A converter for one of the most confusing inconsistencies in audio software: every parametric EQ shapes the same kind of bell filter, but they label its width as Q, as bandwidth in octaves, or as hertz. This tool translates cleanly between Q and octave bandwidth so you can copy an EQ move from one plugin into another, and optionally shows the hertz width at a given centre frequency.
How it works
Q and bandwidth in octaves are tied together by a single, frequency-independent formula. If N is the bandwidth in octaves, then:
Q = √(2^N) / (2^N − 1)
and inverting it to get octaves from Q:
N = log2( (2Q² + 1 + √((2Q² + 1)² − 4Q⁴)) / (2Q²) )
Both forms describe the band’s width between its -3 dB points. Because they are ratios, they hold at any centre frequency. To get the absolute width in hertz you add the centre frequency:
Bandwidth (Hz) = centre frequency ÷ Q
Worked example
You like a 1-octave-wide cut in one EQ but your other plugin only shows Q. Enter
1 octave and the tool returns Q ≈ 1.41. Dial 1.41 into the second plugin
and the bell width matches. At a 1 kHz centre that band is 1000 ÷ 1.41 ≈
709 Hz wide between its -3 dB edges.
Going the other way, a surgical Q = 8 notch comes out to about 0.18 octaves wide — narrow enough to remove a single resonant ring without disturbing neighbouring notes.
Quick reference
| Bandwidth (octaves) | Q (approx) |
|---|---|
| 3.0 | 0.404 |
| 2.0 | 0.667 |
| 1.0 | 1.41 |
| 0.5 | 2.87 |
| 1/3 (graphic EQ) | 4.32 |
| 0.1 | 14.4 |
The pattern is clear: wider bands have lower Q, narrower bands have higher Q. Use a low Q for broad, musical tone-shaping and a high Q to surgically target a single resonance or feedback frequency.
Every calculation runs locally in your browser; nothing is uploaded.