RC Time Constant CalculatorFree online other calculator — instant results, no sign-up
Calculate the time constant of a resistor-capacitor circuit.
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About the RC Time Constant Calculator
The RC Time Constant Calculator calculates how quickly a resistor-capacitor circuit charges or discharges using resistance in kΩ and capacitance in µF. It returns the time constant in seconds, which describes the circuit’s exponential response rather than the time needed to charge completely. With the default resistance of 10 kΩ and capacitance of 100 µF, the result is 1 s.
- Resistance (kΩ)
- The resistance in the capacitor’s charging or discharging path, entered in kiloohms (kΩ), with a default of 10.
- Capacitance (µF)
- The capacitor’s capacitance, entered in microfarads (µF), with a default of 100.
Formula and method
τ = RC; with the entered units, τ (s) = R (kΩ) × C (µF) × 10⁻³. For the defaults: τ = 10 × 100 × 10⁻³ = 1 s.
- τThe RC time constant, measured in seconds (s).
- RThe resistance seen by the capacitor; use ohms (Ω) in τ = RC, or kiloohms (kΩ) with the stated conversion factor.
- CThe capacitance; use farads (F) in τ = RC, or microfarads (µF) with the stated conversion factor.
Convert resistance from kΩ to Ω by multiplying by 1,000, and convert capacitance from µF to F by multiplying by 10⁻⁶. Multiply the converted values to obtain the time constant in seconds. For the default inputs, 10 kΩ becomes 10,000 Ω and 100 µF becomes 0.0001 F, giving τ = 10,000 × 0.0001 = 1 s.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Resistance10 kΩ
- Capacitance100 µF
Results
- Result1 s
How to use the RC Time Constant Calculator
- 1Identify the resistance in the capacitor’s charging or discharging path.
- 2Enter Resistance (kΩ); keep 10 to reproduce the default example.
- 3Enter Capacitance (µF); keep 100 to reproduce the default example.
- 4Run the calculation to obtain the RC time constant in seconds.
- 5Read the result: with 10 kΩ and 100 µF, 1 s is one time constant, not the complete charging time.
Reading your result
The Result row gives the time constant τ in seconds. For the default result of 1 s, a charging capacitor has completed about 63.2% of the change from its initial voltage toward its final voltage after 1 s. When discharging toward zero, about 36.8% of its initial voltage remains after 1 s. A larger result indicates a slower response; a smaller result indicates a faster response.
Limitations
The calculation assumes an ideal, first-order RC circuit with positive, constant resistance and capacitance. The entered resistance should be the effective resistance seen by the capacitor in the relevant circuit configuration, which may include source or load resistance. Component tolerances, capacitor leakage, equivalent series resistance, and temperature or voltage dependence can change the actual response. Circuits with multiple independent energy-storage elements may not have a single time constant. Exact full charge or discharge is approached asymptotically, so the result is not a completion time.
Common questions
Why do 10 kΩ and 100 µF give 1 s rather than 1,000 s?+
The units require conversion: 1 kΩ is 1,000 Ω and 1 µF is 10⁻⁶ F. Their combined conversion factor is 10⁻³, so 10 × 100 × 10⁻³ = 1 s.
Does a result of 1 s mean the capacitor is fully charged after 1 s?+
No. After 1 s, the default circuit has completed about 63.2% of its voltage change. After five time constants—5 s for these defaults—it has completed about 99.3%, often treated as practically settled.
Is the RC time constant the same for charging and discharging?+
Yes, if the capacitor sees the same effective resistance in both paths. If switching changes the resistance, calculate each path separately using its effective resistance.
Why does this calculator not ask for the supply voltage?+
For an ideal linear RC circuit, τ depends only on resistance and capacitance. Supply and initial voltages determine the voltage levels, but not the time constant.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.