RC time constant calculator
1 kΩ times 1 µF gives a time constant of 1 ms, and five of them, 5 ms, is the usual settling window. Enter a resistor and a capacitor to see how fast an RC circuit charges or discharges. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Resistance in ohms
- 1,000
- Capacitance in farads
- 0.000001
- Time constant
- 0.001 second
- Time constant seconds
- 0.001
- Tau
- 0.001
- Settling time
- 0.005
The RC time constant is the product of resistance and capacitance that sets how quickly a first-order resistor-capacitor circuit charges or discharges toward a new steady voltage. One tau equals R times C in seconds when R is in ohms and C is in farads; after one tau a step response reaches about sixty-three percent of its final change, and after five tau it settles to roughly ninety-nine point three percent of the final value. Designers, students, and lab technicians use that pair of numbers every day to size pull-up resistors and filter capacitors, estimate reset-pulse widths, and choose sample delays on analog front ends. This RC time constant calculator accepts non-negative resistance and capacitance, multiplies them with deterministic floating-point arithmetic, and returns the time constant tau together with the conventional five-tau settling time, cleaned inputs, and a short printable expression. Negative resistance or capacitance is rejected with a clear validation error because passive R and C cannot be less than zero; zero is allowed and simply yields a zero time constant.
How the RC time constant and five-tau settling work
In a series resistor and capacitor driven by a voltage step, the voltage across the capacitor approaches its final value exponentially with time constant tau equal to R times C. After one tau the error relative to the final step change has fallen to about thirty-seven percent remaining, after three tau to about five percent, and after five tau to about zero point seven percent, which is why textbooks and design reviews often quote five tau as a practical settling or hold-off window.
The same product governs discharge toward ground through a resistor, debounce RC filters, integrator-like low-pass corners, and many reset or power-on delay networks built from a single resistor and capacitor. Capacitance must be expressed in farads and resistance in ohms so that tau lands in seconds: one kilohm times one microfarad is one millisecond, and one megohm times one nanofarad is one millisecond as well.
Real parts have tolerances, leakage, dielectric absorption, and layout parasitics that shift the effective tau, but the first design step is still the arithmetic product of the nominal R and C. Energy storage, charge Q equals C V, and impedance of a capacitor at a given frequency are different quantities and belong on separate landings so search intent for time constant does not collide with those other formulas.
What you can do with it
Lab homework and scope estimates
Convert a resistor value and a capacitor value into tau and five-tau so students can predict rise and discharge times before measuring on an oscilloscope.
Power-on reset and soft-start windows
Estimate how long to wait after power-up for an RC delay network by taking five times R times C as a practical settling window.
FAQ
What happens if resistance or capacitance is negative?
Passive resistance and capacitance must be ≥ 0 ohms and farads for this model.
Why return five tau as well as tau?
After five time constants a first-order RC step has settled to about 99.3% of its final value, which is a common design rule of thumb for wait windows and filter settle times.
What units should I use?
Resistance in ohms and capacitance in farads. Pass SI values as numbers (e.g. 1000 and 1e-6); prefix strings like '1k' or '1uF' are not parsed.
What does it cost?
The browser tool is free and runs locally.
How far does a capacitor charge after one time constant?
After one tau a step response has covered about sixty-three percent of its final change, after three tau about ninety-five percent, and after five tau roughly 99.3 percent. The same steps apply when it discharges through a resistor.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/elec/rc-time-constant \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"resistance":1000,"capacitance":0.000001}'const res = await fetch("https://api.kit.forhosting.com/elec/rc-time-constant", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"resistance": 1000,
"capacitance": 0.000001
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/rc-time-constant",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"resistance": 1000,
"capacitance": 0.000001
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/rc-time-constant", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"resistance":1000,"capacitance":0.000001}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"resistance":1000,"capacitance":0.000001}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/rc-time-constant", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"resistance": 1000,
"capacitance": 0.000001
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.rc_time_constant",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What you send and what you get back
Send a finite non-negative resistance under the field resistance, expressed in ohms, and a finite non-negative capacitance under the field capacitance, expressed in farads. Numeric values and numeric strings are both accepted so form widgets and spreadsheet paste stay convenient; SI prefix strings such as one kilohm written as text are not parsed, so pass one thousand for a kilohm part and one times ten to the minus six for a microfarad part. Classic classroom checks include one thousand ohms with one microfarad giving tau of one millisecond and five tau of five milliseconds, and one ohm with one farad giving tau of one second. If you need stored charge, energy, impedance at frequency, or series-parallel reduction of resistor banks, treat those as separate tools rather than extending this pure RC product. The same resistance is accepted under the aliases r, R, or ohms, and capacitance under c, C, or farads, but only one key may appear for each quantity in a request. Resistance and capacitance must each be greater than or equal to zero; NaN, Infinity, and negative values fail with invalid_input and are not charged. On success the response includes cleaned resistance and capacitance, time_constant, time_constant_seconds, and tau as the same product R times C, settling_time, settling_time_five_tau, and settling_time_seconds as five times that product, unit set to second, a short expression string showing both tau and five tau, method rc_time_constant, and the formula string tau equals R times C with settling at five R C. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where RC time constant calculators show up in real work
Introductory electronics labs constantly assign RC transient drills so students can convert datasheet kilohms and microfarads into milliseconds of rise or discharge before they move on to scope measurements. Embedded and power bring-up teams size power-on reset and soft-start RC networks by estimating five tau as the wait before deasserting reset or enabling a load switch. Keep charge-only, energy-only, impedance, and multi-element network landings separate so each search intent has one clear page rather than an overloaded multi-formula tool that confuses both students and search engines. Analog front-end and data-acquisition designs choose anti-alias or anti-bounce RC filters by setting tau relative to sample periods and settling accuracy, and a trusted product of R and C keeps notebooks and spreadsheets aligned. Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
CI fixtures and answer keys
Script the same τ = R × C product used in the browser so automated graders and BOM checks share one ground truth for time constants.
Technical questions
What formula does this RC time constant calculator use?
τ = R × C for the time constant in seconds, and t_settle = 5 × R × C for the conventional five-tau settling time of a first-order step response.