RC Circuit Calculator

Calculate an RC time constant, capacitor charging and discharging voltage, and charging current from resistance, capacitance, voltage, and time.

RC charging and discharging

Capacitance is entered in microfarads and converted internally to farads.

Resistance (ohms)

Capacitance (µF)

Source or initial voltage (V)

Elapsed time (s)

About RC circuits

An RC circuit combines a resistor and capacitor, producing a response that changes exponentially rather than instantly. Its defining quantity is the time constant tau = RC, where resistance is measured in ohms and capacitance in farads. One time constant is the time required for a charging capacitor to reach about 63.2 percent of its final voltage, or for a discharging capacitor to fall to about 36.8 percent of its initial voltage. This calculator accepts capacitance in microfarads and applies the required million-to-one conversion automatically. For charging from an ideal DC source, capacitor voltage is Vc = Vs(1 - exp(-t/RC)). Current starts at Vs/R and decays as I = (Vs/R)exp(-t/RC). During discharge from an initial voltage V0, capacitor voltage follows Vc = V0 exp(-t/RC). The displayed source or initial voltage is therefore used as Vs for the charging values and V0 for the discharge value. These equations assume the capacitor begins fully discharged in the charging case and starts at the entered voltage in the discharging case. Time constants make practical circuit behavior easy to estimate. After one time constant a capacitor is 63.2 percent charged; after two it is 86.5 percent charged; after three it is 95 percent charged; and after five it is above 99 percent charged. Engineers commonly treat five time constants as effectively complete. The same milestones work in reverse for discharge, leaving about 36.8, 13.5, 5, and less than 1 percent respectively. RC networks appear in timing circuits, switch debouncing, sensor conditioning, coupling stages, envelope detectors, and first-order filters. For a basic low-pass or high-pass filter, the cutoff frequency is 1/(2 pi RC), so the time constant also controls frequency response. Increasing either resistance or capacitance slows the transient and lowers the cutoff frequency. Decreasing either value makes the circuit respond faster. Real capacitors have tolerance, leakage, equivalent series resistance, and voltage limits, while real sources and loads add resistance that can change the effective time constant. Electrolytic capacitors may vary substantially from their marked capacitance. Use measured component values and include source or load resistance when accuracy matters. The calculator is ideal for education, breadboard planning, component selection, and quick transient checks, but critical designs should also be simulated and verified under worst-case tolerances.

RC circuit examples

These examples use ideal components and a capacitor that starts fully discharged for charging.

Inputs	Key result	Interpretation
1 kΩ, 100 µF, 5 V, 0.1 s	τ = 0.1 s; Vc = 3.1606 V	At one time constant the capacitor reaches 63.2 percent of the source.
10 kΩ, 10 µF, 12 V, 0.2 s	τ = 0.1 s; Vc = 10.376 V	After two time constants charging is about 86.5 percent complete.
2.2 kΩ, 47 µF, 9 V, 0.5 s	τ = 0.1034 s; discharge = 0.0714 V	Nearly five time constants leave less than one percent of the initial voltage.

How to calculate RC behavior

Enter the resistance in ohms and the capacitance in microfarads.
Enter the DC source voltage for charging or initial capacitor voltage for discharging.
Enter the elapsed time in seconds from the switching event.
Select Calculate RC circuit and compare the time constant, voltage, and current results.

RC circuit FAQ

What is an RC time constant?

The time constant is resistance multiplied by capacitance in consistent SI units. It describes the speed of the exponential charging or discharging response.

Why does charging never mathematically reach 100 percent?

The exponential difference from the final voltage continually becomes smaller but never equals zero in the ideal equation. In practice, five time constants is usually treated as fully charged.

Can I enter capacitance in farads?

This calculator expects microfarads, so a value in farads must be multiplied by one million before entry. For example, 0.001 F is entered as 1000 µF.

Does the resistor affect the final capacitor voltage?

In an unloaded ideal DC circuit, resistance changes how quickly the final voltage is reached, not the final voltage itself. A connected load can create a divider and change that final value.

Are real capacitor tolerances included?

No, the result uses the exact values entered and models ideal components. Evaluate minimum and maximum capacitance and resistance values when tolerance matters.