FFT Calculator Online — Free Fast Fourier Transform Tool
Generate a signal, compute its FFT, and visualize the frequency spectrum — entirely in your browser. No uploads, no sign-up.
Time Domain
Amplitude vs. sample index
Frequency Domain (FFT Magnitude)
Magnitude vs. frequency bin
How This FFT Calculator Works
This calculator generates a digital signal of your chosen waveform type and sample count, then computes the Fast Fourier Transform using the Cooley–Tukey radix-2 algorithm — entirely in your browser with JavaScript.
- Signal generation — we synthesize N samples of the selected waveform at the chosen frequency.
- Windowing — the signal is multiplied by the selected window function to reduce spectral leakage.
- FFT — the radix-2 FFT decomposes the windowed signal into complex frequency bins.
- Magnitude spectrum — we compute
|X[k]| = √(re² + im²)for each bin and plot the first N/2 bins (positive frequencies).
The result is a frequency-domain view showing exactly which frequencies are present in the signal and their relative strengths.
Understanding the Output
A pure sine wave at frequency f produces a single spike at bin f. A square wave contains only odd harmonics (f, 3f, 5f, 7f …) with amplitudes decreasing as 1/n. A sawtooth contains all harmonics with amplitudes decreasing as 1/n.
When to Use a Window Function
If your signal is not perfectly periodic within the sample window, select Hann or Hamming to taper the edges and get cleaner spectral peaks. Blackman provides even lower sidelobe leakage at the cost of a wider main lobe.
FFT Calculator FAQ
What is the FFT (Fast Fourier Transform)?
The FFT is an algorithm that efficiently computes the Discrete Fourier Transform (DFT). It converts a time-domain signal into its constituent frequencies, reducing the computation from O(N²) to O(N log N). The most common implementation is the Cooley–Tukey radix-2 algorithm, which this tool uses.
What is the difference between DFT and FFT?
The DFT is the mathematical transform that maps N time-domain samples to N frequency-domain coefficients. The FFT is simply a fast algorithm for computing the DFT — they produce identical results. The FFT requires the sample size to be a power of 2 (64, 128, 256, …).
How do I read the frequency spectrum output?
The x-axis shows the frequency bin index (0 to N/2). Each bin corresponds to a frequency of (bin × sampleRate / N). The y-axis shows the magnitude — taller bars mean more energy at that frequency. A pure sine wave produces a single spike at its frequency.
What are window functions and why should I use one?
Window functions (Hann, Hamming, Blackman) taper the edges of your signal to reduce spectral leakage — artificial frequency content that appears because the DFT assumes the signal repeats. For most real-world analysis, a Hann window is a good default.
Can I use this FFT calculator for audio analysis?
Yes. You can generate standard waveforms (sine, square, sawtooth, triangle) at any frequency and see their harmonic structure. For analyzing your own audio, check our Spectrogram tool or CSV-to-Spectrum converter.
Cite This Tool
APA
Fourier Tools. (2026). FFT Calculator Online — Free Fast Fourier Transform Tool. Retrieved February 17, 2026, from https://fourier.tools/tools/fft-calculator
BibTeX
@misc{fouriertools2026fft,
title = {FFT Calculator Online — Free Fast Fourier Transform Tool},
author = {Fourier Tools},
year = {2026},
url = {https://fourier.tools/tools/fft-calculator},
note = {Accessed: 2026-02-17}
} Embed This Calculator
Copy the snippet below to embed the FFT Calculator on your site or blog.
<iframe src="https://fourier.tools/tools/fft-calculator?embed=1" width="100%" height="520" style="border:1px solid #e5e7eb;border-radius:12px;" loading="lazy" title="FFT Calculator – Fourier Tools" ></iframe>