Attenuation & dB Calculator

dB from power or voltage ratios, and π / T resistive attenuator pad design.

What it does

Converts power or voltage ratios into decibels, and designs resistive π and T attenuator pads for a matched system.

Where it's used

Link budgets, cable loss, protecting receiver inputs and padding signal generators.

How to use it

Choose power ratio, voltage ratio or attenuator pad.
Enter the two powers or voltages, or the attenuation and system impedance.
For a pad, use the nearest 1 % resistors to the results.

The formula explained

dB (power): 10 · log₁₀(P1 / P2)
dB (voltage): 20 · log₁₀(V1 / V2), same impedance
K: voltage ratio for the pad = 10^(dB/20)
Z₀: system impedance (50 Ω for RF, 75 Ω for video)

Worked example

A 6 dB attenuator for a 75 Ω video system.

Inputs:

Calculate: Design π / T attenuator pad
Attenuation: 6 dB
System impedance Z₀: 75 Ω

The calculator gives:

π-pad shunt R1 (×2): 225.7 Ω
π-pad series R2: 56.03 Ω
T-pad series R1 (×2): 24.92 Ω

Tips and common mistakes

3 dB is half the power; 10 dB is one tenth.
Voltage dB uses 20·log, power dB uses 10·log.
Check the pad resistors’ power ratings at your signal level.

Questions people ask

How do I convert decibels back to a ratio?

Power ratio = 10^(dB / 10) and voltage ratio = 10^(dB / 20). A 6 dB pad passes about a quarter of the power, which is half the voltage.

What does dBm mean?

Power compared with 1 milliwatt: 0 dBm = 1 mW, 30 dBm = 1 W and −30 dBm = 1 µW. Adding or subtracting dB from a dBm figure gives another dBm figure.

Should I use a π pad or a T pad?

Both give the same attenuation and the same matched impedance. Choose whichever gives more convenient standard resistor values; π pads are popular on RF boards because both shunt resistors go straight to ground.

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