RC Time Constant Calculator

Calculate τ = R × C, and estimate charge/discharge time to a target percent. Includes unit conversions and Show Work.

How to Use

  1. Enter R (resistance) and C (capacitance) to compute τ.
  2. Choose Charge or Discharge and set a target percent.
  3. Use “Solve For” to force a specific output (τ, time, R, or C).
  4. Open “Show Work” for formulas + steps in base units.
RC Lab View
Visual feedback: charge curve, percent target, and computed timing.
τ
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t
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R
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C
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Status: —
Inputs & Settings
Enter R and C. Optionally compute time to reach a target percent.
Tip: Use kΩ for most timing networks.
Common: 100nF, 1µF, 10µF, 100µF.
Auto computes what it can from your inputs. Use Solve For to force an output.
Charge uses: V/Vmax = 1 − e^(−t/τ). Discharge uses: V/V0 = e^(−t/τ).
Show Work (step-by-step)
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Work is shown in base units: Ω, F, seconds.

RC Reference

Time constant: τ = R × C

  • Charging (ratio): V(t)/Vmax = 1 − e^(−t/τ)
  • Discharging (ratio): V(t)/V0 = e^(−t/τ)
  • Time to percent (charge): t = −τ ln(1 − p)
  • Time to percent (discharge): t = −τ ln(p)
p is a fraction (e.g., 63.2% = 0.632). Typical milestone: ~63.2% at 1τ (charge), ~36.8% at 1τ (discharge).

FAQ

What is an RC time constant?

It’s the characteristic time τ = R×C that sets how quickly a capacitor charges or discharges through a resistor.

What does “63.2%” mean?

After 1τ of charging, the capacitor voltage reaches about 1 − e^(−1) ≈ 63.2% of its final value.

How many time constants to “fully” charge?

Engineers often use ~5τ as “effectively settled” (~99.3% charge, ~0.7% remaining on discharge).

Why is my real circuit different?

Leakage, source resistance, capacitor tolerance/ESR, and load resistance all change the curve compared to the ideal RC model.

Tool Info

Last updated:

Updates may include additional unit support, curve visualization improvements, and edge-case handling.