RC Time Constant Calculator | τ=RC, Cutoff Frequency & Charging Curve

Q: What is the RC time constant?

τ = R×C, the time to charge a capacitor to 63.2% of applied voltage.

Resistance (R)

Capacitance (C)

Applied Voltage (optional)

📏 Resistor Series

📏 Capacitor Series

📊 Time Constant (τ)

1.00 s

Cutoff Frequency (fc)

0.16 Hz

Capacitor Energy

1.25 mJ

Time to 99% (5τ)

5.00 s

R std: 10kΩ

C std: 100µF

Time	% Charge	Voltage

✓ Real‑time · Standard E‑series · Visual curve

📘 RC Circuit Guide

The RC time constant τ = R × C determines how quickly a capacitor charges or discharges. After 1τ, the capacitor reaches 63.2% of the applied voltage; after 5τ, it's considered fully charged (99.3%). The cutoff frequency of an RC low‑pass filter is fc = 1/(2πRC).

❓ FAQ

What is the RC time constant?

τ (tau) is the product of resistance and capacitance (τ = R×C). It represents the time required to charge a capacitor to ~63.2% of the applied voltage.

Why is 5τ considered "fully charged"?

At 5τ, a capacitor reaches 99.3% of the supply voltage, which is practically fully charged for most applications.

How are standard component values chosen?

Resistors follow E24/E12 series; capacitors follow E12/E6 series. The tool matches the closest standard value.

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