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Select a mode
Choose time constant, charging voltage, discharging voltage, or cutoff frequency.
Circuit response workspace
RC Time Constant Calculator
Calculate RC time constant, capacitor charging voltage, discharging voltage, cutoff frequency, and circuit response time with automatic electrical unit conversion.
Instant response time
Charge and discharge
Cutoff frequency
RC Circuit Workbench
Select a mode and enter the known resistance, capacitance, voltage, or time values.
Circuit timing calculation
RC Time Constant
Calculate how quickly a capacitor charges or discharges through a resistor.
τ = RC
How it works
Calculate an RC Circuit in Four Steps
The calculator converts each value to its base SI unit, applies the correct RC formula, and displays practical engineering results.
02
Enter circuit values
Add resistance, capacitance, voltage, and elapsed time where required.
03
Select the units
Use ohms, kilohms, microfarads, nanofarads, milliseconds, and more.
04
Review the result
See the main answer, related response values, and complete formula trace.
RC circuit fundamentals
Understanding the RC Time Constant
An RC circuit contains a resistor and capacitor connected together. The resistor controls how quickly current flows, while the capacitor stores electrical charge.
The time constant, represented by the Greek letter tau, indicates how quickly the capacitor responds. One time constant equals resistance multiplied by capacitance.
Charging after one time constant
The capacitor reaches approximately 63.2% of
the supply voltage after one time constant.
Discharging after one time constant
The capacitor voltage falls to approximately
36.8% of its initial value.
Nearly complete after five time constants
Charging reaches about 99.3%, while discharging
falls to about 0.7%.
RC Circuit Formulas
Time constant
τ = RC
Resistance is measured in ohms and capacitance in farads.
Charging voltage
V(t) = Vs(1 − e−t/RC)
Calculates capacitor voltage while charging from zero.
Discharging voltage
V(t) = V0e−t/RC
Calculates the remaining capacitor voltage during discharge.
Cutoff frequency
fc = 1 ÷ 2πRC
Used for first-order RC low-pass and high-pass filters.
Practical uses
Where RC Calculations Are Useful
RC time calculations are used in timing circuits, filters, sensors, power controls, and electronic design.
Timing Circuits
Estimate delays, pulse lengths, and capacitor response times.
Audio Filters
Calculate the cutoff frequency of low-pass and high-pass filters.
Electronic Design
Select suitable resistor and capacitor values for circuit behavior.
Power-On Delays
Estimate startup delays for relays, transistors, and control circuits.
Testing and Repair
Compare measured capacitor response with expected circuit behavior.
Electrical Learning
Understand exponential charging, discharging, and filter response.
Frequently asked questions
RC Time Constant Calculator FAQs
Common questions about RC time constants, capacitor charging, discharging, and cutoff frequency.
What is an RC time constant?
The RC time constant is the product of resistance and capacitance. It describes how quickly a capacitor charges or discharges through a resistor.
How do I calculate the RC time constant?
Multiply resistance in ohms by capacitance in farads. The resulting time constant is expressed in seconds.
What happens after one time constant?
During charging, the capacitor reaches about 63.2% of the supply voltage. During discharging, it retains about 36.8% of its initial voltage.
How long does a capacitor take to fully charge?
A capacitor theoretically never reaches exactly 100%, but after five time constants it is approximately 99.3% charged and is usually considered fully charged.
How long does a capacitor take to discharge?
After five time constants, the capacitor retains only about 0.7% of its initial voltage and is usually considered effectively discharged.
What units can I use in this calculator?
You can use ohms, kilohms, megohms, farads, millifarads, microfarads, nanofarads, picofarads, seconds, milliseconds, and microseconds.
How is RC cutoff frequency calculated?
RC cutoff frequency is calculated by dividing one by two times pi times resistance times capacitance.
Are real capacitor results always exact?
No. Real results may vary because of resistor tolerance, capacitor tolerance, leakage current, equivalent series resistance, temperature, and measurement conditions.
Electrical design reminder:
This calculator uses ideal RC formulas. Real circuit behavior can
vary because of capacitor leakage, component tolerance, temperature,
parasitic resistance, and measurement conditions.