Low pass / High pass / Band pass Filter Calculator

RC / RLC Filter Calculator (LP • HP • BP)

RC / RLC Filter Calculator (Low-Pass • High-Pass • Band-Pass)

CE Style

Inputs

RC: cutoff; RLC: resonant frequency
Used for gain/phase readout

Key Results

Gain / Phase at ftest

RC LP: H(s)=1/(1+sRC), HP: H(s)=sRC/(1+sRC). RLC BP (series, Vout across R): H(jω)=R/(R+j(ωL−1/(ωC))).

How to Use the RC / RLC Filter Calculator

Design and analyze simple first-order RC low-/high-pass and a series RLC band-pass (Vout across R). Enter values directly or solve from target cutoff / resonance.

1) Pick Filter Type

  • RC Low-Pass (Vout across C).
  • RC High-Pass (Vout across R).
  • RLC Band-Pass (series R-L-C, Vout across R).
For LP/HP the topology is first-order and uses the same cutoff formula; the output node changes.

2) Choose Input Mode

  • RC: enter R & C, or give fc + R (solve C), or fc + C (solve R).
  • RLC: enter R, L, C directly, or give f₀, Q + C (solve R & L), or f₀, Q + L (solve R & C).

3) Enter Values

  • Pick units for R, L, C and frequency.
  • For RC, τ = R·C, fc = 1/(2πRC).
  • For RLC, f₀ = 1/(2π√(LC)), Q = f₀/BW.
Standard E-series parts have tolerances; the results reflect the ideal values.

4) Set Test Frequency

  • Use a convenient ftest to read |H(jω)| and phase.
  • Tip: set ftest = fc (RC) or f₀ (RLC) to see the mid-point behavior.

5) Read Results

  • RC: τ, fc, solved R or C if you chose a target fc.
  • RLC: f₀, Q, bandwidth, and solved component values.

6) Gain & Phase

  • LP: |H| = 1/√(1+(ωRC)²), phase = −tan⁻¹(ωRC).
  • HP: |H| = (ωRC)/√(1+(ωRC)²), phase ≈ tan⁻¹(ωRC) − 90°.
  • BP: |H| = R/√(R²+(ωL−1/ωC)²), phase = −tan⁻¹((ωL−1/ωC)/R).

Quick Checklist

  • Correct filter type & input mode selected
  • Units set for R / L / C / frequency
  • ftest chosen near fc or f₀ for quick readout
  • Account for source/load impedance; first-order formulas assume high-Z load and low-Z source
  • Use E-series parts and re-check targets after rounding
  • For steeper slopes, cascade sections or use active filters
FAQ & Tips

Why does my measured fc differ?
Real sources and loads shift the effective R or C. Buffer the stage or include source/load in your R value.

What Q should I use for BP?
Q ≈ 0.707 for gentle response; Q from 2–10 for narrow band. Higher Q increases peaking and sensitivity to tolerance.

Capacitor type?
Use C0G/NP0 for stable small values; film or tantalum/electrolytic where larger values are required.

How to get a high cutoff with small capacitors?
Reduce R and keep C modest; very small C may be dominated by parasitics.

Copy-Paste Mini Workflow

1) Select filter: LP, HP, or BP (series RLC)
2) Pick an input mode (direct parts or target f_c / f0 with helper)
3) Enter values and units; set a test frequency
4) Read τ, f_c or f0, Q/BW, and |H|/phase at f_test
5) Round to E-series parts and re-check targets