RC Time Constant Calculator

Calculate RC charging and discharging curves, time constants, and voltage at any given time for resistor-capacitor circuits. Get step-by-step formulas and a complete time constant table.

Free to useNo sign-up requiredUpdated Mar 2026

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About RC Time Constant Calculator

The RC Time Constant Calculator computes the time constant (τ) for a resistor-capacitor (RC) circuit and provides a complete analysis of its charging or discharging behavior. Enter your resistance and capacitance values along with the supply voltage to get the time constant, voltage curve chart, time constant table showing voltages at 1τ through 5τ, cutoff frequency, and step-by-step calculation with MathJax formulas.

What Is an RC Time Constant?

The RC time constant, denoted by the Greek letter τ (tau), is the product of the resistance (R) and capacitance (C) in a circuit:

$$\tau = R \times C$$

It is measured in seconds and represents a fundamental timing property of the circuit. After one time constant, a charging capacitor reaches approximately 63.2% of the supply voltage, while a discharging capacitor drops to approximately 36.8% of its initial voltage.

Charging and Discharging Formulas

Charging: When a voltage source is applied to an uncharged capacitor through a resistor, the voltage across the capacitor rises exponentially:

$$V(t) = V_{supply} \times \left(1 - e^{-t/\tau}\right)$$

Discharging: When a charged capacitor is disconnected from the voltage source and connected through a resistor, the voltage decays exponentially:

$$V(t) = V_{initial} \times e^{-t/\tau}$$

Key Time Points

1τ — 63.2% charged / 36.8% remaining
2τ — 86.5% charged / 13.5% remaining
3τ — 95.0% charged / 5.0% remaining
4τ — 98.2% charged / 1.8% remaining
5τ — 99.3% charged / 0.7% remaining (considered fully charged/discharged)

RC Filter Cutoff Frequency

The same RC combination forms a basic low-pass or high-pass filter. The cutoff frequency (−3 dB point) is:

$$f_c = \frac{1}{2\pi \tau} = \frac{1}{2\pi R C}$$

At this frequency, the output signal amplitude drops to 70.7% of the input signal.

How to Use This Calculator

Enter resistance — Input the resistance value and select the appropriate unit (Ω, kΩ, or MΩ).
Enter capacitance — Input the capacitance value and select the unit (F, mF, µF, nF, or pF).
Enter supply voltage — Enter the supply voltage (for charging) or initial capacitor voltage (for discharging).
Select mode — Choose between Charging and Discharging.
Optional: Enter time point — Enter a specific time (in seconds) to calculate the exact voltage at that moment.
Click Calculate — Review the time constant, voltage curve, time constant table, and step-by-step formulas.

Practical Applications

Timer circuits — RC timing is the basis of 555 timer circuits and many delay circuits
Filter design — RC low-pass and high-pass filters are fundamental building blocks in signal processing
Debouncing circuits — RC circuits smooth out mechanical switch bounce in digital electronics
Power supply smoothing — Capacitors in power supplies use RC charging/discharging to reduce ripple
Sensor circuits — Many sensors (e.g., touch sensors) rely on RC time constant measurement

FAQ

What is an RC time constant?

The RC time constant (τ = R × C) is measured in seconds and represents the time for a capacitor to charge to about 63.2% of the supply voltage, or discharge to about 36.8% of its initial voltage. It is the fundamental timing parameter of any resistor-capacitor circuit.

How long does it take to fully charge an RC circuit?

A capacitor is considered fully charged after 5 time constants (5τ), reaching 99.3% of the supply voltage. For many practical applications, 3τ (95%) is sufficient. The exact time depends on the resistance and capacitance values in your circuit.

What is the charging formula for an RC circuit?

The charging voltage is V(t) = V_supply × (1 − e^−t/τ), where V_supply is the source voltage, t is elapsed time, and τ = RC is the time constant. The voltage rises exponentially from 0 toward V_supply.

What is the discharging formula for an RC circuit?

The discharging voltage is V(t) = V_initial × e^−t/τ, where V_initial is the starting voltage and τ = RC is the time constant. The voltage decays exponentially from V_initial toward 0.

How is the RC time constant related to filter cutoff frequency?

The cutoff frequency of a simple RC filter is f_c = 1/(2πRC) = 1/(2πτ). At this frequency, the output signal is attenuated to 70.7% (−3 dB) of the input. This relationship makes RC circuits essential in analog filter design.

Reference this content, page, or tool as:

"RC Time Constant Calculator" at https://MiniWebtool.com/rc-time-constant-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: Mar 18, 2026

Developer API available: Run this tool from your app, automation, or agent with one JSON HTTP request. View API docs

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