RC Time Constant Calculator

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

Inputs

Result

Loading calculator…
—

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.

Related calculators

Capacitance Calculator
C = Q/V or parallel-plate C = ε₀A/d. Compute capacitance, charge stored, or energy.
Inductance Calculator
L = μ₀N²A/ℓ for an ideal solenoid, or V = L(dI/dt) for the defining equation.
Ohm's Law Calculator (Extended)
V = IR, P = IV = I²R = V²/R. Pick which two values you know and solve for the others.
Parallel Resistance Calculator
1/R = 1/R₁ + 1/R₂ + … — find equivalent resistance of resistors in parallel.
Electrical Power Calculator
P = IV — compute power from voltage and current, or from voltage and resistance, or current and resistance.
Wavelength ↔ Frequency Calculator
c = λ × ν. Wavelength in vacuum from frequency or vice versa. c = 3×10⁸ m/s.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Capacitance Calculator
Calc

C = Q/V or parallel-plate C = ε₀A/d. Compute capacitance, charge stored, or energy.

Ohm's Law Calculator (Extended)
Calc

V = IR, P = IV = I²R = V²/R. Pick which two values you know and solve for the others.

Electrical Power Calculator
Calc

P = IV — compute power from voltage and current, or from voltage and resistance, or current and resistance.

Inductance Calculator
Calc

L = μ₀N²A/ℓ for an ideal solenoid, or V = L(dI/dt) for the defining equation.

Parallel Resistance Calculator
Calc

1/R = 1/R₁ + 1/R₂ + … — find equivalent resistance of resistors in parallel.

Time-Off Request Form
Docs

Employee time-off / PTO request — dates, leave type, manager approval signature.