Frequency Bandwidth Calculator — BW, Q Factor & Centre Frequency
Enter the lower and upper cutoff frequencies to get bandwidth, centre frequency, Q factor and fractional bandwidth — for filters, antennas and resonant circuits.
In the same unit as the input frequencies (Hz / kHz / MHz / GHz)
- 1
Identify lower and upper cutoffs
f_low = 88, f_high = 108The smaller frequency becomes the lower cutoff regardless of input order. - 2
Bandwidth
108 − 88 = 20
How does this calculator work?
BW = f₂ − f₁; fc = (f₁ + f₂) / 2; Q = fc / BW (high Q = narrow band); FBW = BW / fc × 100%. Enter both frequencies in the same unit — all outputs match. Q and FBW are dimensionless. Applies to filters, antennas, resonant circuits and any two-cutoff-frequency system.
Formula
How this is calculated
Bandwidth is the simplest of the four results: the absolute difference between the upper cutoff frequency f₂ and the lower cutoff frequency f₁. In filter and amplifier specifications the cutoff frequencies are commonly defined as the −3 dB points — the frequencies at which output power drops to half its peak value, equivalent to a voltage ratio of 1/√2. The bandwidth tells you how wide a frequency range is passed.
The centre frequency fc = (f₁ + f₂) / 2 is the arithmetic midpoint of the pass-band. For a geometrically symmetric filter the geometric mean √(f₁ × f₂) would be more precise, but for narrow bandwidths (FBW < 20%) the two are nearly identical. The Q factor — quality factor — is fc / BW: a high Q means a narrow, selective pass-band relative to the centre frequency; a low Q means broad and less selective. Fractional bandwidth (FBW) expresses the same idea as a percentage of the centre frequency.
This calculator is unit-agnostic: enter both frequencies in the same unit (Hz, kHz, MHz or GHz) and all outputs (BW, fc) are in that same unit. Q factor and fractional bandwidth are dimensionless ratios. The same formulas apply to audio filters, RF pass-bands, antenna bandwidths, and resonant LC circuits — any system characterised by two cutoff or half-power frequencies.
Frequently asked questions
The −3 dB points are the frequencies at which the output power falls to half the peak value (−3 dB ≈ ×0.5 in power; ×0.707 in voltage/current). They define the edges of the usable pass-band for most filters and amplifiers. Enter those two frequencies in this calculator to get the −3 dB bandwidth.
It depends on the application. AM broadcast receivers (bandwidth ~10 kHz at a 1 MHz carrier) have Q ≈ 100 — very selective. Wideband audio equalisers have Q values of 0.5–10. Broadband power-amplifier matching networks may have Q < 1. Higher Q = narrower bandwidth = sharper filtering. For resonant LC circuits Q is also determined by component losses.
Use the arithmetic centre fc = (f₁ + f₂) / 2 for narrow-band systems (FBW < ~20%) — the two expressions give nearly the same result. Use the geometric centre fc = √(f₁ × f₂) for wideband systems where the ratio f₂/f₁ is large (e.g. a filter spanning 1–10 MHz). This calculator uses the arithmetic form, which is the most common engineering convention.
Also known as
TG we-Calculate Editorial Team. (2026). Frequency Bandwidth Calculator — BW, Q Factor & Centre Frequency [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/frequency-bandwidth-calculator
TG we-Calculate Editorial Team. "Frequency Bandwidth Calculator — BW, Q Factor & Centre Frequency." TG we-Calculate. 2026. https://we-calculate.com/calculator/frequency-bandwidth-calculator.
TG we-Calculate Editorial Team, "Frequency Bandwidth Calculator — BW, Q Factor & Centre Frequency," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/frequency-bandwidth-calculator
@misc{wecalculate_frequency_bandwidth_calculator, title = {Frequency Bandwidth Calculator — BW, Q Factor & Centre Frequency}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/frequency-bandwidth-calculator}}, year = {2026}, note = {TG we-Calculate} }
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