The calculation, without hidden assumptions
An ideal first-order RC circuit is governed by τ = R×C. Strong RC work often starts in either direction: calculate tau from known component values, or start from a target timing constant and solve the missing resistor or capacitor. MAXScanner supports all three modes, then uses the solved RC product to show five-tau settling, the −3 dB cutoff 1/(2πRC), and exact charge/discharge target times without pretending real parts are tolerance-free.
How to use this calculator
Choose whether to solve the time constant, resistance or capacitance.
Enter the two known quantities using practical units; reverse-solve modes use the target time constant plus the known component.
Review the solved value, 5τ, cutoff frequency and exact charge/discharge target times, then account for component tolerances in real hardware.
Where people use it
- •RC delay and debounce timing
- •Choosing a resistor or capacitor for a target tau
- •First-order low-pass/high-pass corner checks
- •Capacitor charge and discharge timing
Example: 10 kΩ and 100 µF
R×C = 10,000×0.0001 = 1 second. The same relationship can be solved backward: with 100 µF and a 1 s target tau, R = 10 kΩ.
What the result does not assume
- •Real capacitors and resistors have tolerance, leakage, ESR, source resistance and load effects that can shift measured timing.
- •The cutoff result is the ideal first-order RC pole magnitude; an actual surrounding circuit can change the transfer function.
Frequently asked questions
Can I solve for resistance or capacitance from a target time constant?+
Yes. Choose resistance or capacitance as the target, enter tau plus the other component value, and the calculator rearranges τ = RC.
What happens after one RC time constant?+
An ideal charging capacitor reaches about 63.2% of its final step voltage; during discharge about 36.8% remains.
Is a capacitor fully charged after 5τ?+
Not mathematically. At 5τ an ideal charging capacitor is about 99.33% of its final value, which is often treated as practically settled.
How is RC cutoff frequency related to tau?+
fc = 1/(2πτ), so a larger time constant produces a lower first-order corner frequency.
Semantic next steps
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