RC Time Constant Calculator
Compute the time constant of a resistor-capacitor circuit and see how the capacitor voltage rises while charging.
Ω
F
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Capacitor charges to ~63.2% of supply in one time constant.
- 1
Time constant τ = R × C
10,000 × 0.000001 = 0.010000R in ohms × C in farads — larger RC means slower charge. - 2
Time to ~99.3% charge (5τ)
5 × 0.01 = 0.05 - 3
Capacitor voltage at 1τ
5 × (1 − e⁻¹) = 3.1606
How does this calculator work?
The RC time constant τ equals resistance times capacitance (τ = R × C) in seconds. A capacitor charges to about 63.2% of the supply voltage in one time constant and roughly 99.3% after five. Enter R in ohms and C in farads to get τ and the charging curve.
Formula
How this is calculated
The time constant τ equals resistance R (in ohms) multiplied by capacitance C (in farads), giving a result in seconds. It sets the timescale over which a capacitor charges or discharges through the resistor: larger R or C means slower change.
During charging from a supply voltage Vs, the capacitor voltage follows Vc(t) = Vs × (1 − e^(−t/τ)). After one time constant the capacitor reaches about 63.2% of Vs, after two roughly 86.5%, and after five about 99.3% — which is conventionally treated as fully charged. The curve samples Vc(t) from t = 0 to t = 5τ. The supply voltage is optional and only scales the plotted curve; the time constant itself does not depend on it.
Units must be consistent: enter R in ohms and C in farads (1 µF = 1e−6 F, 1 nF = 1e−9 F). R and C must both be positive; zero or negative values are rejected since they have no physical charging behaviour.
Frequently asked questions
At t = τ the exponential term e^(−1) ≈ 0.368, so Vc = Vs × (1 − 0.368) = 0.632 × Vs, i.e. about 63.2% of the supply voltage.
By convention after about five time constants (5τ), when the capacitor has reached roughly 99.3% of the supply voltage — close enough to treat as fully charged.
No. τ = R × C depends only on resistance and capacitance. The supply voltage only scales the magnitude of the charging curve, not its timing.
Also known as
TG we-Calculate Editorial Team. (2026). RC Time Constant Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/rc-time-constant-calculator
TG we-Calculate Editorial Team. "RC Time Constant Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/rc-time-constant-calculator.
TG we-Calculate Editorial Team, "RC Time Constant Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/rc-time-constant-calculator
@misc{wecalculate_rc_time_constant_calculator, title = {RC Time Constant Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/rc-time-constant-calculator}}, year = {2026}, note = {TG we-Calculate} }
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