RC Circuit Calculator: Time Constant and Capacitor Charge

Online tool for RC circuit calculations: enter resistance and capacitance to get the time constant τ = R·C, a charge/discharge table by τ steps, the full-charge time and the RC filter cutoff frequency. Useful for timers, smoothing, filters and reset circuits.

How to calculate an RC circuit

Enter R and C

Pick units: Ω/kΩ/MΩ for the resistor and pF/nF/µF for the capacitor.

Read τ and the charge time

The time constant, the full-charge time (5τ) and the percentage table update instantly.

Check the filter

The cutoff frequency shows where this same RC pair attenuates a signal by 3 dB.

Work out how fast your capacitor charges and where the RC filter rolls off

Resistance R

Capacitance C

Time constant τ

—

Full charge (5τ)

—

Filter cutoff frequency

—

Charge and discharge by τ steps

τ	Time	Charge	Discharge (left)

Capacitor charge curve

Features

Time constant τ = R·C with automatic units from nanoseconds to minutes

Results scale themselves: 10 kΩ × 100 µF = 1 s, while 1 kΩ × 100 pF = 100 ns.

1τ–5τ table: 63%, 86%, 95%, 98%, 99% charge and the discharge remainder

The classic exponential checkpoints — and why a capacitor counts as fully charged after 5τ.

RC filter cutoff frequency f = 1/(2πRC)

The same R and C as a filter: the −3 dB frequency, identical for low-pass and high-pass.

Charge curve 1 − e^(−t/τ) with τ marks

The plot shows the exponential shape and the 63% point at 1τ — handy for teaching and sanity checks.

FAQ

What is the time constant τ?

τ = R·C is the time a capacitor takes to charge to 63.2% of the supply voltage (or discharge to 36.8%). It sets the scale of the whole exponential.

Why is a capacitor considered charged after 5τ?

After 5τ the voltage reaches 99.3% of its final value — full charge for practical purposes. Exact step values are in the table.

How are charge and discharge related?

They are the same exponential: on discharge 36.8% remains after 1τ and 5% after 3τ. The table shows both columns.

What does the cutoff frequency mean?

f = 1/(2πRC) is where an RC filter attenuates the signal by 3 dB (√2 in amplitude). A low-pass passes below it, a high-pass above it.

Can I use this for a microcontroller reset circuit?

Yes: choose τ so that 1τ–2τ covers the supply settling time. For example, 10 kΩ with 1 µF gives τ = 10 ms.

How do you rate this tool?

Thank you for your rating!

Change my rating

Want to leave a comment?

Want to share more? Leave a comment!

Thank you! Your comment will appear after moderation.

Who is this tool for?

🔌 Electronics Enthusiasts

Published 30 Jul 2026 Author: The Supreme