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RC Time Constant Calculator

Model a first-order resistor-capacitor response. Calculate voltage at a chosen time or find how long it takes to reach a target between the initial and final voltages.

Ideal first-order lumped RC circuit only. Assumes constant R and C, an instantaneous input step and no source resistance, leakage, component tolerance, parasitic elements, loading, saturation or switching delay. Not circuit design or safety advice.

τ = R·C. V(t) = V∞ + (V₀ − V∞)e^(−t/τ). The 10–90% rise or fall time is approximately 2.197τ for this ideal response.

For a target-time calculation, the target must be strictly between the initial and final voltages. Inputs and results are limited to keep calculations finite.

How do you calculate an RC time constant?

The time constant is τ = R × C, with resistance in ohms and capacitance in farads. For a step response, V(t) = V∞ + (V₀ − V∞)e^(−t/τ). After one time constant, the response has completed about 63.2% of its total voltage change.

Frequently asked questions

How do you calculate an RC time constant?

The time constant is τ = R × C, with resistance in ohms and capacitance in farads. For a step response, V(t) = V∞ + (V₀ − V∞)e^(−t/τ). After one time constant, the response has completed about 63.2% of its total voltage change.

Are my circuit values uploaded?

No. Calculations run in this browser, and the values you enter are not uploaded or saved by this tool.

What does one RC time constant mean?

After one τ, a charging capacitor has completed about 63.2% of the voltage step. A discharging capacitor has about 36.8% of its starting voltage-step difference remaining.

How long is the 10–90% rise time?

For an ideal first-order RC response, the 10–90% rise or fall time is ln(9) × R × C, approximately 2.197τ. Real circuits can differ because of source impedance and parasitic effects.

Does this model include the whole circuit?

No. It is a first-order model with one resistance and one capacitance. It does not simulate multiple poles, component tolerances, source impedance, loading, leakage, switching behavior or safety limits.