Calculate RC time constant, cutoff frequency, charge and discharge checkpoints, and series impedance from resistance and capacitance, with optional voltage and frequency analysis.
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Basic circuits
An RC circuit calculator helps translate one resistor-capacitor pair into timing, filter, and impedance behaviour you can act on. This version solves the RC time constant, cutoff frequency, charge and discharge progress at a chosen time, and series impedance at a chosen frequency while keeping the classic 1τ, 3τ, and 5τ checkpoints visible.
This page treats the network as one series resistor-capacitor pair and calculates the time constant, cutoff frequency, capacitive reactance, series impedance, and exponential charge-discharge progress.
An optional supply voltage converts the normalized timing result into actual charge voltage, discharge voltage, current, and stored-energy context.
The RC time constant tells you how quickly the network moves through its exponential charge and discharge response. One time constant means the charge trace has reached about 63.2% of final value and the discharge trace has fallen to about 36.8% of its starting value.
Those checkpoints are useful because they let you estimate timing without plotting the full curve every time.
τ = R × C
Resistance multiplied by capacitance gives the characteristic timing constant of the RC pair.
Charge fraction = 1 - e^(-t/τ); discharge fraction = e^(-t/τ)
These exponential forms determine the charge and discharge percentage at any stated time.
The same R and C values that set timing also define the cutoff frequency and the capacitor's reactance at a chosen analysis frequency. As frequency rises, capacitive reactance falls and the impedance of the series RC pair changes accordingly.
Using cutoff frequency as the default analysis point gives a quick first-pass view even before you enter a custom frequency.
fc = 1 / (2πRC)
The RC pair's cutoff frequency is the reciprocal of 2πRC.
Xc = 1 / (2πfC); |Z| = √(R² + Xc²)
Capacitive reactance and series impedance describe the same pair in the frequency domain.
This calculator is for one ideal series RC pair. It does not model source impedance, diode paths, component tolerances, dielectric absorption, ESR, or more complex filter topologies.
Use it as a planning and educational reference. If the real circuit includes multiple poles, switching behaviour, or significant parasitics, move to the model that captures those effects explicitly.
Frequently asked questions
At one τ, the charging capacitor has reached about 63.2% of its final voltage and a discharging capacitor has about 36.8% of its original voltage left.
Because cutoff is a natural first analysis point for the same RC pair. It provides a useful default impedance and gain check without forcing you to pick a custom frequency first.
Yes. This page models one resistor and one capacitor as an ideal series RC network. More complex RC ladders, filters, or switched circuits need a different model.
Related
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