Introduction
A low pass filter lets low frequency signals through and blocks high frequency ones. This calculator helps you design one fast. Pick your filter type, type in the parts you know, and leave the unknown box blank. The tool solves for it and shows the work.
You can work with four filter types:
- RC low pass — a resistor and a capacitor. The most common passive filter. See also our dedicated RC Filter Calculator.
- RL low pass — a resistor and an inductor.
- Inverting op-amp low pass — an active filter that also adds gain and flips the signal.
- Non-inverting op-amp low pass — an active filter with gain that keeps the signal polarity.
The calculator gives you the cutoff frequency, time constant, phase shift, and more. It also draws Bode plots, pole-zero maps, Nyquist diagrams, and step and impulse responses. You can check how much the filter cuts a signal at any frequency you choose, like 60 Hz hum or switching noise. If you need to work back from a wavelength or period, the Frequency Calculator and Wavelength Calculator pair well with this tool.
Real parts do not come in every value, so the tool snaps your answer to the nearest standard resistor and capacitor sizes. Pick an E6, E12, E24, or E96 series and see the new cutoff frequency and the error. This makes it easy to move from math to a real circuit you can build. Our Resistor Color Code Calculator helps you identify the bands once you have the part in hand.
How to use our Low Pass Filter Calculator
Pick a filter type, type in the values you know, and press Calculate. The tool finds the missing part value, shows the cutoff frequency, time constant, phase shift, step-by-step math, and Bode plots.
Filter type tabs: Choose RC Low Pass, RL Low Pass, Inverting Op-Amp, or Non-Inverting Op-Amp. Each tab shows its own circuit diagram and its own input boxes.
Resistance (R): Type the resistor value and pick Ω, kΩ, or MΩ. You can also type shorthand like 10k or 4.7M. If your resistor is actually a network of parts, work out the total first with the Parallel Resistor Calculator or the general Resistance Calculator.
Capacitance (C): Type the capacitor value and pick pF, nF, µF, mF, or F. Leave it blank if you want the calculator to find it. For series and parallel capacitor combinations, use the Capacitor Calculator.
Inductance (L): On the RL tab, type the coil value and pick pH, nH, µH, mH, or H. Leave it blank to solve for it.
Cutoff Frequency (fc): Type the −3 dB point you want and pick Hz, kHz, or MHz. On the RC and RL tabs, fill in only two of the three boxes and leave one blank.
Desired Gain (Av): On the op-amp tabs, type the passband gain as a plain number, like 10. The non-inverting circuit needs a gain of 1 or more. To convert that gain into decibels, try the dB Calculator.
Anchor resistor (R2, R, or R1): On the op-amp tabs, set one resistor value to lock the design. 10 kΩ works well for most circuits.
Resistor E-Series: Pick E6, E12, E24, or E96 to see the closest real-world part values and how much they shift the cutoff. The Percent Error Calculator is handy if you want to check tolerance stack-ups by hand.
Evaluate at Frequency: Type any frequency, like 50 Hz hum or 100 kHz ripple, then press Evaluate to see the gain, phase, and attenuation there.
Plot and subplot options: Tick the boxes for the magnitude Bode plot, pole-zero map, or Nyquist diagram, and choose phase or group delay for the second plot.
Custom frequency range: Type a start and end frequency for the graphs. Leave both blank to use the auto range of fc/100 to fc×100.
Transient options: Tick the step response, impulse response, final value, or rise time to add them to the results.
Custom simulation time: Type how long the transient plot should run and pick ns, µs, ms, or s. Leave it blank for the auto 5τ window.
What Is a Low Pass Filter?
A low pass filter is a circuit that lets low frequency signals pass through and blocks high frequency signals. Think of it like a screen door: air (low frequencies) gets through, but bugs (high frequencies) do not. Engineers use low pass filters to clean up noise, smooth out signals, and protect audio and sensor circuits from unwanted hiss and buzz.
The Cutoff Frequency
Every low pass filter has a cutoff frequency, written as fc. This is the point where the output signal drops to about 70.7% of the input signal. In decibels, that is a drop of 3 dB, so it is often called the "−3 dB point." Below the cutoff, signals pass almost untouched. Above it, they get weaker and weaker. For a simple one-part (first-order) filter, the signal drops by 20 dB for every 10× jump in frequency.
The Four Filter Types
- RC low pass (passive): A resistor in series with a capacitor to ground. The capacitor shorts high frequencies to ground. Formula: fc = 1 / (2πRC). This is the most common and cheapest low pass filter. It behaves like a frequency-dependent voltage divider.
- RL low pass (passive): An inductor in series with a resistor to ground. The inductor fights fast changes in current, so it blocks high frequencies. Formula: fc = R / (2πL). Used more in power and radio circuits. The Impedance Calculator shows how reactance climbs with frequency.
- Inverting op-amp (active): Adds an amplifier chip. It filters and boosts the signal, but flips it upside down (180° phase shift). Gain = −R2 / R, and fc = 1 / (2πR2C).
- Non-inverting op-amp (active): Also filters and boosts, but keeps the signal right side up. Gain = 1 + R2/R1, and the cutoff comes only from the input R and C.
Passive vs. Active Filters
Passive filters (RC and RL) use only resistors, capacitors, and inductors. They need no power supply and never make the signal bigger — only smaller. Active filters use an op-amp and a power supply. They can boost the signal, and they do not get thrown off when you connect them to another circuit. Basic current and voltage relationships in either case follow from Ohm's Law.
Time Constant, Phase, and Step Response
The time constant (τ) tells you how fast the filter reacts. For RC it is τ = R × C; for RL it is τ = L / R. After one time constant, the output has climbed to 63.2% of its final value. After about five time constants, it is basically done. The phase shift is how much the output lags behind the input in time. At the cutoff frequency, a first-order low pass filter lags by 45°, and far above cutoff it settles near 90°.
Where Low Pass Filters Are Used
- Removing hiss and static from audio signals
- Anti-aliasing filters before an analog-to-digital converter
- Smoothing noisy readings from temperature, pressure, and other sensors
- Cleaning ripple off power supply outputs — size the supply itself with the PSU Calculator
- Blocking radio interference on long cables, where the Wire Resistance Calculator and Voltage Drop Calculator also matter
Picking Real Parts
Math gives you an exact value, but stores only sell certain sizes. These are called E-series values (E6, E12, E24, E96). E24 parts have ±5% tolerance and E96 parts have ±1%. After you round to a real part, your cutoff frequency shifts a little. A good rule: keep resistors between about 1 kΩ and 100 kΩ, then solve for the capacitor. Very small resistors waste power, and very large ones pick up noise. Use the Resistor Calculator to explore combinations, the LED Resistor Calculator for indicator circuits, and the Trace Width Calculator when you lay the design out on a board.