rc circuit frequency calculator

What This RC Circuit Frequency Calculator Does

This tool calculates the cutoff frequency (also called corner frequency or -3 dB frequency) of an RC circuit. Whether you are building a simple low-pass filter to smooth noise or a high-pass filter to block DC, the same cutoff formula applies.

Enter your resistor value and capacitor value, choose units, and the calculator instantly returns:

• Cutoff frequency in Hz (automatically scaled to kHz/MHz when helpful)
• Angular cutoff frequency in rad/s
• Time constant (τ = RC)
• Equivalent normalized component values in ohms and farads

Formula Used

Main equation

The RC cutoff frequency is:

f_c = 1 / (2πRC)

Where:

• f_c = cutoff frequency (Hz)
• R = resistance (ohms)
• C = capacitance (farads)

Useful rearranged forms

• R = 1 / (2πf_cC)
• C = 1 / (2πf_cR)

These are useful when you know your target frequency and want to pick a resistor or capacitor.

How to Use the Calculator

1. Enter the resistor value.
2. Select the resistance unit (Ω, kΩ, or MΩ).
3. Enter the capacitor value.
4. Select the capacitance unit (F, mF, µF, nF, or pF).
5. Click Calculate Frequency.

The displayed cutoff frequency is where output amplitude becomes about 70.7% of passband amplitude (or half power, i.e., -3 dB).

Quick Design Examples

Example 1: Audio low-pass filter

Suppose you choose R = 10 kΩ and C = 100 nF. The cutoff is approximately 159.15 Hz. Frequencies below this are mostly passed, and higher frequencies are increasingly attenuated.

Example 2: AC coupling high-pass filter

For R = 100 kΩ and C = 1 µF, cutoff is around 1.59 Hz. This is useful when you want to remove DC offset while preserving slow-changing signals.

Low-Pass vs High-Pass RC Circuits

The same RC pair can create different behavior depending on where you take output:

• Low-pass: output across capacitor, passes low frequencies.
• High-pass: output across resistor, passes high frequencies.

In both cases, cutoff frequency remains 1/(2πRC).

Component Selection Tips

Choose practical resistor values

Common resistor values in E12/E24 series are easiest to source (e.g., 1k, 2.2k, 4.7k, 10k, 22k). Staying between roughly 1 kΩ and 1 MΩ is often a good starting range in many analog designs.

Match capacitor type to application

• Ceramic capacitors: great for small values (pF to nF), compact and affordable.
• Film capacitors: stable and low distortion for precision/audio paths.
• Electrolytic capacitors: larger values (µF+), polarity-sensitive in most cases.

Common Mistakes to Avoid

• Mixing units (for example, entering µF but assuming nF).
• Forgetting that cutoff is not a hard wall—response changes gradually.
• Ignoring source/load impedance, which can shift real-world cutoff frequency.
• Using overly large resistor values and introducing noise sensitivity.

FAQ

Is this calculator valid for both low-pass and high-pass RC filters?

Yes. The cutoff formula is identical for first-order passive RC low-pass and high-pass filters.

What does -3 dB mean in practical terms?

At cutoff, output voltage is about 0.707 of input in amplitude. Power is reduced to half, which is why cutoff is often called the half-power point.

Can I use this for timing circuits too?

Absolutely. The calculator also shows the time constant τ = RC, which is central in charging/discharging behavior, delays, and pulse shaping circuits.