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dB Attenuation Calculator

Find signal attenuation in decibels.

How it is calculated

The tool compares output power with input power: dB = 10 × log₁₀(Pout ÷ Pin). Pin and Pout are powers in the same unit (mW, W, µW, any unit works because only the ratio matters). It also shows the plain ratio Pout ÷ Pin to four decimals. The sign tells you the direction: a negative dB value is a loss (attenuation), a positive one is gain, and 0 dB means the power is unchanged. Both powers must be greater than zero, because the logarithm of zero is undefined; otherwise no result is produced. The factor 10 applies to power. For voltage or current ratios the factor would be 20, so do not enter voltages here.

Worked example

A cable carries 10 mW in and delivers 2.5 mW out. The ratio is 2.5 ÷ 10 = 0.25 (shown as 0.2500). The base-10 logarithm of 0.25 is about −0.602, and multiplying by 10 gives -6.02 dB. So the cable loses about 6 dB, which means three quarters of the power is gone.

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Common mistakes

  • Entering voltages or currents instead of powers; the 10-based formula would then understate the decibels by half, since the voltage version uses 20.
  • Swapping input and output, which flips the sign: a 6 dB loss appears as +6 dB gain.
  • Mixing units, such as Pin in watts and Pout in milliwatts, which shifts the answer by 30 dB.
  • Entering dBm values as powers. dBm is already logarithmic; convert it to milliwatts first or simply subtract the two dBm values.
  • Typing 0 for a fully blocked signal. The tool returns nothing for zero power; use a very small positive number instead.

About this calculator

This calculator finds signal loss in decibels using dB = 10·log₁₀(Pout/Pin), the standard power-ratio decibel formula (for voltage or amplitude ratios the multiplier becomes 20 instead of 10, since power scales with the square of voltage). A negative result means attenuation — the signal lost power passing through a cable, connector, or medium — while a positive result means gain. The logarithmic scale is used because signal power in real systems spans many orders of magnitude, which decibels compress into a manageable range.

RF and telecom engineers use this to specify cable, connector, and free-space path loss in link budgets, audio engineers use it for mixer and amplifier gain staging, and network technicians use it to verify that fiber or coax runs meet spec after installation. Anywhere a spec sheet lists loss “per 100 meters” or “per connector” in dB, this is the formula behind that number.

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Comments (1)

  • Editorial team

    Decibels add where ratios multiply, which is why they suit link budgets. Run each segment separately, such as a cable and then a connector, and add the dB figures to get the total loss. Cable loss also scales with length in dB, not in ratio: 6 dB over 100 m becomes about 12 dB over 200 m, a quarter of the power twice over, so one sixteenth. Which kind of link are you budgeting, and what loss per length does your datasheet give?

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