Calculator
Example data table
| Scenario | Input | Attenuation | Output | Remaining ratio |
|---|---|---|---|---|
| Short cable loss | 0 dBm | 3 dB | -3 dBm | Power ≈ 0.5000 |
| Long coax run | 10 dBm | 12 dB | -2 dBm | Power ≈ 0.0631 |
| Voltage drop | 1.000 V | 6 dB | 0.501 V | Voltage ≈ 0.5012 |
| Heavy attenuation pad | 0 dBW | 20 dB | -20 dBW | Power ≈ 0.0100 |
Formula used
- Power attenuation (dB): A = 10 · log10(Pin / Pout)
- Voltage/current attenuation (dB): A = 20 · log10(Vin / Vout)
- Power ratio: Pout/Pin = 10(−A/10)
- Voltage ratio: Vout/Vin = 10(−A/20)
- Total loss with length: Atotal = Abase + (loss per meter) · (meters)
How to use this calculator
- Select a mode: power or voltage/current, and what you know.
- Enter the input level and select the correct unit.
- Enter either the output level or the attenuation value.
- Optionally add loss per length and a path length.
- Optionally set a target output to see your margin.
- Press Calculate to show results above the form.
- Use the CSV/PDF buttons to export the result card.
dB Attenuation Guide
1) Decibels summarize large changes
Decibels compress ratios into small numbers. A 3 dB loss halves power (50%). A 10 dB loss leaves one‑tenth power (10%). A 20 dB loss leaves one‑hundredth power (1%). In practice, small dB changes can still matter greatly.
2) Power and voltage use different factors
Power attenuation uses 10·log10(Pin/Pout). Voltage or current uses 20·log10(Vin/Vout). Therefore 6 dB is about 0.501× voltage, but about 0.251× power.
3) Reference points prevent unit mistakes
Remember: 0 dBm = 1 mW, 30 dBm = 1 W, −30 dBm = 1 µW. Also, 0 dBV = 1.000 V RMS and 0 dBu = 0.775 V RMS. These anchors make results easier to verify.
4) Length-based loss adds in dB
Extra loss is linear in dB: (loss per length) × (length). Example: 0.20 dB/m over 30 m adds 6 dB. If connectors add 1.5 dB total, include it as additional attenuation.
5) Typical attenuation numbers
Short audio leads over a few meters are often under 1 dB at line level. Long RF coax can be many dB, especially as frequency rises. Single‑mode fiber is often near 0.35 dB/km at 1310 nm and about 0.2 dB/km at 1550 nm. For quick intuition, 2 dB leaves about 63.1% power, and 12 dB leaves about 6.3% power.
6) Pads protect sensitive stages
Common fixed pads are 3, 6, 10, and 20 dB. A 10 dB pad drops power to 10% and voltage to 31.6%. A 20 dB pad drops power to 1% and voltage to 10%.
7) Margin turns loss into a design check
With a target output, the margin (dB) equals allowable loss minus total loss. Positive margin means your link meets the target. Negative margin means you must raise input, shorten the run, or choose lower‑loss components. This mirrors a basic link‑budget check used in RF, audio, and optical systems.
8) “Remaining percent” depends on family
In power mode, remaining percent is remaining power × 100. In voltage/current mode, it is remaining amplitude × 100. That is why 3 dB can mean 50% power yet about 70.8% voltage.
FAQs
1) What is the difference between attenuation and gain?
Attenuation is a positive dB value that reduces a signal. Gain is a positive dB value that increases a signal. Mathematically they are the same ratio; only the sign and the direction of change differ.
2) Why does power use 10 and voltage use 20?
Power is proportional to the square of voltage (or current) for a fixed impedance. Using 20·log10 on voltage converts the squared relationship into the correct power ratio on a dB scale.
3) Can I use negative dB values in this calculator?
Yes. Input or output levels expressed in dBm, dBW, dBu, or dBV can be negative. However, linear units like watts or volts must be greater than zero for logarithms and ratios to work.
4) What does 3 dB mean in simple terms?
In power terms, 3 dB is about half the power. In voltage/current terms, 3 dB is about 0.707× the amplitude. The calculator shows both ratios so you can interpret results correctly.
5) How do I include connector or splitter losses?
Add them as extra attenuation. If you have 0.5 dB per connector and use three connectors, add 1.5 dB. Splitters, couplers, and filters can be treated the same way if you know their insertion loss.
6) When should I enter loss per length?
Use loss per length when you know a cable, fiber, or waveguide specification like dB/m or dB/km. Enter both the loss rate and the run length to estimate additional path loss alongside your base attenuation.
7) What does a negative margin mean?
A negative margin means the total loss exceeds what your target output allows. The output will fall below the target, so you may need a stronger source, a shorter path, or lower‑loss components.