What is the difference between dB and dBm?

dB is a relative unit that expresses a ratio between two quantities. dBm is an absolute unit referenced to 1 milliwatt, so 0 dBm equals exactly 1 mW and +30 dBm equals 1 W.

How do I convert dBm to watts?

Power in milliwatts is P(mW) = 10^(dBm/10). To get watts, divide by 1000, or use P(W) = 10^((dBm-30)/10). For example, +30 dBm = 10^(0) = 1 W.

When do I use 10·log10 versus 20·log10?

Use 10·log10 for power ratios (watts) and 20·log10 for amplitude ratios (voltage, current, sound pressure). The factor of two comes from power being proportional to the square of amplitude.

Why is +3 dB roughly double?

10·log10(2) ≈ 3.01 dB, so doubling the power is about +3 dB. For voltage, doubling is +6 dB because 20·log10(2) ≈ 6.02 dB.

What does a negative dB value mean?

A negative dB means the output is smaller than the reference — attenuation rather than gain. For example, -3 dB is about half the power and -20 dB is one hundredth of the power.

trysys.jp

📶Reference

dB / dBm Converter

Convert dBm to watts and turn power or voltage ratios into decibels.

dBm

dBm ⇔ Power

dBm

Power leveldBm

dBm

P

Powerwatts

dB

Ratio ⇔ dB

×

Ratiolinear

×

dB

DecibelsdB

dB

Quick reference

Common dB and dBm values worth memorising.

Value	Meaning