RC Time Constant Calculator

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 milliseconds. 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.

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%.
• Forgetting that the result is in milliseconds, not seconds.
• Ignoring source resistance and tolerances, so a real capacitor charges slower than the ideal figure.