LC Low-Pass Filter Calculator
Calculate cutoff frequency, inductance, or capacitance for 2nd-order passive filters.Assumes ideal components with zero parasitics.
How to Use This Calculator
This tool solves three common filter design problems in seconds. You can calculate the cutoff frequency if you know your inductor and capacitor values, find the inductance you need for a target frequency, or determine the capacitance required to hit a specific cutoff.
Step 1: Pick Your Goal
Start by choosing what you want to solve for in the first dropdown menu. The calculator offers three modes: calculate cutoff frequency, calculate inductance, or calculate capacitance.
If you already have components on hand and need to know what cutoff frequency they’ll give you, select the first option. If you’re designing from scratch with a target frequency in mind, pick one of the other two based on which component value you already know.
Step 2: Choose Your Filter Layout
The second dropdown lets you select the circuit arrangement. You’ll see two options: series inductor with shunt capacitor or shunt capacitor with series inductor.
Most power supply and audio applications use the first layout (inductor in series, capacitor to ground). The second layout is less common but useful when you need the capacitor to block DC at the input.
Step 3: Enter Your Known Values
Type your numbers into the input fields that appear. Each field has two parts: a number box on the left and a unit selector on the right.
Enter the numeric value first, then click the unit dropdown to pick the right scale. For inductance, you can choose nanohenries, microhenries, millihenries, or henries. For capacitance, the options are picofarads, nanofarads, microfarads, or farads. For frequency, you’ll see hertz, kilohertz, megahertz, or gigahertz.
Filter Topology Guide
The term topology just means how you arrange the inductor and capacitor in the circuit. This calculator supports two standard layouts, and your choice affects how the filter interacts with the rest of your system.
Series Inductor, Shunt Capacitor (LC Layout)
This arrangement puts the inductor in the signal path between input and output. The capacitor connects from the output side to ground, creating a shunt element that diverts high frequencies away from the load.
Think of it like a traffic pattern where the inductor acts as a speed bump in the main road. High frequency signals hit that bump and get redirected to ground through the capacitor instead of continuing to the output.
Shunt Capacitor, Series Inductor (CL Layout)
This configuration flips the order by placing the capacitor first. The capacitor connects from the input to ground, and the inductor sits in series between that connection point and the output.
The capacitor hits incoming signals first and shunts high frequencies to ground immediately. Lower frequencies pass through to the inductor, which provides additional filtering before reaching the load.
LC Filter Formulas
Three equations do all the work in this calculator. Each one solves for a different variable, but they all come from the same fundamental relationship between inductance, capacitance, and resonance frequency.
Finding the Cutoff Frequency
Use this equation when you know both component values and want to calculate where the filter starts rolling off:
fc = 1 / (2π√LC)
The cutoff frequency (fc) comes out in hertz when you plug in inductance (L) in henries and capacitance (C) in farads. The pi constant (approximately 3.14159) and the factor of 2 come from the resonance math that defines how LC circuits behave.
Calculating the Inductance
Rearrange the cutoff equation to solve for L when you have a target frequency and a known capacitor:
L = 1 / ((2πfc)²C)
Inductance (L) comes out in henries when frequency (fc) is in hertz and capacitance (C) is in farads. The squared term in the denominator means small changes in frequency create large changes in the required inductance.
Determining the Capacitance
Flip the equation one more time to find C when you know the target frequency and have an inductor on hand:
C = 1 / ((2πfc)²L)
Capacitance (C) comes out in farads when frequency (fc) is in hertz and inductance (L) is in henries. The relationship is identical to the inductance formula, just with L and C swapped in their positions.
Typical Filter Frequencies
Use these ranges as your initial target when you’re beginning a design. You’ll adjust based on your actual component values, load requirements, and how the filter performs in your specific circuit.
Looking for an op-amp-based alternative? Try the Butterworth low-pass filter calculator.
Audio Bass Crossovers: 80 to 250 Hertz
Bass management in speaker systems typically uses cutoff frequencies in this range to split low-frequency content between subwoofers and mid-bass drivers. The exact crossover point depends on your speaker cabinet design and how much overlap you want between drivers.
| Crossover Point | Typical Use |
| 80 Hz | Standard home theater subwoofer crossover, handles deep bass and kick drum fundamentals |
| 100 Hz | Common two-way system split, protects smaller woofers from excessive cone excursion |
| 120 Hz | Compact speaker systems where the main driver can’t reproduce very low frequencies |
| 150 Hz | Car audio subwoofer crossover, accounts for cabin acoustics and smaller enclosures |
| 200-250 Hz | Upper limit for bass management, transitions into mid-bass territory |
Power Supply Filtering: 10 to 100 Kilohertz
Switching power supplies generate high-frequency ripple that needs filtering before it reaches sensitive loads. The cutoff frequency sits well below the switching frequency but high enough that the filter components stay reasonably sized.
| Cutoff Frequency | Application Context |
| 10 kHz | Low-frequency switching regulators, mains-powered linear supply ripple reduction |
| 20-50 kHz | Common range for buck and boost converter output filters |
| 100 kHz | Higher switching frequency converters (300 to 500 kilohertz switching) |
Audio ADC Anti-Aliasing: 20 to 24 Kilohertz
Analog-to-digital converters need input filtering to prevent frequencies above the nyquist limit from folding back into the audible spectrum. The cutoff sits just above the highest frequency you want to capture, creating a buffer zone before the sampling rate’s nyquist frequency.
| Cutoff Frequency | Sample Rate | Reasoning |
| 20 kHz | 44.1 kHz (CD quality) | Captures full audible range, leaves 2.05 kHz transition band to nyquist |
| 22 kHz | 48 kHz (professional audio) | Standard studio recording, wider transition band for gentler filter slopes |
| 24 kHz | 48 kHz or higher | High-resolution recording, accommodates extended frequency response microphones |