Q = fc/BW. Higher Q means a narrower bandwidth relative to the centre frequency. A Q of 10 means the bandwidth is 1/10th of fc. High-Q filters are very selective.

How do I build a bandpass filter?

Use a series RLC circuit (bandpass at resonance), cascade a high-pass and low-pass filter, or use an active filter with an op-amp. The RLC approach gives the highest Q for passive designs.

What is the centre frequency?

The geometric mean of the lower and upper cutoffs: fc = √(fL × fH). For narrow bandwidths, fc ≈ (fL + fH)/2 (arithmetic mean is close to geometric mean).

Band Pass Filter Calculator

This calculator requires JavaScript. f_c = √(f_L × f_H) | BW = f_H − f_L | Q = f_c/BW

f_c = √(f_L×f_H)

BW = f_H − f_L

Q = f_c/BW

Band Pass — Enter Lower and Upper Cutoff

f_L Lower Cutoff

f_H Upper Cutoff

Enter frequency limits or RLC values

Band Pass Results

Centre fc
—
Hz

Bandwidth

—

Q Factor

—

f_L (−3dB low)

—

f_H (−3dB high)

—

ω_c

—

rad/s

Figure 1: A band pass filter passes a range of frequencies between f_L and f_H, attenuating everything outside this band. The centre frequency f_c = √(f_L×f_H) and bandwidth BW = f_H − f_L.

Table of Contents

Fundamentals

What Is a Band Pass Filter?
How It Works

Worked Examples

Voice Band Filter (300-3400 Hz)
AM Radio Tuner

Deep Dive

Q Factor and Selectivity
Building a Bandpass from LP + HP

Reference

Frequently Asked Questions
Related Filter Calculators

What Is a Band Pass Filter?

A band pass filter passes a range of frequencies between a lower cutoff (f_L) and an upper cutoff (f_H), rejecting everything outside this band. It is used in radio receivers, audio equalisers, and communication systems to select a specific frequency range. The Filter Cutoff Frequency Calculator covers the fundamental cutoff frequency calculation that defines every band pass filter.

How It Works

Band Pass Filter: f_c = −3 dB point where output power drops to 50%.
Below f_c: passband | Above f_c: passband

The filter response is determined by the component values and circuit topology. First-order filters (single RC or RL) give −20 dB/decade roll-off. Second-order (LC or cascaded) give −40 dB/decade. The RC Filter Calculator shows how different component combinations affect the response.

Worked Example — Voice Band Filter (300-3400 Hz)

Given: R = 10 kΩ, C = 100 nF

f_c = 1/(2π × 10000 × 100×10⁻⁹) = 159.2 Hz

The passband extends around the centre frequency.

The RL Filter Calculator handles the individual stages.

Worked Example — AM Radio Tuner

Given: L = 10 mH, C = 1 µF

f_c = 1/(2π√(0.01 × 10⁻⁶)) = 1592 Hz

Q Factor and Selectivity

Q factor determines how selective the bandpass is. Higher Q = narrower bandwidth = more selective. The LC Filter Calculator handles the complementary filter type.

Building a Bandpass from LP + HP

The notch depth depends on how precisely the component values are matched. A deeper notch requires tighter tolerances. The Low Pass Filter Calculator covers the active version with op-amp gain.

Resonant Frequency Calculator: The Resonant Frequency Calculator calculates the resonant frequency that defines the centre of an RLC bandpass filter.

Frequently Asked Questions

What is the −3 dB point?

−3 dB is where the output power is half the input power (voltage is 70.7% of input). This is the universally agreed definition of cutoff frequency for all filter types.

Can I use this for audio?

Yes. Audio frequencies range from 20 Hz to 20 kHz. Choose component values that place f_c within this range for audio filtering applications.

What order filter do I need?

First-order (−20 dB/dec) is adequate for gentle roll-off. For sharper transitions, use second-order (−40 dB/dec) or higher. The Butterworth Filter Calculator designs multi-order responses.

Does the filter introduce delay?

Yes. All filters introduce phase shift and group delay. First-order filters have constant group delay (linear phase approximation). Higher-order filters can have significant group delay variation near fc.

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Last updated: March 2026