RC Circuit Calculator
Use this RC circuit calculator to compute the characteristic frequency of the RC circuit. You can also use it as a capacitor charge time calculator (RC time constant calculator) or an RC filter calculator. Depending on your need, the RC circuit can serve as a low pass filter or high pass filter.
What is an RC Circuit?
The RC circuit is a fundamental electrical circuit in which a resistor of resistance R is connected in series with a capacitor of capacitance C. Such a circuit is characterized by a time constant τ (tau) and a cutoff frequency fc, and has two primary applications:
- Low or high pass filter — suppresses signals below or above the cutoff frequency
- Energy storage — the capacitor stores and releases electrical energy
RC Time Constant (τ)
The time constant τ = R × C defines how quickly the capacitor charges or discharges. After one time constant, the capacitor charges to approximately 63.2% of the source voltage. After five time constants (5τ), the capacitor is considered fully charged (~99.3%).
| Time | Charge Level |
|---|---|
| 1τ | 63.2% |
| 2τ | 86.5% |
| 3τ | 95.0% |
| 4τ | 98.2% |
| 5τ | 99.3% |
RC Filter Calculator
The cutoff (characteristic) frequency fc = 1 / (2π × R × C) determines which signals can pass through the circuit:
- Low-pass filter: Signals below fc pass freely; signals above fc are suppressed
- High-pass filter: Signals above fc pass freely; signals below fc are attenuated
- Signals near fc are partially transmitted (the transition is gradual, not sharp)
Combining a low-pass and high-pass filter creates a bandpass filter — widely used in audio equipment, radio receivers, and signal processing.
Formulas
- Time constant: τ = R × C [seconds]
- Cutoff frequency: fc = 1 / (2π × τ) [Hz]
- Capacitor voltage at time t: V(t) = Vs × (1 − e−t/τ)
Units
This calculator supports both metric (SI) and American unit systems. Resistance can be entered in Ω, kΩ, or MΩ. Capacitance can be entered in F, mF, µF, nF, or pF. Results are automatically scaled to the most appropriate unit prefix.
Typical RC Circuit Values
- Audio low-pass filter: R = 10 kΩ, C = 1.6 µF → fc ≈ 10 Hz
- Audio high-pass filter: R = 10 kΩ, C = 16 nF → fc ≈ 1 kHz
- Power supply decoupling: R = 100 Ω, C = 100 µF → τ = 10 ms
- RF filter: R = 50 Ω, C = 100 pF → fc ≈ 32 MHz