Starting from vector artwork
SVG is the best possible source for a favicon because it has no resolution. A logo drawn as paths can be rendered at 16 pixels or 1600 pixels with equally crisp edges, and it usually carries a transparent background by default. The catch is that neither Windows nor the ICO format understands vectors, so at some point the drawing has to be rasterised. How and at what size that happens determines the quality of the icon.
How the converter rasterises your SVG
The file is loaded into an image element and drawn onto a canvas at the size the SVG declares: its width and height attributes, or the viewBox if those are missing. If the SVG declares no size at all, the tool falls back to 512 by 512 pixels. That single rendering becomes a PNG, and it is then scaled to 16, 32, 48, 64 and 256 pixel squares, each saved as PNG and written into one .ico container. So the pipeline is vector to one bitmap, then bitmap to five sizes, not five separate vector renders.
The practical consequence: an SVG with width and height set to 24, which is common for icon sets exported from design tools, is rendered at 24 pixels first and then upscaled to 256. That produces a blurry large entry. Before converting, open the SVG in a text editor and either remove the width and height attributes, leaving only the viewBox, or set them to 512. Either way every entry becomes a clean downscale.
Features that may not survive the trip
The browser renders SVG faithfully for shapes, gradients, masks and text set in installed fonts. Things that commonly break are external references: linked images, stylesheets loaded by URL, and fonts fetched from the web, none of which are available when a file is rendered from a local blob. Text in a font you do not have installed falls back to a default face. Convert text to outlines in your editor to avoid surprises. Scripts inside the SVG are not executed, and CSS animations are captured at their first frame.
Designing for 16 pixels
Vector precision does not help when a stroke is thinner than one pixel at the final size. A 1 pixel line in a 512 pixel drawing becomes a faint grey smear at 16 pixels. Favicons that read well at tab size use a single bold shape, generous negative space and at most two or three colours. Many brands keep a separate simplified mark for this purpose. If yours has fine detail, make a reduced version of the SVG and convert that instead of the full logo.
Typical output size
An SVG logo of 4 KB with flat fills typically becomes an ICO of 15 to 35 KB, most of it in the 256 pixel entry. Gradients and drop shadows push that towards 60 to 80 KB. These figures are typical, not guaranteed. A non-square viewBox gets stretched to fit each square, so adjust the viewBox to equal width and height, with the artwork centred, before converting.
Consider shipping the SVG as well
Modern browsers accept SVG directly in a link rel icon tag, and it scales perfectly on every display. The recommended setup is both: the SVG for current browsers and the .ico from this page as a fallback for older ones and for Windows shortcuts. The favicon generator assembles the full set. For a plain raster export of the same drawing at any size, the SVG to PNG converter uses the same rendering path. Nothing is uploaded at any point; the SVG is rendered and packed entirely on your device.