁₀ rather than 10 × log₁₀, because power goes as the square of amplitude. Doubling a voltage into the same impedance is 6.0206 dB, which is the same as quadrupling the power.

What does a negative dBm mean?

Less than a milliwatt. −70 dBm is 100 picowatts, a typical received signal on a fringe Wi-Fi connection. Receiver sensitivity is quoted in negative dBm for exactly this reason: the numbers stay small and addable where watts would be a string of zeroes.

dBm to Watts Calculator

With value 37, dbm to watts comes to 5.0119 W — power. It is reached in 4 steps, the last of which is 5011.8723 / 1000, and each one is printed on the page with its numbers filled in. The formula is the one published by BIPM, not an approximation fitted to it.

Converts dBm to watts and milliwatts, and back — the logarithmic power scale radio, fibre and audio equipment is specified in.

Formula and sources checked 8 September 2026 · How we check

Convert dBm to power, Value 37

5.0119 W

Power for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

ConvertValuedBm in one direction, watts in the other.

Power

5.0119 W

Power in dBm: 37 dBm
Power in milliwatts: 10 ^ (37 / 10)5,011.872 mW
Power in watts: 5011.8723 / 10005.012 W
Power in dBW: 37 - 307 dBW

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Worked example

37 dBm is 5.0119 watts, or 5011.87 milliwatts. The reference is the m: dBm counts decibels above one milliwatt, so 0 dBm is exactly 1 mW and 30 dBm is exactly 1 W. That fixed anchor is what separates dBm from plain dB, which is a ratio and has no absolute value at all.

How to work it out yourself

1.Every 3 dB is roughly double the power and every 10 dB is exactly ten times it. 20 dBm is 100 mW, 30 dBm is 1 W, 40 dBm is 10 W.
2.Add and subtract rather than multiply. An amplifier with 13 dB of gain fed 17 dBm puts out 30 dBm, and a 6 dB splitter loss takes it back to 24 dBm.
3.dBm and dBW differ by exactly 30, because a watt is a thousand milliwatts and ten times log₁₀(1000) is 30.

The formula

Power in dBm
Power in milliwatts10 ^ (37 / 10)
Power in watts5011.8723 / 1000
Power in dBW37 - 30

Source: BIPM — the SI Brochure, on the decibel and logarithmic quantities, NIST Special Publication 811 — guide for the use of the SI

Questions people actually ask

What is the difference between dB and dBm?: dB is a ratio between two powers and says nothing about size — 3 dB of gain doubles whatever went in. dBm is an absolute level, measured against one milliwatt. You can add a dB figure to a dBm figure, but adding two dBm figures is meaningless.
Why is 3 dB "double"?: Because 10 × log₁₀(2) is 3.0103, and the scale is rounded in conversation. Doubling power is 3.0103 dB exactly, so a chain of ten "3 dB" doublings is 30.103 dB, not 30 — small, but it is why link budgets are worked in the exact figures.
How do I convert a voltage ratio?: Voltage and other amplitude quantities use 20 × log₁₀ rather than 10 × log₁₀, because power goes as the square of amplitude. Doubling a voltage into the same impedance is 6.0206 dB, which is the same as quadrupling the power.
What does a negative dBm mean?: Less than a milliwatt. −70 dBm is 100 picowatts, a typical received signal on a fringe Wi-Fi connection. Receiver sensitivity is quoted in negative dBm for exactly this reason: the numbers stay small and addable where watts would be a string of zeroes.

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