RC Time Constant Calculator

τ = RC. Time for an RC circuit to charge to ~63% (or discharge to ~37%) of final value.

Inputs

Result

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General calculation reads

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How to use this calculator

  • Enter resistance in ohms.
  • Enter capacitance in farads.
  • Read τ in seconds and ms.

About this calculator

The RC time constant τ = RC characterizes how fast an RC circuit responds. After 1τ a charging capacitor reaches 1−1/e ≈ 63% of its final voltage; after 5τ it's at 99.3% (essentially "settled"). For discharge, the same constant governs how fast voltage falls to 1/e ≈ 37% of starting value.

Frequently asked

Why 63%?+
1 − 1/e ≈ 0.632. After τ seconds, a charging cap is at 63.2% of final voltage. Comes from solving the exponential charging equation.
When is the cap "fully" charged?+
Mathematically never — it approaches asymptotically. Practically, 5τ gives 99.3%, which is "fully" for design purposes.
Real-world example?+
A 10 kΩ × 1 µF circuit has τ = 10 ms. Used in debouncing buttons, simple filters, oscillator timing.
How does this relate to LR circuits?+
For an inductor with resistance, τ = L/R. Same exponential shape, current rises (or falls) with this time constant.
What about an RLC circuit?+
More complex — can oscillate. The natural frequency is f = 1/(2π√(LC)) and damping depends on R.

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