HesapHanem

RC Time Constant Calculator

Find the RC time constant.

How it is calculated

The time constant of a resistor–capacitor circuit is τ = R × C. Resistance R is entered in ohms and capacitance C in microfarads (µF); the tool multiplies by 10⁻⁶ to get seconds and shows τ in the unit that fits: s, ms, µs or ns. After one τ a charging capacitor reaches about 63.2% of the supply voltage, and a discharging one falls to about 36.8%. After 5τ the capacitor is more than 99% charged, which is the usual rule for a full charge, and the tool shows that as the second value (5 × τ). Both inputs must be zero or positive. The result assumes an ideal resistor and capacitor with a constant voltage source.

Worked example

A 10,000 Ω (10 kΩ) resistor and a 100 µF capacitor. τ = 10,000 × 100 × 10⁻⁶ = 1 s, which the tool shows as 1.00 s. Full charge time = 5 × 1 s = 5.00 s.

Try this example →

Common mistakes

  • Entering the capacitance in farads or nanofarads when the field takes microfarads; 100 nF must be typed as 0.1.
  • Entering the resistance in kilohms: 10 kΩ must be typed as 10000.
  • Reading τ as the full charge time; at one τ the capacitor is only at about 63%.
  • Overlooking the unit label next to the result; it switches between s, ms, µs and ns depending on size.
  • Ignoring source resistance and tolerances, so a real capacitor charges slower than the ideal figure.

About this calculator

This calculator finds the RC time constant (τ = R × C) of a resistor-capacitor circuit, along with the full-charge time of 5τ, after which the capacitor is more than 99% charged. One time constant is always the time it takes to cover 63.2% of the remaining change.

Electronics hobbyists and engineering students use it to design timing circuits, debounce switches, and set filter cutoff frequencies, since the RC constant directly determines how fast a circuit responds to a voltage change. Resistance in ohms and capacitance in microfarads produce both times in the unit that fits (s, ms, µs or ns).

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    The time constant is also the key to a simple RC low-pass filter, whose cutoff frequency is 1 divided by 2π times τ in seconds. A τ of 1 ms, or 1,000 µs, puts the cutoff near 159 Hz. For a debounce or delay circuit, also check the logic threshold of the input you drive, since the voltage only crosses it after some fraction of τ. Is your circuit a timer, a debounce or a filter?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators