Attenuation Loss Calculator

Calculator

Calculation method

Pick the option matching your available measurements.

Rounding (decimal places)

Affects displayed values and downloads.

Power unit (shared)

Used where input power is provided.

Input power (Pin)

Enter the transmitted or launched power.

Input power unit

dBm is referenced to 1 mW.

Output power (Pout)

Enter the received or measured power.

Output power unit

You can use a different unit than Pin.

Power loss formula

A_dB = 10 · log₁₀(P_in/P_out)

If you know attenuation in dB, switch to “Output values from known dB attenuation”.

Formula used How to use

Example data table

These examples show common attenuation scenarios. Enter the same values above to verify.

Scenario	Inputs	Attenuation (dB)	Notes
Power ratio	Pin = 10 mW, Pout = 1 mW	10	10× drop in power equals 10 dB loss.
Voltage ratio	Vin = 1 V, Vout = 0.5 V	6.0206	Half voltage equals ~6.02 dB (matched impedance).
Coefficient & distance	α = 0.2 dB/km, L = 10 km	2	Good for cable/fiber segments with published specs.
Components	0.35 dB/km × 5 km + 2×0.5 + 4×0.1 + 1	4.15	Sum all dB terms to get total link loss.

Formula used

A_dB = 10 · log₁₀(P_in/P_out) for power measurements.
A_dB = 20 · log₁₀(V_in/V_out) for voltage (matched impedance).
A_dB = α · L when α is already in dB per length.
α_dB/length = 8.686 · α_Np/length to convert nepers to decibels.
P_out = P_in / 10^A_dB/10 and V_out = V_in / 10^A_dB/20.

Note: dB is logarithmic, so multiple losses add by simple summation.

How to use this calculator

Select a calculation method that matches your available data.
Enter inputs (power, voltage, coefficient, distance, or component losses).
Pick units and rounding, then press Calculate.
Review the result section that appears above the form.
Use Download CSV or Download PDF to save the details.

In-page article (no accordion)

1) What attenuation loss means in practice

Attenuation is the reduction of signal strength between an input and an output. In decibels (dB), it compares two levels on a logarithmic scale, which makes wide ranges easy to handle. For example, a 10 dB loss means the output power is 10× lower than the input power, while 3 dB is roughly a 2× power drop.

2) Power-based loss: the most direct measurement

When you measure transmitted and received power, use A_dB = 10·log₁₀(P_in/P_out). If P_in=10 mW and P_out=1 mW, the loss is 10 dB. If the output is 5 mW, the loss is 10·log₁₀(10/5)=3.0103 dB, a common “half-power” reference.

3) Voltage-based loss and the 20× rule

Voltage ratios use 20·log₁₀(V_in/V_out) because power is proportional to voltage squared when impedance is matched. A 2× voltage drop (1 V to 0.5 V) produces about 6.0206 dB. If impedances differ, convert to power first for accurate results.

4) Coefficient and distance for cables and fiber

Many media are specified with a coefficient such as 0.2 dB/km to 0.4 dB/km for low-loss optical fiber at common wavelengths. Total loss is simply A = α·L. For a 10 km run at 0.2 dB/km, the medium contributes 2 dB before connector or splice losses.

5) Nepers conversion when specs are not in dB

Some references use nepers (Np). Convert with 1 Np ≈ 8.686 dB. If a datasheet lists 0.02 Np/m, that is about 0.1737 dB/m. Over 30 m, the total becomes roughly 5.21 dB.

6) Building a realistic link budget with components

Real links include fixed losses. Typical field values might be connectors at 0.2–0.75 dB each and fusion splices around 0.05–0.1 dB. Add a margin (often 1–3 dB) for bends, aging, temperature, and measurement uncertainty. Because dB adds, budgeting stays simple even with many elements.

7) Interpreting results: ratios and “percent remaining”

This calculator reports dB plus linear ratios. Power remaining is 100·10^−A/10. With a 20 dB loss, only 1% of power remains. Voltage remaining is 100·10^−A/20; a 20 dB loss leaves about 10% voltage. Use these to sanity-check whether received levels meet system sensitivity.

8) Common checks before you trust the number

Confirm units (mW vs dBm), verify the measurement reference plane, and keep input/output values positive. For component mode, ensure length units match the coefficient (dB/km) and that connector/splice counts reflect the real path. If your computed loss is negative, you may be measuring gain rather than attenuation.

FAQs

1) What does a positive dB value indicate here?

A positive attenuation value means loss: the output level is lower than the input level. If you expect amplification, the same math describes gain, but this calculator labels the result as attenuation.

2) Why does power use 10·log10, but voltage uses 20·log10?

Power ratios use 10·log10 directly. Voltage ratios use 20·log10 because power is proportional to voltage squared when impedance is matched. If impedances differ, convert measurements to power first.

3) Can I mix Pin in dBm and Pout in mW?

Yes. Select the correct unit for each field. The calculator converts everything to a common internal reference before computing the ratio, so mixed units are handled safely.

4) How do I compute output power if I only know total dB loss?

Use the “Output values from known dB attenuation” mode. It applies Pout = Pin / 10^(A/10). Optionally, it also computes Vout = Vin / 10^(A/20) for matched impedance.

5) What is a reasonable loss per connector or splice?

Many designs assume connectors around 0.2–0.75 dB each and fusion splices around 0.05–0.1 dB. Your actual values depend on hardware quality, cleanliness, and installation practices.

6) Why add an extra margin in component calculations?

A margin covers bends, aging, temperature shifts, and measurement uncertainty. Even if the theoretical loss is low, a 1–3 dB margin helps keep the link reliable over time.

7) My result seems too high. What should I check first?

Recheck units and decimal placement, confirm length units, verify connector/splice counts, and ensure Pin and Pout are measured at the correct points. Also confirm you are not double-counting the same loss.