Attenuation Calculator — Signal Loss in dB
Calculate signal attenuation in decibels from measured input and output levels. Works for power (10·log₁₀) or amplitude/voltage (20·log₁₀) signals. Also gives the attenuation coefficient (dB/m) and the distance at which power halves.
Signal type
m
Positive = loss; negative = gain (amplification)
- 1
Signal ratio (input ÷ output)
100 ÷ 10 = 10 - 2
Attenuation = 10 × log₁₀(ratio)
10 × log₁₀(10) = 10Factor of 10 for power quantities.
How does this calculator work?
Attenuation (dB) = 10·log₁₀(P_in/P_out) for power, or 20·log₁₀(V_in/V_out) for voltage/amplitude. Positive = loss; 3 dB ≈ half power. Divide total dB by path length for the attenuation coefficient (dB/m). Enter input, output and distance to get all three values at once.
Formula
How this is calculated
Attenuation is the reduction in signal strength as it travels through a medium — cable, fibre-optic, air, tissue, or any lossy path. The result is expressed in decibels (dB), a logarithmic ratio. For power quantities (watts, milliwatts) the formula is 10·log₁₀(P_in/P_out); for amplitude or voltage quantities (volts, electric field) it is 20·log₁₀(A_in/A_out). The factor-of-2 difference arises because power is proportional to the square of amplitude.
A positive dB value means loss (output is weaker than input), while a negative value indicates gain (amplification). The attenuation coefficient α = A / d gives loss per unit length, useful for comparing different cables or media. For a uniform medium, signal decays exponentially with distance — the XY plot shows this decay curve.
Common reference points: 3 dB ≈ half power (or 70.7% amplitude), 10 dB = one-tenth power, 20 dB = one-hundredth power. Typical fibre-optic cables attenuate at 0.2–0.5 dB/km at 1550 nm; coaxial cables at MHz frequencies range from 2–20 dB/100 m depending on frequency and gauge.
Frequently asked questions
Use 10·log₁₀ when comparing power quantities (watts, milliwatts, intensity). Use 20·log₁₀ when comparing amplitude, voltage, current, or field strength — because power is proportional to amplitude squared, so the 2× factor appears in the log.
3 dB loss means the output power is approximately half the input power (more precisely 10^(−3/10) ≈ 50.1%). It is also called the half-power point and is widely used to define bandwidth of filters and amplifiers.
A negative dB result means amplification — the output is stronger than the input. This happens with active devices like amplifiers or optical amplifiers, not passive cables. Check your input/output values if you get a negative result unexpectedly.
Also known as
TG we-Calculate Editorial Team. (2026). Attenuation Calculator — Signal Loss in dB [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/attenuation-calculator
TG we-Calculate Editorial Team. "Attenuation Calculator — Signal Loss in dB." TG we-Calculate. 2026. https://we-calculate.com/calculator/attenuation-calculator.
TG we-Calculate Editorial Team, "Attenuation Calculator — Signal Loss in dB," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/attenuation-calculator
@misc{wecalculate_attenuation_calculator, title = {Attenuation Calculator — Signal Loss in dB}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/attenuation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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