Band-Pass Filter Calculator

This band-pass filter calculator works out the centre frequency, bandwidth and quality factor of a filter from its two cutoff frequencies. A band-pass filter is a circuit that passes a range of frequencies while blocking everything above and below it, and you will find one in radios, audio gear, instrumentation and communications equipment. You enter the lower cutoff frequency f1 and the upper cutoff frequency f2 in hertz, the minus 3 dB points where the filter's output power has dropped to half. From these the calculator returns the centre frequency f0, the geometric mean of the two cutoffs, the bandwidth, which is simply f2 minus f1, and the quality factor Q, the centre frequency divided by the bandwidth. Q shows how narrow and selective the passband is: a high Q means a tightly focused band around the centre frequency, while a low Q describes a broad, forgiving filter. Change either cutoff and the three results recalculate straight away, so you can see how tightening or widening the band affects selectivity. It suits electronics students checking their working, hobbyists designing filter circuits, and engineers running a quick sanity check before detailed simulation. The maths assumes an ideal filter defined purely by its stated minus 3 dB cutoffs, and results are rounded for display, so treat the figures as a design estimate rather than a substitute for measured or simulated performance.

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