A component-value calculator for DIY speaker builders, hi-fi tinkerers and car-audio installers designing passive crossover networks. Enter the crossover frequency, driver impedance and slope, and get the exact capacitor and inductor values.
How it works
A passive crossover uses reactive components — capacitors and inductors — whose impedance changes with frequency to route highs to the tweeter and lows to the woofer.
For a first-order filter (6 dB/octave):
high-pass capacitor: C = 1 / (2π · fc · R)
low-pass inductor: L = R / (2π · fc)where fc is the crossover frequency and R is the driver impedance. At 3000 Hz into 8 Ω the high-pass cap is about 6.6 µF and the low-pass inductor about 0.42 mH.
Second- and third-order filters add components and follow standard Butterworth alignment tables (for example the 2nd-order coefficients C = 0.1125 / (R · fc) and L = 0.2251 · R / fc). These give a maximally flat summed response when the two sections cross.
Worked example: 3-way speaker system
Suppose you are building a 3-way speaker with a woofer, midrange, and tweeter, all nominally 8 Ω. You choose a low crossover at 400 Hz (woofer to midrange) and a high crossover at 3,500 Hz (midrange to tweeter), using 2nd-order Butterworth filters throughout.
Low crossover at 400 Hz, 8 Ω:
- High-pass cap for midrange: approximately
C = 0.1125 / (8 × 400) = 35.2 µF - Low-pass inductor for woofer: approximately
L = 0.2251 × 8 / 400 = 4.5 mH
High crossover at 3,500 Hz, 8 Ω:
- High-pass cap for tweeter: approximately
C = 0.1125 / (8 × 3500) = 4.0 µF - Low-pass inductor for midrange: approximately
L = 0.2251 × 8 / 3500 = 0.51 mH
Round each value to the nearest standard component. For the 35.2 µF woofer cap, combining a 33 µF and a 2.2 µF in parallel (35.2 µF) hits the target precisely.
Choosing a slope
| Order | Slope | Trade-off |
|---|---|---|
| 1st | 6 dB/oct | Simplest, minimal phase shift, but lots of overlap between drivers |
| 2nd | 12 dB/oct | Common choice; good driver protection; introduces 180° phase shift |
| 3rd | 18 dB/oct | Steep rolloff; excellent isolation; more parts and greater phase rotation |
For home hi-fi builds, 2nd-order Butterworth is the most commonly chosen topology because it balances rolloff steepness against parts count and phase behaviour. Car-audio installers often use 1st-order high-pass filters on tweeters to minimise phase issues in acoustic reflections off glass and metal.
Practical tips for real builds
Impedance is not flat: a nominal 8 Ω tweeter may measure 12–20 Ω near resonance and rise above 20 Ω at very high frequencies. A Zobel network — a resistor and capacitor in series — placed across the driver flattens the impedance curve so the crossover behaves predictably.
Inductor core matters: air-core inductors avoid magnetic saturation and keep distortion low, but they need thicker wire for low DCR on woofer sections. Ferrite-core inductors are compact but saturate at high current, introducing harmonic distortion on peaks.
Standard component values: capacitors are typically available in the E6 or E12 series. Combine values in parallel (adds) or series (use the reciprocal formula) to hit precise targets. Every calculation runs locally in your browser; nothing is sent to any server.