rc lpf calculator

What this RC LPF calculator does

This rc lpf calculator helps you quickly design a basic first-order low-pass filter made from one resistor and one capacitor. It gives you the key values that matter in real circuits:

• Cutoff frequency (the -3 dB point)
• Time constant (τ = RC)
• Optional attenuation and phase shift at a test frequency

If you are smoothing sensor noise, reducing PWM ripple, or limiting high-frequency content before an ADC input, this is one of the most common filter calculations you will use.

RC low-pass filter equation

For a passive RC low-pass filter, the cutoff frequency is:

f_c = 1 / (2πRC)

Where:

• R is resistance in ohms (Ω)
• C is capacitance in farads (F)
• f_c is cutoff frequency in hertz (Hz)

What does cutoff mean?

At cutoff, output amplitude is about 70.7% of input amplitude (0.707 × Vin), which corresponds to -3 dB. Below cutoff, the signal mostly passes. Above cutoff, attenuation increases gradually at about -20 dB per decade for this first-order filter.

How to use this calculator

Step-by-step

• Enter resistor value and choose Ω, kΩ, or MΩ.
• Enter capacitor value and choose F, mF, µF, nF, or pF.
• Click Calculate.
• Optionally enter a test frequency to see gain (%) and phase shift (degrees).

The calculator converts everything to SI units internally, so mixed unit inputs are handled automatically.

Worked examples

Example 1: Basic anti-noise filter

Suppose R = 10 kΩ and C = 100 nF. Then τ = RC = 0.001 s (1 ms), and cutoff is approximately 159.15 Hz. This is a typical value for taming high-frequency noise on slow sensor signals.

Example 2: Smoother output for PWM

If your PWM base frequency is 20 kHz, you might choose a cutoff far below that carrier frequency to improve smoothing. For instance, R = 4.7 kΩ and C = 1 µF gives cutoff near 33.86 Hz. That produces strong attenuation at 20 kHz while still letting very slow changes through.

Practical design tips

• Component tolerance matters: 5% resistors and 10% capacitors can move actual cutoff noticeably.
• Source and load impedance affect behavior: a real source resistance or heavy load can shift effective R.
• Watch capacitor type: electrolytic, ceramic, and film capacitors each have different leakage and stability traits.
• Use buffering for precision: add an op-amp buffer if loading changes your expected response.
• Cascade stages for steeper roll-off: two RC stages increase attenuation slope beyond a single-pole filter.

Common mistakes

• Mixing units (e.g., entering 100 as if it were nF but leaving unit at µF)
• Forgetting that first-order filters are gradual, not brick-wall filters
• Ignoring the interaction with ADC sample-and-hold input currents
• Placing very large R values in noisy environments, increasing susceptibility to interference

Quick takeaway

An RC low-pass filter is simple, cheap, and effective for many analog conditioning tasks. Use this rc lpf calculator to get fast, consistent values, then validate your design in simulation or measurement if precision is important.