# RC Time Constant Calculator

URL: https://speedycalculator.com/calculators/physics/rc-time-constant/
Last updated: 2026-10-25

> The RC circuit calculator finds the time constant τ = RC, the time to charge to about 63% and to 99% (5τ), and the cut-off frequency of an RC filter.

## Inputs

- Resistance (Ω, kΩ, MΩ)
- Capacitance (µF, nF, pF, mF, F)

## Results

- Time constant τ
- Time to ~99% (5τ)
- Cut-off frequency

## Formula

```
τ = R × C
f_c = 1 ÷ (2πRC)
```

## Explanation

RC circuits set timing in oscillators, debounce switches and filter signals.

### How to use the RC Time Constant Calculator

- **Resistance**
- **Capacitance**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Multiply resistance by capacitance for τ. A capacitor is about 99.3% charged after five time constants.

### Inputs explained

FieldWhat to enterResistanceUnits: Ω, kΩ, MΩCapacitanceUnits: µF, nF, pF, mF, F

### What each result means

- **Time constant τ** = [Resistance] × [Capacitance]
- **Time to ~99% (5τ)** = 5×[Time constant τ]
- **Cut-off frequency** = 1÷(2×pi×[Resistance]×[Capacitance])

### Example values

How the main result changes when **Resistance** changes and everything else stays at the example values above:

Resistance (Ω)Time constant τ (s)5,0000.0057,5000.007510,0000.0112,5000.012515,0000.01520,0000.02

## Worked example

Inputs — Resistance: 10,000 Ω; Capacitance: 1 µF.

Results — **Time constant τ = 0.01 s**; Time to ~99% (5τ) = 0.05 s; Cut-off frequency = 15.92 Hz.

## FAQ

**What happens after one τ?**
The capacitor reaches about 63.2% of the supply voltage.

**Is the cut-off frequency the same for high-pass and low-pass?**
Yes, f_c = 1/(2πRC) for both.

**What formula does the RC Time Constant Calculator use?**
τ = R × C.