Advanced Filter Cutoff Frequency Calculator

Calculator inputs

Filter class

Response type

Input voltage

Resistance

Resistance unit

Capacitance

Capacitance unit

Inductance

Inductance unit

Test frequency

Sweep start frequency

Sweep end frequency

Plot points

Example data table

Filter class	Response	Resistance	Reactive element	Cutoff frequency
RC	Low-pass	1 kΩ	0.1 µF	1591.549431 Hz
RC	High-pass	4.7 kΩ	10 nF	3386.275385 Hz
RL	Low-pass	47 Ω	10 mH	748.028141 Hz
RL	High-pass	220 Ω	2.2 mH	15915.494309 Hz

Formula used

RC filters: cutoff frequency is fc = 1 / (2πRC). The angular cutoff is ωc = 1 / RC, and the time constant is τ = RC.

RL filters: cutoff frequency is fc = R / (2πL). The angular cutoff is ωc = R / L, and the time constant is τ = L / R.

For both ideal first-order filters, the magnitude at cutoff is 1 / √2, which equals about 0.707 of the input amplitude or -3.01 dB.

How to use this calculator

Select the filter class: RC or RL.
Choose whether you want a low-pass or high-pass response.
Enter resistance and the correct unit.
Enter capacitance for RC filters or inductance for RL filters.
Add an input voltage if you want the tool to estimate output voltage.
Provide a test frequency to evaluate gain and phase at one point.
Set sweep start, sweep end, and plot points for the response graph.
Press Calculate cutoff frequency to show results above the form.
Use the download buttons to save a CSV summary or PDF snapshot.

FAQs

1. What is cutoff frequency?

Cutoff frequency is the point where a filter’s output falls to 70.7% of the input amplitude. In power terms, it marks the half-power boundary and is commonly called the minus three decibel point.

2. Why do low-pass and high-pass filters share the same cutoff equation?

For ideal first-order RC and RL designs, the cutoff depends on the same component balance. The response shape changes, but the boundary appears where resistance equals the component reactance.

3. Which units can I enter?

You can enter resistance in ohms, kilo-ohms, or mega-ohms. Capacitance supports farads through picofarads. Inductance supports henrys, millihenrys, and microhenrys for practical filter work.

4. Why is the output 0.707 of input at cutoff?

At cutoff, the filter magnitude equals one divided by square root of two. That value is about 0.707, which corresponds to a gain of minus 3.01 decibels.

5. What phase shift appears at cutoff?

An ideal first-order low-pass filter reaches about minus 45 degrees at cutoff. An ideal first-order high-pass filter reaches about plus 45 degrees at the same boundary.

6. Can this calculator be used for active filters?

Yes, when the active stage behaves like an ideal first-order section and the effective RC time constant is known. It is not intended for higher-order, multiple-feedback, or strongly loaded networks.

7. Why does the graph use a logarithmic frequency axis?

Filter behavior usually spans many decades of frequency. A logarithmic axis makes low and high regions readable together and matches how Bode plots are normally interpreted.

8. What happens if I double the reactive component?

Doubling capacitance in an RC filter halves the cutoff frequency. Doubling inductance in an RL filter also halves the cutoff frequency, assuming resistance stays unchanged.