RC Filter Calculator

We can use this RC filter calculator to instantly show us the cutoff frequency just by entering resistor value and capacitor value. It is very useful for students and engineers who want to build filters for signal processing, amplifier input, noise reduction, waveform shaping and many other projects.

What Values We Have to Enter

So first thing is resistor value which we must enter in ohms. We can enter values like 1000, 4700, 10000 etc for 1k, 4.7k, 10k respectively. No need to write "k" or symbol, just enter the real number in ohms.

Second thing is capacitor value. In older calculators we had to enter in farads which was very confusing. But here we made it simple. We just enter directly in microfarads which we normally use in real circuits. For example we can enter 0.1, 0.47, 1, 10 etc for 0.1µF, 0.47µF, 1µF, 10µF and so on.

So now we enter R in ohms and C in microfarads and then press the calculate button. That is all. The output will show the cutoff frequency in Hz.

Formula That We Use

So the formula used in background JavaScript is based on standard RC filter cutoff equation:

Fc = 1 / (2 × π × R × C)

But since we are entering capacitor in microfarads so we convert it internally to farads by dividing by 1,000,000.

That means that real formula used is:

Fc = 1 / (2 × π × R × (C / 1,000,000))

This formula works for both low-pass filter and high-pass filter. That depends on actual circuit connection, not on formula. Like for low-pass we put resistor first then capacitor to ground. For high-pass we put capacitor first then resistor to ground. But cutoff frequency is same for both.

Example Use

Suppose we enter:

R = 1000 (that is 1k ohm)
C = 1 (that is 1 microfarad)

Then the formula becomes:

Fc = 1 / (2 × π × 1000 × 0.000001)
Fc = around 159.15 Hz

So the calculator will instantly show us 159.15 Hz as the cutoff point. That means all signal frequency below 159Hz will pass easily in low-pass and above 159Hz will pass in high-pass.

Why This Calculator is Useful

This tool is very helpful when we are designing audio filters, amplifier input filters, PWM smoothing or even power supply ripple filtering. It helps us to find the exact Fc value and match it with our application needs. Also since it accepts microfarad directly, we do not get confused with too many decimal zeros like 0.000001 etc.

This tool works for normal RC circuits. It does not cover complex active filters or multi-stage filters. But for basic analog projects this is more than enough.