RC Circuit Time Constant Calculator

Analyze charging and discharging behaviors in Resistor-Capacitor (RC) circuits with our advanced RC Time Constant Calculator. Determine the exact circuit time constant (τ), graph exponential voltage curves over time, and pinpoint voltage and charge status at any custom millisecond marker. Perfect for physics students simulating transient responses across RC filters and timing circuits.

Please fill out Resistance, Capacitance, and Voltage limits clearly.

Resistance (R)

Capacitance (C)

Supply Voltage (V₀)

Time Constant (τ)

0 s

63.2% Charge Time

0 s

1τ elapsed

99.3% Fully Charged

0 s

5τ elapsed

Charging Phase

Discharging Phase

Evaluate at Custom Time (t)

V(t) = 0 V

The Importance of the RC Time Constant

In direct current (DC) electronics, capacitors take a certain amount of time to fully charge up or discharge when exposed to a fixed voltage. This timeline is mathematically dictated by the product of the circuit’s total resistance and total capacitance, represented by the Greek letter tau (τ = R × C).

Universal RC Charging Rules

At 1τ (One Time Constant): A charging capacitor will predictably reach 63.2% of its supply voltage. Conversely, a discharging capacitor will have dropped down to 36.8% of its initial voltage.
At 5τ (Five Time Constants): A capacitor is considered “fully charged” or “fully discharged” for practical engineering purposes, residing at 99.3% completion. Due to the nature of exponential decay curves, mathematically it never reaches 100%, but 5τ is the accepted industry standard.