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electronics Engineering Reference Matrix

dB vs dBm: Dimensionless Ratio Gain vs Absolute Power Sizing

Decibel (dB) is a dimensionless logarithmic ratio expressing the relative change (gain or loss) between two power or voltage signals. In contrast, dBm is an absolute unit of power referenced to exactly 1 milliwatt (0 dBm = 1.0 mW; 30 dBm = 1.0 Watt). In RF communications, fiber optics, and audio engineering, confusing relative dB gain with absolute dBm signal strength leads to receiver saturation.

Governing Formula: Relative Ratio: dB = 10 * log10(P2 / P1); Absolute Power: dBm = 10 * log10(P_mW / 1 mW)

Technical Comparison & Conversion Matrix

Power Level dBm (Absolute) Milliwatts (mW) Watts (W)
Cellular Receiver Sensitivity Limit -100 dBm 0.0000000001 mW 0.1 picowatt
Wi-Fi Signal Floor (Marginal) -75 dBm 0.0000316 mW 31.6 picowatts
Wi-Fi Signal Strength (Excellent) -50 dBm 0.00001 mW 10 nanowatts
Optical Fiber Transceiver Output 0 dBm 1.0 mW 0.001 Watt
Standard Bluetooth Class 2 +4 dBm 2.5 mW 0.0025 Watt
Wi-Fi Router Max EIRP (FCC) +20 dBm 100 mW 0.100 Watt
Commercial Handheld Two-Way Radio +37 dBm 5,000 mW 5.0 Watts
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Live Calculator: 4-Band Resistor Color Code Decoder & Tolerance Calculator

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Computed Output

Nominal Resistance (Ohms)
1,000 Ω
Standard Engineering Notation
1.0 kΩ
Tolerance Percentage
±5.0%
Permissible Resistance Range
950.0 Ω - 1,050.0 Ω

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4-Band Resistor Color Code Decoder & Tolerance Calculator

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Frequently Asked Questions

Why is adding dB to dBm mathematically valid?

Because dB is a logarithmic multiplier. In logarithmic domain, multiplication becomes addition: Output (dBm) = Input Power (dBm) + Amplifier Gain (dB) - Cable Attenuation (dB). A 0 dBm input fed into a +20 dB amplifier yields exactly +20 dBm (100 mW).

What is the 3 dB rule of thumb?

Every +3 dB increase represents an approximate doubling of power (10^(3/10) = 1.995). Conversely, a -3 dB loss represents a 50% reduction in power.