RC Circuit Calculator
Calculate the time constant of a resistor-capacitor (RC) circuit and see how the voltage charges or discharges over time.
What is an RC circuit?
An RC circuit contains a resistor (R) and a capacitor (C) connected together. When a voltage is applied, the capacitor does not charge instantly — it charges (or discharges) gradually, following an exponential curve.
The time constant formula
The key value is the time constant, τ = R × C, measured in seconds when R is in ohms and C is in farads. After one time constant, a charging capacitor reaches about 63.2% of the supply voltage; after five time constants it is considered essentially fully charged (or discharged).
The voltage at any moment follows V(t) = V0(1 – e-t/τ) when charging, or V(t) = V0 · e-t/τ when discharging. This calculator applies that formula directly using the resistance, capacitance, and elapsed time you enter.
This tool is for general educational and planning purposes; real components have tolerances that affect exact timing.