📉 RC Filter Calculator

Key formulas

fc = 1 / (2π × R × C)
τ = R × C = 1 / (2π × fc)
Gain (LP): H = 1 / √(1+(f/fc)²)
Gain (HP): H = (f/fc) / √(1+(f/fc)²)
Rolloff: −20 dB/decade (1st order)

Low-pass filter

• Passes frequencies below fc, attenuates above
• At fc: signal is −3 dB (70.7% of input)
• At 10×fc: −20 dB (10% of input)
• Uses: audio treble cut, ADC anti-aliasing, PWM smoothing, noise reduction
• Output taken across C

High-pass filter

• Passes frequencies above fc, attenuates below
• At fc: −3 dB (same as low-pass)
• At fc/10: −20 dB
• Uses: AC coupling, bass cut, DC blocking, differentiation
• Output taken across R

Practical tips

• Load resistance forms a divider with R — use a buffer (op-amp) after the filter
• For steeper rolloff, cascade multiple RC stages (adds −20 dB/decade per stage)
• Preferred R values: 1 kΩ – 100 kΩ for audio/signal work
• Use NP0/C0G ceramic caps for accurate, stable fc