RC Time Constant Calculator
Calculate RC time constant τ, 5τ settling time, cutoff frequency and capacitor charge/discharge voltage for resistor-capacitor circuits.
⏱️ Timing Rule: One time constant is τ = R × C. A capacitor reaches about 63.2% in 1τ and is practically settled after about 5τ. Use with the 555 Timer Calculator, Reactance Calculator, Capacitor Code Calculator and LC Resonance Calculator.
⏱️ Resistor + Capacitor → Time Constant → Charge / Discharge Behavior
Resistance
Capacitance
Use Case
For a simple RC low-pass filter, cutoff frequency is fc = 1 / (2πRC).
Presets:
Final / Supply Voltage
Resistance
Capacitance
Time
Mode
Initial Voltage
Charging uses Vc(t)=Vf(1-e^-t/RC). Discharging uses Vc(t)=V0e^-t/RC.
Examples:
Target Time Constant
Solve For
Known Resistor
Known Capacitor
Preferred Series
Use this tab when you know one component and want the other one for a target delay or cutoff.
Targets:
📐 Formula Reference
Time Constant
τ = R × C
Cutoff Frequency
fc = 1 / (2πRC)
Charging Voltage
Vc(t) = Vf × (1 − e^(-t/RC))
Discharging Voltage
Vc(t) = V0 × e^(-t/RC)
What is an RC time constant calculator?
An RC time constant calculator finds how fast a capacitor charges or discharges through a resistor. It is useful for timing delays, reset circuits, debounce networks, filters and capacitor discharge estimates.
How long does a capacitor take to charge?
A capacitor reaches about 63.2% of the final voltage after 1τ, about 95% after 3τ and about 99.3% after 5τ. Real circuits may vary because of capacitor tolerance, leakage and source/load resistance.
RC low-pass cutoff frequency
The cutoff frequency of a simple RC low-pass filter is calculated using fc = 1 / (2πRC). Increasing either resistance or capacitance lowers the cutoff frequency.
❓ Frequently Asked Questions
One time constant means the capacitor has moved about 63.2% of the way toward its final value during charging, or dropped to about 36.8% during discharging.
After 5 time constants, a capacitor is over 99% charged or discharged, so it is treated as practically settled in many circuits.
Choose values whose product is 1 second, such as 10kΩ × 100µF, 100kΩ × 10µF or 1MΩ × 1µF. Check leakage and tolerance for long delays.
A 555 timer uses capacitor charging and discharging, but the timing formulas are different because the capacitor charges between fixed threshold levels.