Low Pass Filter Calculator
Welcome to Omni's low pass filter calculator. Whether you're designing an entire sound system complete with a bass boost, or just want to remove highfrequency noise, the lowpass filter calculator can help you create the perfect lowpass filter circuit for your needs. Read on to learn:
 What a lowpass filter is;
 The difference between passive and active lowpass filters; and
 Whether inductors (that offer inductive reactance) can be used for lowpass filters.
How do I use the lowpass filter calculator?
Using the lowpass filter calculator is easy! Here's how:

Select the filter type you're designing. Based on this choice, the lowpass filter calculator can magically transform into an RC lowpass filter calculator, an opamp lowpass filter calculator, and others. We offer the following filter types:
 An RC lowpass filter;
 An RL lowpass filter;
 An noninverting opamp lowpass filter; and
 An inverting opamp lowpass filter.

Input the values you are using. The passive (RC and RL) filters allow component values and the desired cutoff frequency. To the active (opamp) filters, you can also add a gain to your AC signal. Read on if you want to learn how they work!
💡 The lowpass filter calculator is omnidirectional. You can enter whatever values you know and the calculator will work out the rest according to the selected filter type.
What is a lowpass filter?
A lowpass filter is an electronic circuit that removes higherfrequency components from a given AC signal. In other words, it blocks high frequencies and lets low frequencies pass — hence the name "lowpass filter".
Let's illustrate a lowpass filter's frequency response (a fancy word for how a filter amplifies or dampens signals of certain frequencies). Take a look at the typical Bode plot for a lowpass filter below:
In the graphic above, we can see that a lowpass filter's frequency response is relatively flat up to $f_c$, after which it descends quickly. That point $f_c$ is called the cutoff frequency and it's the defining parameter of a lowpass filter.
💡 The lowpass filter has an ideal (theoretical) and a real (practical) version. Ideally, the cutoff frequency $f_c$ marks the sharp transition point between the Bode plot's flat and sloped regions. In reality, that transition is gradual, and $f_c$ instead marks where the filter's frequency response hits the $3\ \text{dB}$ mark.
Past the cutoff frequency, the filter's frequency response drops at a slope of 20 decibels per decade — or, equivalently, the amplitude of a signal passing through the filter decreases by a factor of 10 for every tenfold increase in the signal's frequency.
💡 Note, that the explanation above is mostly true for all lowpass filters, even though the ones we discuss in this article are only firstorder filters. The only difference between firstorder and higherorder filters (such as RLC circuits) is that their highfrequency signal response drops at a higher rate than 20 decibels per decade.
When dealing with signals containing more than one frequency, a lowpass filter will remove the highfrequency components while leaving lowfrequency components untouched. For audio systems, this would typically mean the treble is dampened, and the bass is seemingly amplified.
Different types of lowpass filters — passive vs. active lowpass filters
While all lowpass filters perform the same function, many different lowpass filter circuits exist. They are split into two categories, passive and active, and this dichotomy can be categorized further:
 Passive lowpass filters are built with only the three linear passive components: the resistor, the capacitor, and the inductor. They include:
 RC lowpass filters; and
 RL lowpass filters.
 Active lowpass filters can be built with active components, most notably the operational amplifier (or opamp). They include:
 Inverting opamp lowpass filters; and
 Noninverting opamp lowpass filters.
The RC lowpass filter
The RC lowpass filter consists of a resistor (with resistance $R$) and a capacitor (with capacitance $C$) in the configuration shown below:
The RC lowpass filter is probably the most wellknown passive lowpass filter. It is simple to design and build thanks to the simple formula for its cutoff frequency $f_c$:
The RC lowpass filter takes advantage of the reactive properties of the capacitor, whose impedance $Z_C$ decreases as the signal frequency $f$ increases:
Higher frequencies can easily pass through the capacitor and skip the load at $v_\text{out}$; lower frequencies are blocked from flowing through the capacitor and must instead travel through the output terminals. In this way, lower frequencies are delivered to the load, and higher frequencies are filtered out.
The RL lowpass filter
We haven't seen any inductors yet, but don't worry — inductors can be used for a lowpass filter just as easily as capacitors and resistors! Similar to the RC filter, the RL lowpass filter is another passive filter, and is constructed with a resistor $R$ and an inductor $L$ in this configuration:
Its cutoff frequency can be determined with this formula:
Inductors behave in the opposite way to capacitors — their impedance $Z_L$ grows with the frequency $f$ of the signal it's conducting:
As a result, the inductor in the RL lowpass filter blocks higher frequencies from ever reaching $v_\text{out}$, while allowing lower frequencies to pass through the inductor and reach the output.
The inverting opamp lowpass filter
The inverting opamp lowpass filter is an active filter, meaning it doesn't use just passive components (resistors, capacitors, and inductors). This particular filter incorporates an operational amplifier (opamp) that feeds back into itself with the feedback resistor $R_f$ and capacitor $C$. It's designed as follows:
Lucky for us, the formula for the cutoff frequency is simple:
Because opamps are powered by an external voltage source that is independent of the input signal, the inverting opamp lowpass filter introduces a gain $G$ by which the input signal $v_\text{in}$ will be multiplied to obtain $v_\text{out}$:
It's important to note that the inverting opamp lowpass filter's gain $G$ is negative. Therefore, your output signal $v_\text{out}$ will be flipped to be exactly 180° out of phase with the input signal $v_\text{in}$ — that's why it's called an "inverting filter". For some circuits (like audio systems) this effect doesn't matter much (as speakers don't care about polarity) but in other applications, the flip must be kept in mind. If you want to avoid this flip, then jump on over to the section on NONinverting opamp lowpass filters.
💡 Remember that opamps have a maximum DC voltage that can be supplied to their rails — consult the component's datasheet to find it. Whatever its value, this DC supply voltage limits the output of your opamp filters. If your gain $G$ is too large, or you supply $v_\text{in}$ with signals that are too large, your output will be distorted.
The noninverting opamp lowpass filter
The noninverting opamp lowpass filter doesn't flip the signal like the inverting opamp filter does — its output $v_\text{out}$ retains the polarity of its input $v_\text{in}$.
The formula for its cutoff frequency is:
As an active filter, the noninverting opamp lowpass filter also introduces a gain $G$:
Because of the $1$ in $G = 1 + R_f/R_g$, the gain will always be at least $1$, i.e. $G\ge1$. So, if you were planning on reducing the amplitude of your signal with $G < 1$, you might want to consider the inverting opamp lowpass filter instead.
FAQ
What does a lowpass filter do? Why use a lowpass filter?
Lowpass filters block high frequencies and admit low frequencies. With suitable cutoff frequencies, lowpass filters can be applied to:
 Sound engineering, specifically amplifier design;
 Noise reduction (useful in telecommunications); and
 Biomedical devices, such as vital sign monitoring and pacemakers.
What is the cutoff frequency of a lowpass filter?
A lowpass filter's cutoff frequency, f_{c}, is the frequency at which the filter's gain is −3dB. Frequencies lower than the cutoff frequency are admitted through the filter, and higher frequencies are blocked. For the typical RC lowpass filter, f_{c} = 1 / (2πRC).
How do I build a lowpass filter?
To build a lowpass filter, follow these easy steps:
 Select a suitable filter type (RC, RL, opamp, etc.).
 Determine your desired cutoff frequency, f_{c}.
 Calculate the components' values based on the above.
What components do I need for a 1 kHz lowpass filter?
You can build an RC lowpass filter with a cutoff frequency of 1 kHz using a 3.3 kΩ resistor and a 47 nF capacitor (which are standard resistor and capacitor values). Such a circuit will deliver an exact cutoff frequency of
f_{c} = 1 / (2π × 3.3 kΩ × 47 nF) = 1.0261 kHz
Remember to keep components' tolerances in mind — consider measuring them with a multimeter!