RC Filter Calculator

Q: Which E-series should I pick for resistors and capacitors?

For most projects, E24 for resistors and E6 for capacitors work well. E24 resistors are common and cheap with ±5% tolerance. Capacitors have wider tolerances, so E6 or E12 is usually enough. Use E48 or E96 resistors only when you need very precise values.

Q: What do the Bode plots show?

The magnitude plot shows how much the filter reduces the signal at each frequency, measured in decibels (dB). The phase plot shows how much the filter shifts the timing of the signal at each frequency, measured in degrees. Together they give you a complete picture of filter behavior.

Updated Jun 6, 2026 By Jehan Wadia

Introduction

An RC filter is a simple circuit made from just two parts: a resistor (R) and a capacitor (C). It is used to block certain frequencies while letting others pass through. A low-pass RC filter allows low frequencies through and cuts off high frequencies. A high-pass RC filter does the opposite — it blocks low frequencies and lets high ones pass. The point where the filter starts to cut off signals is called the cutoff frequency.

This RC filter calculator helps you find the cutoff frequency, resistance, or capacitance of a first-order RC filter. Just enter any two of the three values, and the tool solves for the missing one using the formula f = 1 / (2π × R × C). It also shows the nearest standard resistor and capacitor values from common E-series tables, so you can pick real parts for your circuit. The calculator displays Bode plots for both magnitude and phase response, giving you a clear picture of how your filter behaves across a wide range of frequencies. It works for both low-pass and high-pass filter types. If you need to understand basic voltage and current relationships first, our Ohms Law Calculator is a great starting point.

How to Use Our RC Filter Calculator

This calculator helps you find the cutoff frequency, resistance, or capacitance of an RC filter. Enter any two values and it will solve for the third. It also shows Bode plots and nearest standard component values.

Filter Type: Pick either Low-Pass or High-Pass. A low-pass filter lets slow signals through and blocks fast ones. A high-pass filter does the opposite.

Cutoff Frequency (f): Enter the frequency where the filter starts to reduce the signal. You can type a number and pick Hz, kHz, or MHz from the dropdown. Shorthand like 10k also works. For understanding frequency and signal behavior, you may also find our Wavelength Calculator useful.

Resistance (R): Enter the resistor value in your circuit. Pick Ω, kΩ, or MΩ from the dropdown. You can use shorthand like 10k, 2M2, or 4R7. If you need to identify a resistor's value from its color bands, try our Resistor Color Code Calculator.

Capacitance (C): Enter the capacitor value in your circuit. Pick pF, nF, or µF from the dropdown. Shorthand like 4u7, 2n2, or 22p is supported. For more detailed capacitor analysis including energy storage and charge, check out our Capacitor Calculator.

Resistor E-Series: Choose a standard resistor series like E12, E24, E48, or E96. The calculator will show the closest real resistor you can buy.

Capacitor E-Series: Choose a standard capacitor series like E3, E6, or E12. The calculator will show the closest real capacitor you can buy.

Calculate Button: Press this button after entering your values. The results section will show the solved value, time constant, phase, nearest standard parts, and Bode plots for both magnitude and phase response.

An RC filter is a simple electric circuit made from two parts: a resistor (R) and a capacitor (C). These two components work together to block some frequencies of a signal while letting others pass through. The point where the filter starts to block signals is called the cutoff frequency. At this frequency, the output signal drops to about 70.7% of the input, which equals a loss of 3 decibels (dB). To learn more about decibel calculations, see our dB Calculator.

There are two main types of first-order RC filters. A low-pass filter lets low frequencies pass and blocks high frequencies. A high-pass filter does the opposite — it blocks low frequencies and lets high frequencies pass. Both types use the same formula to find the cutoff frequency: f = 1 / (2π × R × C).

The time constant (τ) of an RC filter equals R × C. It tells you how fast the circuit responds to changes. A smaller time constant means a faster response and a higher cutoff frequency. A larger time constant means a slower response and a lower cutoff frequency.

Every first-order RC filter has a roll-off rate of 20 dB per decade. This means that for every tenfold increase in frequency past the cutoff point, the signal strength drops by 20 dB. At the cutoff frequency itself, the phase shift is 45 degrees.

This calculator lets you enter any two of the three values — frequency, resistance, or capacitance — and solves for the missing one. It also finds the nearest standard E-series component values so you can pick real parts that are available for purchase. The Bode plots show you how the filter's gain and phase change across a wide range of frequencies. For related circuit design tools, you might also want to explore our Voltage Divider Calculator, Parallel Resistor Calculator, LED Resistor Calculator, or Impedance Calculator. If you're working with power circuits, our Power Calculator and Amps to Watts Calculator can also be helpful.

Frequently asked questions

What does 'leave one blank to solve' mean?

The calculator needs two values to find the third. Enter any two of the three fields (frequency, resistance, capacitance) and leave the one you want to find empty. Press Calculate and the tool fills in the missing value for you.

What happens if I fill in all three fields?

If you enter all three values, the calculator treats the field you edited most recently as the unknown. It recalculates that value using the other two. This way you can quickly update one parameter without clearing a field.

How does the shorthand input work?

You can type values using letter shortcuts instead of typing full numbers. For resistance, 10k means 10,000 Ω, 2M2 means 2.2 MΩ, and 4R7 means 4.7 Ω. For capacitance, 4u7 means 4.7 µF, 2n2 means 2.2 nF, and 22p means 22 pF. For frequency, 10k means 10 kHz. The unit dropdown updates automatically when you use these shortcuts.

What is an E-series and why should I use it?

E-series are standard lists of component values that manufacturers actually make and sell. If your calculation gives a result like 7,283 Ω, you cannot buy that exact resistor. The E-series selector finds the closest standard value you can purchase. E12 has fewer choices, E96 has many. Pick the series that matches the tolerance you need.

Which E-series should I pick for resistors and capacitors?

For most projects, E24 for resistors and E6 for capacitors work well. E24 resistors are common and cheap with ±5% tolerance. Capacitors have wider tolerances, so E6 or E12 is usually enough. Use E48 or E96 resistors only when you need very precise values.

What does the deviation percentage mean in the results?

The deviation tells you how far off the cutoff frequency will be when you use standard E-series parts instead of the exact calculated values. A deviation of +2.5% means the actual cutoff frequency will be 2.5% higher than your target. Smaller deviation means a closer match.

What is the time constant shown in the results?

The time constant, written as τ (tau), equals R × C. It tells you how fast the filter responds to a sudden change in signal. After one time constant, the output reaches about 63% of its final value. A smaller τ means a faster response and higher cutoff frequency.

What do the Bode plots show?

The magnitude plot shows how much the filter reduces the signal at each frequency, measured in decibels (dB). The phase plot shows how much the filter shifts the timing of the signal at each frequency, measured in degrees. Together they give you a complete picture of filter behavior.

What does the -3 dB line on the magnitude plot mean?

The -3 dB line marks the cutoff frequency. At this point, the output power is half the input power, and the output voltage is about 70.7% of the input. This is the standard way engineers define where a filter starts working.

Why is the phase shift 45 degrees at the cutoff frequency?

At the cutoff frequency, the resistor and capacitor contribute equally to the circuit behavior. This balance creates a phase shift of exactly 45 degrees. For a low-pass filter it is −45°, and for a high-pass filter it is +45°.

What does 20 dB per decade roll-off mean?

It means that for every 10× increase in frequency past the cutoff point, the signal drops by 20 dB (about 10× weaker). This is a fixed property of all first-order RC filters. If you need a steeper roll-off, you must use higher-order filters with more stages.

Can this calculator design a bandpass or notch filter?

No. This tool is for first-order RC filters only, which are either low-pass or high-pass. A bandpass filter needs two stages — one low-pass and one high-pass. You can use this calculator twice with different cutoff frequencies to design each stage separately.

Does the calculator account for real-world component tolerances?

The calculator uses ideal values and does not simulate tolerance effects like temperature drift or parasitic properties. However, the E-series feature helps you pick real components, and the deviation percentage shows how close your actual filter will be to the target.

How do I choose between a low-pass and high-pass filter?

Use a low-pass filter when you want to keep slow, steady signals and remove fast noise or interference. Use a high-pass filter when you want to remove slow drift or DC offset and keep fast-changing signals. The choice depends on which frequencies you want to pass through.

What units should I use for my inputs?

You can use any unit the dropdowns offer. The calculator converts everything internally. Pick whatever is most natural for your values. For example, use kΩ for resistors in the thousands, nF for small capacitors, and kHz for audio-range frequencies. The shorthand input also auto-selects the right unit for you.

RC Filter Calculator

Introduction

How to Use Our RC Filter Calculator