RC Filter Calculator
Design low-pass and high-pass RC filters with cutoff frequency, time constant, and E24 value recommendations.
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Input
Result
Formulas
fc = 1 ÷ (2π × R × C)
τ = R × C
R = 1 ÷ (2π × fc × C)
C = 1 ÷ (2π × fc × R)
At fc, output power is -3dB (half power)
Understanding RC Filters
RC filters are among the simplest and most widely used electronic filter circuits, consisting of just a resistor and a capacitor. They are fundamental building blocks in signal processing, audio engineering, and power supply design. A low-pass RC filter allows signals below the cutoff frequency to pass while attenuating higher frequencies, whereas a high-pass RC filter does the opposite — blocking low frequencies and passing high ones. Both share the same cutoff frequency formula, making this calculator versatile for either configuration.
Formulas
These four equations are interrelated — knowing any two of fc, R, and C lets you calculate the third. The time constant τ determines how quickly the filter responds to changes: after one time constant, a step input reaches approximately 63.2% of its final value. At the cutoff frequency, the output power drops to half (-3 dB) of the input power.
How to Use This Calculator
Frequently Asked Questions
What is the cutoff frequency of an RC filter?
The cutoff frequency (fc) is the frequency at which the filter's output power drops to half of its input power, equivalent to a -3 dB reduction in amplitude ratio. For an RC filter, fc = 1 / (2πRC). Below this frequency, a low-pass filter passes signals with minimal attenuation; above it, signals are increasingly blocked. For a high-pass filter, the behavior is reversed.
What is the difference between low-pass and high-pass RC filters?
In a low-pass RC filter, the capacitor is connected to ground and the output is taken across it — low-frequency signals pass while high frequencies are shunted to ground. In a high-pass RC filter, the resistor is connected to ground and the output is taken across it — high-frequency signals pass while low frequencies are blocked. Both use the same cutoff frequency formula, but their attenuation behavior is opposite.
What is the time constant τ in an RC circuit?
The time constant τ (tau) equals R × C and represents the time it takes for the capacitor to charge to approximately 63.2% of the applied voltage (or discharge to 36.8%). After 5τ, the capacitor is considered fully charged (99.3%). The time constant also determines the filter's roll-off rate: the first-order RC filter attenuates at 20 dB/decade (6 dB/octave) above the cutoff frequency.
Why use E24 standard resistor and capacitor values?
The E24 series provides 24 values per decade with approximately 10% spacing, covering the most commonly available resistor and capacitor values. Using standard values ensures components are readily available, cost-effective, and interchangeable across suppliers. This calculator recommends the nearest E24 value and shows the resulting deviation from your target cutoff frequency so you can evaluate whether it meets your design tolerance.
How do I choose between RC, RL, and RLC filters?
RC filters are the simplest and most common for low-frequency applications — they use inexpensive components and are easy to design. RL filters use inductors instead of capacitors, which can be bulky at low frequencies but are preferred in high-current or high-frequency applications. RLC filters include both inductors and capacitors, enabling sharper roll-off (second-order, 40 dB/decade) and resonant behavior, but are more complex to design and tune.