RC Time Constant Calculator
Find the RC time constant and cutoff frequency.
Timing an RC circuit
A resistor and capacitor together set a characteristic timescale, the time constant tau. It governs how fast the capacitor charges and where an RC filter rolls off — core knowledge for electronics.
A 1 kΩ resistor with a 1 microfarad capacitor gives a time constant of 1 millisecond and a cutoff frequency near 159 Hz. After five time constants the capacitor is over 99% charged.
Useful and private
It computes tau and the cutoff in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What is the time constant?
The product of resistance and capacitance, tau. After one tau a capacitor charges to about 63% of the supply, and after five tau it is essentially full.
What is the cutoff frequency?
The frequency where an RC filter halves the signal power, equal to one over two-pi-R-C.
What units are used?
Resistance in ohms and capacitance in farads give the time constant in seconds.
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