Q Factor Calculator
Calculate the quality factor of a resonant circuit and the selectivity it provides.
About Q Factor Calculator
Q can be measured from the response shape or predicted from component values, and the two should agree. It expresses how much energy is stored relative to that lost each cycle, which is why low resistance gives high Q. High Q means sharp selectivity but also long ringing, so filter and antenna design always trades the two against each other.
Formula
Q Factor
Q = f₀ ÷ bandwidth = 2πf₀L ÷ R
- Q
- Ratio of energy stored to energy dissipated per cycle
- half-power frequencies
- Points where output falls 3 dB below the peak
Worked example
With the default values below, Q Factor Calculator returns Q From Frequency and Bandwidth of 20.0000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Resonant frequencyminimum 0.01 Hz
- 100,000 Hz
- Measured bandwidthminimum 0 Hz
- 5,000 Hz
- Series resistanceminimum 0 ohm
- 5 ohm
- Inductanceminimum 0 H
- 0.0001 H
Results
- Q From Frequency and Bandwidth
- 20.0000
- Q From Component Values
- 12.5664
- Bandwidth Implied by the Component Q
- 7,957.747 Hz
- Upper Half-Power Frequency
- 102,500.000 Hz
- Lower Half-Power Frequency
- 97,500.000 Hz
Step by step
- 1Values used
- 2Q Factor
- 3Q From Frequency and Bandwidth= 20.0000
- 4Q From Component Values= 12.5664
- 5Bandwidth Implied by the Component Q= 7,957.747 Hz
- 6Upper Half-Power Frequency= 102,500.000 Hz
- 7Lower Half-Power Frequency= 97,500.000 Hz
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Q Factor calculated?
Q = f₀ ÷ bandwidth = 2πf₀L ÷ R Q can be measured from the response shape or predicted from component values, and the two should agree. It expresses how much energy is stored relative to that lost each cycle, which is why low resistance gives high Q.
Why does Q Factor matter?
High Q means sharp selectivity but also long ringing, so filter and antenna design always trades the two against each other.
What values do I need to enter?
This calculator takes 4 inputs: Resonant frequency, Measured bandwidth, Series resistance, Inductance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.