Insertion Loss Calculator (dB loss of a cable, filter or connector from power or voltage)

Understanding the insertion loss calculator — concept and principle

This calculator finds the insertion loss of a cable, filter, connector, attenuator or other component, in decibels, from readings of power or voltage at the load with and without the device in place. It also shows the share of power that gets through. RF technicians, audio engineers and fibre-optic installers use it daily.

Losing a fifth of the power is just under 1 dB; every 3 dB would halve it.

How it is calculated

Power readings: IL = 10 log₁₀(P_without ÷ P_with) dB

Voltage readings across the same load: IL = 20 log₁₀(V_without ÷ V_with) dB

Power reaching the load = P_with ÷ P_without × 100%

Both readings must use the same unit. A positive IL is a loss; a negative value means a gain.

The principle

Decibels express ratios on a logarithmic scale, so losses in a chain simply add. Power ratios use 10 log₁₀. Power across a fixed resistance is proportional to the voltage squared, so the same ratio expressed in voltages uses 20 log₁₀. Insertion loss compares the power delivered to the load with and without the device, using the same source and load, so it includes both the energy the device absorbs and the energy it reflects back towards the source.

Worked example

Default inputs: power readings, 10 mW without the device, 8 mW with it.

Ratio = 8 ÷ 10 = 0.8, so 80% of the power reaches the load and 20% is lost.
IL = 10 × log₁₀(10 ÷ 8) = 0.969 dB.
Equivalent voltage ratio = √0.8 = 0.8944.
The value is positive, so it causes a loss.

Each point is the calculator re-run with only “With the device inserted: power (mW) or voltage (V)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

With the device inserted: power (mW) or voltage (V)	Insertion loss (dB)
2	6.99
4	3.979
6	2.218
8	0.969
10	0
12	-0.792
16	-2.041

How to read the result

3 dB means about half the power gets through and 10 dB means a tenth. A good connector typically loses only a fraction of a decibel, while a long cable at high frequency can lose many decibels. Add the dB figures of components in series to get the total loss.

Common mistakes

Using 20 log for power readings or 10 log for voltages.
Measuring with different source or load conditions in the two readings.
Mixing W and mW between the two readings.
Confusing insertion loss with return loss, which measures reflection.

Keep in mind: Insertion loss depends on frequency and on the source and load impedances. Measure at the frequency and impedance where the device will be used.

Frequently asked questions

What does 1 dB of loss mean?

About 79% of the power gets through.

Can insertion loss be negative?

Yes, if the device amplifies, as with an active filter or amplifier.

How do I find the total loss of several parts?

Add their insertion losses in dB.

Sources: Standard definitions of the decibel (10 log₁₀ for power, 20 log₁₀ for voltage across equal resistances) and of insertion loss

Checked 9 Nov 2026. Definition of the decibel for power ratios (10 log₁₀) and for voltage ratios across equal resistances (20 log₁₀); insertion loss as the ratio of load power before and after insertion — standard definitions.

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