CSS Box Model
The box model explains why a declaration of width alone may not match the size you measure. In this border-box example the content width is 224 pixels.
How it works
- A box includes content, padding, border, and margin. Use the developer-tools box diagram to see which layer creates a gap.
- Prefer logical properties such as padding-inline when a layout should adapt to different writing directions.
A practical example
The HTML supplies the content; the CSS below styles it. Open both together in the playground, then change the lesson rules at the end of the stylesheet.
<article class="card"><span class="badge">This Saturday</span><h2>Neighbourhood repair club</h2><p>Bring a small project. Share a useful skill.</p><a href="#booking">View the workshop</a></article>
body { margin: 0; padding: 24px; font-family: system-ui, sans-serif; color: #25332d; background: #f4f7f2; }
.card { padding: 20px; background: white; border: 1px solid #d4dfd6; }
h2 { margin-top: 0; }
p { line-height: 1.6; }
a { color: #17634c; }
.badge { display: inline-block; margin-bottom: 12px; font-size: .8rem; }
.layout { display: grid; gap: 16px; }
.photo { width: 100%; height: 160px; background: linear-gradient(135deg, #e4af6b, #668cba); }
button, input, select { font: inherit; padding: 8px; }
label { display: block; margin-block: 12px 4px; }
/* This lesson */
.card { width: 280px; padding: 24px; border: 4px solid #367b5a; box-sizing: border-box; }
The box model explains why a declaration of width alone may not match the size you measure. In this border-box example the content width is 224 pixels.