RC Time Constant Calculator
Calculate the RC time constant, charge times and the filter cutoff frequency that follows.
Five time constants counts as fully charged
A capacitor reaches 63.2% of the supply in one time constant, 86.5% in two and 99.3% in five. It never mathematically arrives at 100%, so engineering practice treats 5τ as complete. Discharge follows the identical curve in reverse.
What this page needs from you
This page asks for resistance in Ω and capacitance in µF. From those it returns time constant τ in s, together with charged to 63.2% (1τ), charged to 95% (3τ), charged to 99.3% (5τ) and filter cutoff frequency. Every figure updates as you type, and nothing you enter leaves your browser.
Check the power rating, not just the value
Choosing a component by its resistance or capacitance alone leaves out the rating that actually determines whether it survives. Work out the power the part will dissipate and pick something rated for at least twice it: a resistor run at its limit gets hot enough to drift out of tolerance and to discolour the board.
The formula
τ = R · C; cutoff f = 1 / (2πRC)
Every result on this page comes from this formula. Nothing is rounded until the final display, so figures match a spreadsheet to the cent.
How to use the RC Time Constant Calculator
- Fill in each field with your own figures — the defaults are only there as an example.
- The result updates as you type, so there is no button to press.
- Check the breakdown under the main result for the supporting numbers.
- Use the copy button to take the result with you, or share the link to reload the same inputs.
RC Time Constant Calculator — frequently asked questions
Is this rc time constant calculator free to use?
Yes. Every calculator on the site is free, needs no sign-up, and runs entirely in your browser, so your figures never leave your device.
How accurate is the result?
The maths is exact for the formula shown on the page. Accuracy in the real world depends on your inputs and on assumptions the formula cannot see, so treat the output as a well-grounded estimate rather than a quote.
Can I use it on my phone?
Yes. The page is responsive, weighs a few kilobytes and works offline once loaded, so it behaves the same on a phone as on a desktop.
What people search for
This page answers each of these searches. They are the exact phrases visitors use to find the rc time constant calculator.
- rc time constant calculator
- rc filter cutoff
- tau calculator electronics
- capacitor charge time constant
- rc circuit calculator
- rc time constant
About this calculator
Written and reviewed by The Samsung Calculator editorial team. Every calculator is written against a published formula, reviewed against at least one independent reference implementation, and dated when it changes.
Last reviewed September 14, 2026 · Calculation runs client-side · No data collected · Report an error