dB Attenuation Calculator

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.

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.