Attenuation Calculator
Enter your input and output signal levels to find attenuation in decibels, or switch to Cable / Fiber mode to budget a full link using length, connector count, and splice count. All three modes support reverse-solve: fill in any two values and the third is calculated for you. Results update as you type.
Formula
Worked example
A fiber run of 10 km with a coefficient of 0.2 dB/km, 2 SC connectors (0.5 dB each), no splices, and a 3 dB safety margin: cable loss = 0.2 x 10 = 2 dB; connector loss = 2 x 0.5 = 1 dB; total = 3 dB; with margin = 6 dB. Equivalently, 1 V in and 0.5 V out gives 20 x log10(1/0.5) = 6.02 dB attenuation.
What is attenuation?
Attenuation is the reduction in signal strength as it travels through a medium or passes through a component. It appears in every branch of signal transmission: electrical cables lose energy to resistance, optical fibers scatter and absorb light, and radio waves spread out over distance. The effect is always measured in decibels (dB), a logarithmic unit that compresses the enormous range of practical signal ratios into manageable numbers. A 3 dB loss means output power is half the input; a 10 dB loss means it is one-tenth; a 20 dB loss cuts it to one-hundredth. Because the dB scale is additive, you can simply sum the losses from cable length, connectors, splices, and components to get the total link loss.
Voltage vs. power decibels: why two formulas?
Decibels always compare power ratios: dB = 10 x log10(Pout/Pin). Voltage is related to power through Ohm's law (P = V^2/R), so the voltage formula picks up an extra factor of 2: dB = 20 x log10(Vout/Vin). This is only valid when input and output impedances are equal - the standard assumption for most RF, audio, and telecommunications work. In power-limited systems (transmitter budgets, optical link budgets), always use the 10 x log10 formula. For voltage dividers, amplifier gain, and audio level matching, use the 20 x log10 formula. The two are consistent: a 6 dB voltage loss is also a 6 dB power loss because squaring the voltage ratio doubles the logarithm, which is exactly canceled by halving the multiplier (20 vs. 10).
Cable and fiber link budgets
A link budget adds up every source of loss between transmitter and receiver to confirm the signal arrives above the receiver's sensitivity threshold. The three main contributors are: (1) propagation loss - the attenuation coefficient of the medium multiplied by its length; (2) connector loss - each mated connector pair introduces a small but real insertion loss; (3) splice loss - every mechanical or fusion joint adds loss, fusion being far lower. To these you add a safety margin (typically 3-6 dB) to cover aging, future repairs, temperature changes, and measurement uncertainty. If the total budgeted loss exceeds the system's power budget (transmit power minus minimum receive power), you need to shorten the run, upgrade to a lower-loss medium, reduce connector count, or add an amplifier.
Reverse-solving: finding the required output level
Sometimes you know the allowed loss and the input level, and you need the minimum acceptable output level. For voltage: Vout = Vin x 10^(-A/20). For power: Pout = Pin x 10^(-A/10). Enter a target attenuation in the "Target attenuation" field (and leave the output level blank) to use this mode. It is useful when specifying a passive attenuator pad, checking whether a receiver meets its sensitivity specification, or working backwards from a link margin to a maximum cable length.
Typical attenuation reference values
| Medium / event | Typical loss | Notes |
|---|---|---|
| Single-mode fiber (1310 nm) | 0.35 dB/km | ITU-T G.652D |
| Single-mode fiber (1550 nm) | 0.20 dB/km | Lowest loss window |
| Multimode fiber (OM3/OM4, 850 nm) | 2.0-3.5 dB/km | Depends on launch conditions |
| RG-6 coaxial (100 MHz) | 3.5 dB/100 m | Standard CATV quad-shield |
| RG-58 coaxial (100 MHz) | ~5 dB/100 m | Thin 50-ohm cable |
| LMR-400 coaxial (1 GHz) | ~2.2 dB/100 m | Low-loss RF coax |
| Fusion splice (fiber) | 0.02-0.1 dB | Per splice event |
| Mechanical splice (fiber) | 0.1-0.5 dB | Per splice event |
| SC / LC fiber connector | 0.3-0.75 dB | Per mated pair |
| BNC / F-type connector (coax) | 0.1-0.3 dB | Per mated pair |
Common attenuation benchmarks. Exact values vary by frequency, cable grade, and connector quality.
Frequently asked questions
What is the difference between 10 log and 20 log in the dB formula?
Use 10 x log10 when comparing power quantities (watts, milliwatts). Use 20 x log10 when comparing field quantities such as voltage or current, because power is proportional to the square of voltage, and the square introduces a factor of 2 in the logarithm (log(x^2) = 2 log(x)). The two are equivalent: a 3 dB power loss is exactly the same as a 3 dB voltage loss in a matched-impedance system - power halved is voltage reduced by 1/sqrt(2), and 20 x log10(1/sqrt(2)) = 10 x log10(1/2) = -3 dB.
What does -3 dB mean?
A -3 dB level is the half-power point: output power is exactly 50% of input. In a voltage context it corresponds to about 70.7% (1/sqrt(2)) of the input voltage. The -3 dB point is widely used as the definition of a filter's cutoff frequency and an amplifier's bandwidth edge.
How do I convert attenuation in dB back to a voltage or power ratio?
For power: ratio = 10^(dB/10). For voltage: ratio = 10^(dB/20). For example, 6 dB of voltage attenuation gives a ratio of 10^(6/20) = 1.995, meaning output is about half the input.
What is a typical acceptable fiber optic link loss?
This depends on the optical power budget of your transceiver pair. A typical short-reach SFP might have 0 dBm launch power and -20 dBm sensitivity, giving a 20 dB power budget. With a 3 dB safety margin you have 17 dB to allocate to cable, connectors, and splices. For a single-mode fiber at 0.2 dB/km with 2 connectors (0.5 dB each) and no splices, that allows up to (17 - 1) / 0.2 = 80 km.
Does attenuation change with frequency?
Yes, for all real media. In coaxial cable, attenuation rises approximately with the square root of frequency (skin effect) at lower frequencies and more steeply at higher frequencies due to dielectric losses. In optical fiber, attenuation varies by wavelength - it is lowest around 1550 nm for silica fiber (the "third window"). For acoustic and radio signals in the atmosphere, higher frequencies are attenuated more by absorption and scattering. This calculator uses a single user-supplied coefficient, so enter the value appropriate for your operating frequency.