Last updated October 13, 2026 · Report an error
When a capacitor charges or discharges through a resistor, it doesn't happen instantly — it follows an exponential curve at a rate set by the circuit's time constant. This calculator solves for whichever value — resistance, capacitance, or time constant — is missing.
How it works
Enter any 2 of the 3 values (resistance in ohms, capacitance in microfarads, or time constant in milliseconds) and leave the one you want to solve for blank. The calculator applies τ = RC.
- Enter resistance (Ω) — leave blank to solve for this.
- Enter capacitance (µF) — leave blank to solve for this.
- Enter time constant (ms) — leave blank to solve for this.
- Click Calculate to see your results.
Examples
A simple RC timing circuit
A 1 kΩ resistor in series with a 100 µF capacitor has a time constant of 100 ms — after about 5 time constants (500 ms), charging or discharging is considered essentially complete.
Who should use it
- Electronics coursework on transient circuit behavior.
- Estimating timing for simple RC-based delay, debounce, or fade circuits.
Industry applications
- Electronics and circuit design
- Timing circuits and signal conditioning
Advantages
- Solves for any of the three variables, not just forward time-constant calculation.
- Simple, exact formula for ideal RC circuit behavior.
Limitations
- Assumes an ideal resistor and capacitor — real components have some additional parasitic effects not captured here.
Common mistakes to avoid
- Mixing up the RC time constant (this tool) with the RC Low-Pass Filter's cutoff frequency — they're related but answer different questions.
- Forgetting that "5 time constants" is a practical rule of thumb, not an exact "fully charged" point — the exponential curve technically never reaches exactly 100%.
Best practices
- Use the "5 time constants ≈ settled" rule of thumb for practical timing estimates, like an LED-fade or debounce circuit.
- If you need cutoff-frequency behavior instead of timing behavior, use the dedicated RC Low-Pass Filter Calculator.
Tips
- Pair this with the RC Low-Pass Filter Calculator and Capacitor Energy Calculator to see the same RC pair's frequency, timing, and energy behavior together.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
For charging, it's the time to reach about 63.2% of the final voltage; for discharging, it's the time to drop to about 36.8% of the starting voltage — not the total time to fully charge or discharge. A common rule of thumb treats about 5 time constants as essentially complete.
They're two different ways of describing the same RC pair — the time constant (τ = RC) describes charging/discharging speed in the time domain, while cutoff frequency (fc = 1/(2πRC)) describes filtering behavior in the frequency domain. They're mathematically related (fc = 1/(2πτ)) but answer different practical questions.
A larger resistance limits how fast charge can flow into or out of the capacitor, so charging/discharging takes longer — the opposite relationship from an RL circuit, where bigger resistance shortens the time constant.