RC circuits set timing in oscillators, debounce switches and filter signals.
How to use the RC Time Constant Calculator
- Resistance
- Capacitance
Results update as you type, and every field accepts the units shown in its menu.
How it works
Multiply resistance by capacitance for τ. A capacitor is about 99.3% charged after five time constants.
Inputs explained
| Field | What to enter |
|---|---|
| Resistance | Units: Ω, kΩ, MΩ |
| Capacitance | Units: µF, nF, pF, mF, F |
What each result means
- Time constant τ = [Resistance] × [Capacitance]
- Time to ~99% (5τ) = 5×[Time constant τ]
- Cut-off frequency = 1÷(2×pi×[Resistance]×[Capacitance])
Example values
How the main result changes when Resistance changes and everything else stays at the example values above:
| Resistance (Ω) | Time constant τ (s) |
|---|---|
| 5,000 | 0.005 |
| 7,500 | 0.0075 |
| 10,000 | 0.01 |
| 12,500 | 0.0125 |
| 15,000 | 0.015 |
| 20,000 | 0.02 |
Formula
τ = R × Cf_c = 1 ÷ (2πRC)Worked example
Inputs — Resistance: 10,000 Ω; Capacitance: 1 µF.
Results — Time constant τ = 0.01 s; Time to ~99% (5τ) = 0.05 s; Cut-off frequency = 15.92 Hz.
Frequently asked questions
What happens after one τ?
The capacitor reaches about 63.2% of the supply voltage.
Is the cut-off frequency the same for high-pass and low-pass?
Yes, f_c = 1/(2πRC) for both.
What formula does the RC Time Constant Calculator use?
τ = R × C.