Calculators
RC Time Constant Calculator
Calculate the RC time constant τ = R × C, solve for resistance or capacitance, and find charge or discharge time to a target voltage locally in your browser.
Use the RC time constant τ = R × C to find a missing resistance, capacitance, or time constant, or to find how long a capacitor takes to charge to or discharge to a target voltage. Enter positive values with matching unit families, then choose which quantity to solve for. Calculations run locally in your browser; your values are not uploaded or stored.
This is the value being calculated.
Find τ = R × C and the time to charge or discharge.
The time constant of a resistor-capacitor circuit is τ = R × C, the time for the capacitor voltage to reach about 63.2% of the supply while charging or fall to about 36.8% while discharging. Enter resistance, capacitance, and the time constant in any convenient units to solve for the missing quantity, or enter resistance, capacitance, a target voltage, and the supply voltage to find how long the capacitor takes to charge to or discharge to that level. For example, 10 kΩ with 100 µF gives τ = 1 s, and charging that circuit toward 10 V reaches about 6.321 V after one time constant. Charge and discharge times use the ideal exponential model t = -τ × ln(1 - Vc/Vs) and t = τ × ln(Vs/Vc). The result is reported in automatically scaled units. This assumes an ideal resistor and capacitor with no leakage, tolerance error, or parasitic effects. All calculations run locally in your browser; your values are not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is the RC time constant?
The time constant τ (tau) of a resistor-capacitor circuit is τ = R × C, where R is resistance in ohms and C is capacitance in farads. It is the time for a charging capacitor to reach about 63.2% of the supply voltage, or a discharging capacitor to fall to about 36.8% of its starting voltage. After about five time constants the capacitor is considered fully charged or discharged.
How do I find resistance or capacitance from the time constant?
Rearrange τ = R × C. For resistance use R = τ ÷ C, and for capacitance use C = τ ÷ R. For example, a 1 ms time constant with 100 nF gives R = 0.001 ÷ 0.0000001 = 10,000 Ω (10 kΩ), and a 1 ms time constant with 10 kΩ gives C = 0.001 ÷ 10000 = 100 nF.
How is charge or discharge time calculated?
A charging capacitor follows Vc = Vs × (1 - e^(-t/τ)), so solving for time gives t = -τ × ln(1 - Vc/Vs). A discharging capacitor follows Vc = Vs × e^(-t/τ), so t = τ × ln(Vs/Vc). The target voltage Vc must be greater than zero and lower than the supply voltage Vs. Charging toward 10 V through 10 kΩ and 100 µF (τ = 1 s) reaches about 6.321 V after 1 s.
Which units can I use?
Resistance accepts ohms (Ω), kilo-ohms (kΩ), mega-ohms (MΩ), and giga-ohms (GΩ); capacitance accepts picofarads (pF), nanofarads (nF), microfarads (µF), millifarads (mF), and farads (F); the time constant accepts nanoseconds (ns), microseconds (µs), milliseconds (ms), and seconds (s). Results are shown in automatically scaled units. Voltages are entered in volts.
What are the limitations of this model?
The calculator assumes an ideal resistor and capacitor with no leakage, no tolerance error, and no parasitic inductance or capacitance, and a constant supply voltage. Real components, leakage, and source impedance shift the actual timing. It is an educational estimate for design intuition, not a substitute for measurement or simulation, and all calculations run locally in your browser.