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Physics Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

RC Time Constant Calculator

You can calculate the RC time constant, resistance, capacitance, elapsed time or capacitor voltage, and see the charge percentage at each time constant plus the cutoff frequency.

Runs in your browserFree · No sign-up · No watermark
R, C → τRuns in your browser
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At a glance

Works with
R, C → τ
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Physics Tools

About RC Time Constant Calculator

An RC circuit's behavior is set by a single time constant, τ = R·C. It is the time to reach about 63.2% of the final voltage when charging, and the time to fall to about 36.8% of the initial voltage when discharging, regardless of the supply voltage.

This calculator solves for τ, R, C, elapsed time or voltage, converts resistor and capacitor prefixes, and reports the charging percentage at one through five time constants along with the cutoff frequency f_c = 1/(2πRC). Both charging and discharging are covered, and the same RC pair gives the first-order filter cutoff.

What this tool does

The details that matter when you use RC Time Constant Calculator for real work.

  • Five solve directions

    Calculate the time constant, resistance, capacitance, elapsed time or capacitor voltage from the remaining values.

  • Charge and discharge

    A phase selector switches between the rising charging curve and the falling discharging curve.

  • Time-constant progress table

    The percentage reached at 1τ through 5τ is listed, with the actual time for each row.

  • Cutoff frequency

    f_c = 1/(2πRC) is reported for the same pair, useful for first-order RC filters.

  • Unit conversions

    Resistance in Ω/kΩ/MΩ, capacitance in F/µF/nF/pF and time in s/ms/µs.

  • Formula and substitution

    The time-constant and exponential equations are printed with the converted values.

How to use RC Time Constant Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Choose what to calculate

    Select the time constant, resistance, capacitance, elapsed time or voltage.

  2. 02

    Enter the known values

    Use the unit selectors for resistance, capacitance and time; the tool converts to SI units.

  3. 03

    Pick charge or discharge

    For time and voltage modes, choose whether the capacitor is charging from zero or discharging from the supply voltage.

  4. 04

    Read the result and percentages

    The headline gives the solved quantity, and the table shows the charge reached at each time constant.

  5. 05

    Check the working

    The formula panel shows τ = RC, the exponential equation and the cutoff frequency with the values substituted.

How this tool works

Works with
R, C → τ
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Assumes ideal, constant resistance and capacitance driven by a DC step, with no leakage, ESR or inductance.
  • Describes first-order RC behavior only; multi-pole filters and loaded circuits need a fuller analysis.
  • The cutoff frequency refers to a first-order RC filter and does not replace a full frequency-response calculation.

Privacy and file handling

Resistance, capacitance, voltage and time values are processed entirely in your browser. Nothing is uploaded, stored or logged.

Read the full privacy policy

RC Time Constant Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
What is the time constant of an RC circuit?
The time constant is τ = R × C, with resistance in ohms and capacitance in farads. It sets the speed of the exponential charge and discharge curves: larger resistance or capacitance gives a longer time constant and slower changes.
Why does a capacitor reach 63.2% after one time constant?
The charging curve is V(t) = V₀(1 − e^(−t/τ)), so at t = τ the fraction is 1 − e^(−1), which equals about 0.632. That is why 63.2% is quoted as the one-time-constant point for charging.
How long does a capacitor take to charge fully?
In theory it approaches the supply voltage asymptotically and never fully arrives. In practice five time constants reach about 99.3% of the final value, so circuits are usually treated as settled after 5τ.
How do I calculate the cutoff frequency of an RC circuit?
Use f_c = 1/(2πRC). With R = 10 kΩ and C = 100 µF, RC = 1 s and the cutoff frequency is about 0.159 Hz. The same pair also has a time constant of 1 s, showing how τ and f_c are reciprocal measures.
How do I find the charging time for a target voltage?
Rearrange the charging equation: t = −τ · ln(1 − V/V₀). Selecting the time mode does this automatically from the target percentage, and the same relation with ln(V/V₀) gives the discharge time for a remaining voltage.
Does the supply voltage change the time constant?
No. τ = RC depends only on the resistance and capacitance, while the supply voltage sets the final voltage and the scale of the curve. A higher supply reaches a higher voltage in the same percentage of the time constant.
What happens when the capacitor discharges?
The voltage follows V(t) = V₀·e^(−t/τ), falling to about 36.8% of the starting voltage after one time constant and to about 0.7% after five. The same RC product controls both directions of the exponential change.
Where are RC time constants used in practice?
They appear in first-order low-pass and high-pass filters, debounce networks, timers, coupling between amplifier stages and the front ends of many sensors. The time constant tells you how fast the circuit responds to a step change in input.

Background reading

Format guides that explain the trade-offs behind RC Time Constant Calculator.