RC Time Constant Calculator

Work out the time constant, charge time, and filter cutoff frequency of an RC circuit from resistance and capacitance (tau = R x C).

This RC time constant calculator is for students and engineers working with resistor-capacitor circuits: timing delays, debounce circuits, smoothing and filtering, and the simple low-pass and high-pass filters at the heart of audio and sensor electronics. If you need to know how fast a capacitor charges through a resistor, how long before a signal settles, or where a filter starts to roll off frequencies, this tool gives all three from two values you already have.

When a capacitor charges or discharges through a resistor, it does not change instantly. The time constant, written with the Greek letter tau, is the resistance times the capacitance: tau = R x C. After one time constant the capacitor reaches about 63% of the way to its final voltage, and after five time constants it is within about 1%, which engineers treat as fully charged. The same R and C set the cutoff frequency of a simple filter, fc = 1 / (2 x pi x R x C), the point where the filter begins to noticeably cut the signal. The calculator computes the time constant, the practical five-tau settling time, and the cutoff frequency together. It works in kilohms and microfarads, whose product conveniently lands in milliseconds.

Take a 10 kilohm resistor and a 1 microfarad capacitor. The time constant is 10 times 1, which is 10 milliseconds. Five time constants is 50 milliseconds, so the capacitor is effectively fully charged about a twentieth of a second after you apply voltage. The same pair forms a filter with a cutoff of 1 divided by (2 x pi x 10,000 ohms x 0.000001 farads), about 15.9 hertz - low enough to smooth out anything faster while letting slow changes through. Swap in a 1 kilohm resistor with a 100 microfarad capacitor and the time constant jumps to 100 milliseconds, a much slower circuit.

Capacitor and resistor behavior is universal, so the formulas are the same everywhere. The conventions you meet on components are international too: capacitor values are often printed as a three-digit code (for example 104 means 100,000 picofarads, or 0.1 microfarads), and tolerance is shown by a letter. Mains frequency differs by region, 50 hertz in much of the world and 60 hertz in North America, which matters when you design a filter to reject mains hum, but the RC math itself does not change.

• Mixing units. Kilohms with microfarads gives milliseconds; if you enter plain ohms and farads expecting milliseconds, the result is off by huge factors.
• Assuming the capacitor charges instantly or linearly. The charge curve is exponential, fast at first and then slowing as it approaches the final voltage.
• Treating one time constant as fully charged. It only reaches about 63%; allow about five time constants for the circuit to settle.
• Ignoring the capacitor's tolerance, which is often 10% or 20%, so the real time constant varies from the ideal figure.
• Forgetting the resistance of whatever drives the circuit. A high source resistance adds to R and slows charging beyond the calculated value.

Timing in electronics is the point, so the practical rule is to design for about five time constants whenever you need a circuit to settle fully before the next event - whether that is a switch debounce, a reset pulse, or a sample on an analog input. For filters, place the cutoff frequency clearly below the noise you want to remove and clearly above the signal you want to keep, leaving margin for component tolerance so the filter still does its job at the edges of its rated values.

• RC time constant - Reference (retrieved 2026-06-11)