What Are UI Icons and How Do You Choose and Use the Right Icon Set?

UI icons are small, simplified symbols that represent actions, objects, or states inside a user interface — for example a magnifying glass for search or a trash can for delete. They differ from logos (brand identity), illustrations (explanatory or decorative art), and app icons (launcher artwork on a device home screen). You choose an icon set by matching one consistent style to your product, then checking coverage, license, and format before integrating it. This guide explains the decisions in order, using Slim Icons as a concrete example of an open-source set.

What separates UI icons from other visual assets

Because UI icons repeat at small sizes, they must stay legible when scaled down and consistent when placed next to each other. A logo can be detailed; a UI icon usually cannot.

Style choices that define an icon set

Outline vs filled

Outline icons use strokes with open interiors; filled icons use solid shapes. Outline sets tend to read as lighter and more modern in dense interfaces, while filled versions often carry more visual weight and can work better for active or selected states. Many design systems use one set for default states and the other for emphasis.

Stroke width

Stroke width is the thickness of the line in an outline icon, usually measured in pixels. It must be consistent across the whole set, or icons will look mismatched. Slim Icons exposes this directly: the site shows a Stroke Width control set to 1.5 px, plus a Color control and a Reset option, so you can preview the set at your chosen thickness before downloading.

Corner radius and grid size

Corner radius is how rounded the line ends and corners are; grid size is the underlying square (commonly 24×24) that every icon is drawn on. A shared grid is what keeps icons optically aligned in a toolbar or list. If a set mixes grids, icons will appear to jump in size even when their bounding boxes match.

How to evaluate an icon set before committing

Run through these checks in order:

1. Consistency — Do all icons share one stroke width, corner style, and grid? Preview them side by side, not one at a time.
2. Coverage — List the icons your product actually needs (navigation, actions, status, empty states). A set with 200 icons like Slim Icons covers common interface needs but may not include niche domain symbols.
3. License — Confirm whether the set is free, open source, or paid, and what attribution or redistribution terms apply. Slim Icons describes itself as a free & open source icon library and links to GitHub, which is where you verify the actual license terms.
4. Format — SVG is the standard for UI work because it scales cleanly and can be recolored with CSS. Icon fonts are an older alternative; they are harder to align and less accessible, so prefer SVG unless you have a specific constraint.
5. Maintenance — Is the set actively updated? A GitHub repository with contribution and icon-request paths, as Slim Icons provides, signals ongoing upkeep.

Integrating icons into a design system

• Store icons as SVG components, not loose files, so they inherit size and color from your design tokens.
• Standardize sizes — pick two or three (for example 16, 20, 24 px) and use only those.
• Bind color to tokens so icons match text and states automatically.
• Keep a naming convention (for example icon-search, icon-trash) so designers and developers refer to the same thing.

Keeping icons accessible

• Decorative icons (next to a visible text label) should be hidden from screen readers with aria-hidden="true".
• Meaningful icons (icon-only buttons) need an accessible name, such as aria-label="Search" or visually hidden text.
• Do not rely on color alone to convey state; pair it with shape or text.
• Check contrast between the icon and its background against WCAG contrast guidance.

Common pitfalls

• Mixing styles — pulling one filled icon into an outline set breaks visual consistency instantly.
• Scaling issues — raster icons blur when enlarged; SVG avoids this, but only if the source is vector.
• Unclear licensing — "free" does not always mean open source or unrestricted. Check the repository license before shipping.
• Inconsistent stroke width — if you customize one icon's thickness, match it across the set. Slim Icons' stroke control makes this easy to preview but you still need to apply it uniformly.
• Overusing icons — an icon for every label adds noise; use them where they speed recognition.

A quick decision path

1. Define your icon needs and sizes.
2. Shortlist sets that match your visual style (outline or filled, stroke weight).
3. Verify coverage, license, and SVG availability.
4. Preview the full set at your target size and stroke width — Slim Icons lets you adjust stroke and color before downloading.
5. Integrate as components, add accessible names, and document usage in your design system.

If you need a lightweight, open-source outline set with adjustable stroke width and a GitHub contribution path, Slim Icons is a reasonable starting point. If your product needs dense domain-specific symbols or a filled style, evaluate other sets against the same checklist before deciding.