Is one time constant a full charge?

No. An ideal capacitor reaches about 63.2 percent after one time constant and approaches its final voltage asymptotically.

Does the cutoff apply to both low-pass and high-pass filters?

Yes for ideal first-order RC forms using the same R and C. Their response differs on either side of the shared cutoff frequency.

Electronics / LOCAL TOOL

RC Time Constant & Filter Calculator

Calculate RC time constant, filter cutoff, and charge/discharge thresholds.

Runs locally — input is never uploaded

RC valuesIdeal first-order circuit

Resistance Capacitance Selected threshold (%)

ResultReady when you are

τ = R C

f_{c} = \frac{1}{2 π R C}

WHAT IT DOES

Read timing and cutoff from the same R and C pair.

Calculate the RC time constant, the ideal first-order minus-three-decibel cutoff, common charging milestones, and the exact charge and discharge time for a selected percentage threshold.

Why use it?

Use it to estimate an ideal first-order timing or filter response and distinguish a time constant from the time to reach an arbitrary threshold.

How to use this tool

Enter resistance, capacitance, and a threshold strictly between 0% and 100%, then calculate.
For example, 10 kΩ and 100 nF produce a 1 ms time constant and a cutoff near 159.15 Hz.
Read charging percentage as the fraction of final voltage reached, and discharging percentage as the fraction still remaining. A 90% charge and a 90% remaining discharge do not take the same time.
Account separately for source/load impedance, component tolerances, and leakage when comparing the ideal result with a real circuit.

Details and limitations

Charging and discharging use the exponential step-response equations rather than assuming every threshold occurs at one time constant.
The selected percentage means fraction of final voltage while charging and fraction of initial voltage remaining while discharging.
Source resistance, load impedance, capacitor ESR and leakage, dielectric absorption, and component tolerance are outside the ideal one-R one-C model and are named with the result.

WORKED EXAMPLES

The same percentage describes different timing questions

Calculate at a 90% threshold with these R and C values. Compare reaching 90% of the final voltage with decaying to 90% of the initial voltage.

Resistance

10 kΩ

Capacitance

100 nF

Threshold

Expected result

Time constant 1 ms, cutoff approximately 159.15 Hz, charge to 90% approximately 2.303 ms, and discharge to 90% remaining approximately 105.36 µs. Neither threshold should be assumed to take exactly one time constant.

COMMON QUESTIONS

RC Calculator questions

Is one time constant a full charge?: No. An ideal capacitor reaches about 63.2 percent after one time constant and approaches its final voltage asymptotically.
Does the cutoff apply to both low-pass and high-pass filters?: Yes for ideal first-order RC forms using the same R and C. Their response differs on either side of the shared cutoff frequency.