cutoff frequency calculator

What is cutoff frequency?

Cutoff frequency is the point where a filter starts to significantly attenuate a signal. For first-order filters, this point is conventionally defined at -3 dB, where output power is half the input power and output voltage is approximately 70.7% of the input.

In practical electronics, cutoff frequency helps you decide which frequencies pass through a circuit and which frequencies get reduced. Whether you're building an audio tone control, cleaning sensor noise, or designing a basic communications stage, cutoff frequency is one of the first values you calculate.

Common formulas for first-order filters

RC filter

For both RC low-pass and RC high-pass filters, the cutoff frequency is:

f_c = 1 / (2πRC)

• R in ohms (Ω)
• C in farads (F)
• f_c in hertz (Hz)

RL filter

For RL low-pass or RL high-pass filters, cutoff frequency is:

f_c = R / (2πL)

• R in ohms (Ω)
• L in henries (H)
• f_c in hertz (Hz)

How to use this cutoff frequency calculator

1. Select the filter type: RC or RL.
2. Enter the resistance value and choose its unit.
3. Enter capacitance (for RC) or inductance (for RL) and choose its unit.
4. Click Calculate.
5. Read the cutoff frequency, angular frequency, and time constant in the result panel.

Quick design examples

Example 1: RC low-pass for noise reduction

Suppose you choose R = 1 kΩ and C = 100 nF. Then: f_c ≈ 1 / (2π × 1000 × 100e-9) ≈ 1591.55 Hz. Frequencies much higher than ~1.6 kHz are attenuated.

Example 2: RL high-pass stage

If R = 220 Ω and L = 10 mH, then: f_c ≈ 220 / (2π × 0.01) ≈ 3501.41 Hz. Frequencies above this region pass more effectively for a high-pass arrangement.

Practical engineering tips

• Component tolerance matters: A 5% resistor and 10% capacitor can noticeably shift your cutoff point.
• Load interaction: Real source and load impedances modify the effective R value.
• Parasitics: At high frequency, PCB trace capacitance and inductor resistance affect behavior.
• Simulation first: Validate with SPICE before hardware if your margin is tight.

Frequently asked questions

Is cutoff frequency the same as resonant frequency?

No. Cutoff frequency usually describes the -3 dB point of a filter section. Resonant frequency typically refers to LC or RLC circuits where reactive effects create a peak or natural oscillation point.

Can I use this for both low-pass and high-pass?

Yes, for first-order RC and RL circuits the same cutoff formula applies. What changes is component placement and whether frequencies above or below cutoff are passed.

Why is there a time constant in the output?

Time constant links frequency and transient behavior. For RC, τ = RC; for RL, τ = L/R. It tells you how quickly the circuit responds to changes and is useful for step response analysis.

Conclusion

A reliable cutoff frequency calculation is the foundation of filter design. Use this tool to quickly estimate first-order RC and RL behavior, then refine your design with tolerance checks, simulation, and measurement. Small parameter changes can produce meaningful frequency shifts, so precise inputs and practical validation are always worth the extra effort.