Capacitor Calculator
Answer
For AC, Xc = 1 / (2π f C) (ohms). For timing, τ = R × C (seconds); many RC curves hit ~90% at 2.2 τ. Enter capacitance in farads — use the unit converter if you think in microfarads.
Calculator
How it works
Capacitors show up in two quick questions: how much they impede AC, and how fast an RC node moves. This page answers both without pulling in ESR models.
Fill capacitance in farads, optional frequency for Xc, optional series resistance for τ. The checks remind you when inputs are missing. For inductors at the same frequency, use the reactance calculator on this site.
Parameters used on this page
| Parameter | Value | Source |
|---|---|---|
| Capacitive reactance | Xc = 1 / (2π f C) | Steady-state sinusoidal excitation |
| Time constant | τ = R C | First-order RC step response |
| 90% settling | ≈ 2.2 τ | 1 − e^(−t/τ) = 0.9 |
Worked example
1 µF at 1 kHz: Xc ≈ 159 Ω. The same capacitor with 1 kΩ gives τ = 1 ms and 2.2 τ ≈ 2.2 ms to approach full charge on a step.
Pitfalls
- Using picofarads in the field labelled F. 100 nF is 1×10⁻⁷ F, not 100.
- Applying Xc at frequencies where ESL and ESR dominate — this is ideal capacitance only.
- Forgetting that discharge through a GPIO may add series resistance not in your τ.
FAQ
What is the reactance of 100 nF at 50 Hz?
Xc = 1 / (2π × 50 × 100×10⁻⁹) ≈ 31.8 kΩ.
How do I get τ for a reset RC?
Multiply R and C in ohm-farads. 10 kΩ × 100 nF = 1 ms.
Why 2.2 τ?
It is a common rule of thumb for ~90% of a charging exponential; precise thresholds need the exact exponential.
Can I leave frequency blank?
Yes. Provide R to get τ only, or f to get Xc only, or both.
Related
Sources
- Linear AC reactance and first-order RC timing.