Cutoff Frequency Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A cutoff frequency result can look convincing even when one unit or assumption is off. Electrical and magnetic results can shift sharply with geometry, frequency, phase, material properties, and whether the quantity is a magnitude or signed value. This page keeps the calculation narrow enough to trace the answer back to the values you enter.

What this calculator does

The Cutoff Frequency Calculator connects Resistance (R), Capacitance (C), Inductance (L) to the page’s cutoff frequency. Supporting values are included only when they follow from the same relationship, so you can compare the headline with the quantities behind it.

How to use it

Enter Resistance (R), Capacitance (C), Inductance (L) using the units shown beside each field. Set Circuit type to match the solve path you want. Keep all values from the same physical case, then check the headline result and any supporting values before changing one input at a time for comparison.

How the calculation works

For an RC circuit, fc = 1/(2πRC). For an RL circuit, fc = R/(2πL). The circuit-type selector determines which reactive component enters the calculation.

Example

With the default setup (Circuit type = RC circuit; Resistance (R) = 1000 Ω; Capacitance (C) = 1 µF; Inductance (L) = 0.1 H), the page reports cutoff frequency of 159.154943 Hz. This is a useful baseline: change one input and confirm the new value follows the proportionality in the formula.

How to interpret the result

Read the cutoff frequency with its sign, magnitude, phase, frequency, geometry, and unit as applicable. A field, reactance, power factor, loss, or flux value should be compared only with a quantity defined in the same way.

Limitations and notes

Real electrical and magnetic systems can add parasitics, finite geometry, temperature dependence, nonlinear materials, frequency-dependent losses, tolerances, and measurement uncertainty beyond the ideal relationship shown here. Recheck Resistance (R), Capacitance (C) first if the result looks surprising, because an incorrect unit or definition there can dominate rounding error. Safety-critical or standards-based work still needs the applicable design rules and independent verification.