Physics

Instant, private, and free

RC Circuit Calculator.

Calculate the time constant and cutoff frequency of a first-order RC circuit.

On-device calculationNo signup
01

Set your values

Results update as you type.

Time constant: 0.001 s

Time constant

0.000000s
Cutoff frequency: 159.154943 Hz

Cutoff frequency

0.000000Hz

Results update automatically as you type.

Use Cases

Designing RC filters

Determine the cutoff frequency for low-pass or high-pass filters in audio and signal processing circuits.

Example: Set R=1 kΩ and C=100 nF to get a cutoff frequency of about 1.59 kHz.

Analyzing transient response

Estimate how quickly a capacitor charges or discharges in timing circuits, such as in oscillators or delay circuits.

Example: With R=10 kΩ and C=10 µF, the time constant is 0.1 seconds.

Frequently Asked Questions

What is the time constant in an RC circuit?
The time constant (τ) is the time required for the voltage across the capacitor to charge or discharge to approximately 63.2% of its final value. It is calculated as the product of resistance (R) and capacitance (C): τ = R × C.
How is the cutoff frequency calculated?
The cutoff frequency (f_c) is the frequency at which the output power drops to half its maximum, corresponding to -3 dB. It is calculated as f_c = 1 / (2π × R × C).
What units should I use for resistance and capacitance?
Enter resistance in ohms (Ω) and capacitance in farads (F). The calculator will compute the time constant in seconds and the cutoff frequency in hertz (Hz).

Tips & Common Mistakes

Tips

  • Use standard resistor and capacitor values to achieve a desired time constant or cutoff frequency.
  • For accurate results, ensure resistance is in ohms and capacitance in farads; convert units if necessary.
  • Remember that the time constant equals R×C, and the cutoff frequency is 1/(2πRC).
  • Check your circuit's tolerance: real components have ±5% or ±10% tolerance, affecting the actual values.

Common Mistakes to Avoid

  • Forgetting to convert units, e.g., using microfarads instead of farads, which leads to incorrect results.
  • Confusing time constant with cutoff frequency; they are inversely related but not the same.
  • Assuming the calculator accounts for load or source resistance; it only uses the given R and C values.

Last updated: August 13, 2026