RC Filter Calculator

What this calculator does

The RC Filter Calculator provides fast estimates using standard electrical relationships. Enter Resistance (R), Capacitance (C) to compute results for real-world electrical scenarios. Use it for quick checks, comparisons, and documentation while planning.

Inputs explained

  • Resistance (R): Resistor value in Ohms (Ω).
  • Capacitance (C): Capacitor value in Farads (F), µF, nF, or pF.

How it works / Method

The calculator applies the listed formulas to the values you enter and then formats the result in standard units. It uses first-order filter relationships to relate component values to cutoff frequency or time constant.

Formulas used

  • τ = R x C
  • f c = 1 / (2πRC)
  • X C = 1 / (2πfC)

Units: R in ohms (ohm) C in farads (F)

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RC Filter Calculator

Low-pass & high-pass filter calculator

Ω
Result
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Formulas

ττ = R × C
fcfc = 1 / (2πRC)
XCXC = 1 / (2πfC)

Time Constant Response

Time Charge Discharge
1τ 63.2% 36.8%
2τ 86.5% 13.5%
3τ 95.0% 5.0%
5τ 99.3% 0.7%

Related

RL FilterOhm's LawVoltage Divider

Step-by-step example

Scenario: Design a low-pass filter with R=10kΩ and C=100nF.

Formula: fc = 1 / (2π x R x C)

  1. fc = 1 / (2π x 10000 x 0.0000001)
  2. fc = 1 / 0.00628
  3. fc = 159 Hz

Result: 159 Hz cutoff frequency

Use cases

  • Estimate cutoff frequency for simple RC or RL filters.
  • Compare how R, C, or L values shift the response.
  • Document first-order filter behavior for reports.
  • Create quick checks before deeper circuit analysis.

Assumptions & limitations

  • Results are estimates and may be rounded; small differences are expected.
  • Inputs assume steady-state values; transient and surge effects are not modeled.
  • Voltage can vary by supply, load, and location; use measured values when possible.
  • Power factor varies by load type; use a realistic value for inductive equipment.
  • Wiring rules and safety codes vary by jurisdiction; consult a licensed electrician for installation decisions.

Sources & references

Related calculators

Frequently Asked Questions

The RC Filter Calculator estimates first-order filter behavior such as cutoff frequency or time constant. It is designed for quick checks of simple RC or RL filters. You can compare how R, C, or L values shift the response. The results assume ideal components and steady-state sinusoidal signals. Use it for planning, then validate with measurements or simulation.

Cutoff frequency is the point where output is about 3 dB down from the passband level. For RC filters, f_c = 1/(2 pi R C). For RL filters, f_c = R/(2 pi L). These formulas assume a simple one-pole network and light loading. Use measured component values if accuracy matters. This calculator is intended for planning and comparison, not for final engineering approval.

Real components have tolerances and temperature drift. A 5% resistor and 10% capacitor can shift cutoff frequency noticeably. Inductors vary with core material and current. The calculator uses nominal values and does not include tolerance bands. Treat the output as an estimate and document tolerance limits. This calculator is intended for planning and comparison, not for final engineering approval. This calculator is intended for planning and comparison, not for final engineering approval.

Yes, source and load impedances can change the effective resistance. That shifts the cutoff frequency and changes the slope. Include known source or load impedance in the effective R value. If loading is significant, use a full circuit model. This calculator assumes light loading for simplicity. This calculator is intended for planning and comparison, not for final engineering approval. This calculator is intended for planning and comparison, not for final engineering approval.

This tool targets passive RC and RL filters. Active filters use op-amps and different transfer functions. Their cutoff frequency depends on the full topology and gain. Use this calculator for rough guidance only. Follow the specific active filter equations for final design. This calculator is intended for planning and comparison, not for final engineering approval. This calculator is intended for planning and comparison, not for final engineering approval.

Results assume ideal components and steady-state conditions. Parasitics and component variation shift real responses. High-frequency effects can be significant. Use the output for planning and comparison only. Validate with measurement or simulation for critical designs. This calculator is intended for planning and comparison, not for final engineering approval. This calculator is intended for planning and comparison, not for final engineering approval.