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The CSS border box is the rectangle bounded by the outer border edges in the CSS box model. It includes the content area, padding and borders, while excluding margins.^[ref]

For a simple unfragmented box, let the CSS content box be $120$ by $40$ CSS pixels, padding be $8$ pixels on every side, and borders be $2$ pixels on every side. The border dimensions areAdding six-pixel left and right margins would give a CSS margin box width of $152$, without changing the $140$-pixel CSS border box width.

The sizing edge matters when reversing this calculation. With box-sizing: border-box; width: 140px and the same padding and borders, the content width is $120$ pixels. With content-box sizing, a declaration width: 140px would instead produce a $160$-pixel CSS border box width, assuming no other constraint.

A CSS border box is a layout region, not a claim that every point inside it is opaque or that nothing paints outside it. Border style and color affect appearance, and CSS overflow concerns content extending beyond a box. Distinguishing geometry from painting avoids treating a screenshot's visible ink as a complete measurement of layout size.

The CSS box model describes the content, padding, border and margin regions used to lay out generated boxes. These define a CSS content box, CSS padding box, CSS border box and CSS margin box respectively. A box's declared width must be interpreted together with the applicable sizing rules.^[ref]

For a simple horizontal box with content width $w$, left/right padding $p_l,p_r$, border widths $b_l,b_r$ and margins $m_l,m_r$, the total margin-box width isWith $w=100$, padding ten on each side, borders two on each side and margins ten on each side, the total is 144 CSS pixels. Its border box is 124 pixels wide.

CSS box-sizing changes which area a width declaration sizes. With the same padding and borders but box-sizing: border-box; width: 100px, the border box is 100 pixels and the content width is 76 pixels in this example. Margins are still outside that declared border-box size.^[ref]

The example assumes ordinary nonnegative fixed horizontal values. Vertical margins can participate in CSS margin collapsing, and an inline element split across lines can generate several fragments. The box model supplies the regions; the formatting context determines how particular boxes are generated and positioned.

CSS box-sizing is the box-sizing property that selects which box edge applicable length and percentage sizes refer to. With content-box, such sizes describe the CSS content box; with border-box, they include padding and borders in the CSS border box. Margins remain outside both.^[ref]

For example, use width: 200px; padding: 16px; border: 2px solid, with no other constraints. Under content-box sizing, the content width is $200$ and the CSS border box width isCSS pixels. Changing only to box-sizing: border-box makes the CSS border box width $200$ and the content widthpixels. This is why a width declaration cannot be interpreted independently of its sizing edge.

The content width cannot become negative. If the declared border-box width is only $20$ pixels but total horizontal padding and borders require $36$, the content width floors at zero and the actual border box is $36$ pixels wide. Border-box sizing does not make arbitrarily thick padding disappear.^[ref]

CSS box-sizing changes size interpretation rather than making every property apply to every box. A CSS non-replaced element generating an ordinary inline box still has its width applicability rules. CSS minimum size, CSS maximum size and the surrounding layout also remain relevant to the final CSS used value.

The CSS content box is the innermost box region bounded by the content edges, inside padding, border and margins. It is the area assigned for the box's content by the applicable layout rules.^[ref]

Suppose the content width is $120$ CSS pixels, with eight-pixel left/right padding and two-pixel left/right borders. Then the CSS padding box width is $136$ and the CSS border box width is $140$ pixels. Six-pixel margins on both sides increase the CSS margin box width to $152$ pixels. These are different measurements of the same simple box, not competing answers to one unspecified width question.

With box-sizing: content-box; width: 120px, the declared width requests that content dimension. With box-sizing: border-box; width: 120px and the same padding and borders, the requested border dimension instead leaves $120-16-4=100$ pixels for content, before any further constraints.

A content area is not a guarantee that all painted content fits inside it. A long unbroken item or a descendant can cause CSS overflow. Also, an ordinary inline element can fragment across CSS line boxes; its geometry should not be reduced to one fixed-width rectangle merely because a width declaration appears in its style. Read the content region together with CSS box-sizing, box kind and the layout context.

The CSS padding box is the rectangle enclosed by the outer padding edges. It contains the CSS content box and padding, but excludes the border and margins.^[ref]

For content dimensions $120$ by $40$ CSS pixels and eight-pixel padding on all sides, the padding dimensions areA further two-pixel border on each side makes the CSS border box $140$ by $60$ pixels. Padding is thus space inside the border rather than an external gap between neighboring boxes.

This distinction explains a common design change. Increasing padding while keeping a fixed content width enlarges the outer box. Increasing padding inside a fixed border-box width instead leaves less room for content, subject to the zero floor and other sizing constraints of CSS box-sizing. The same padding declaration can therefore have different consequences for text wrapping depending on the sizing edge.

The CSS padding box can also serve as a reference in particular layout rules, such as some cases of CSS absolute positioning. Use the applicable rule rather than assuming all geometric references mean the CSS content box. Padding geometry alone likewise does not define every painted or interactive boundary; those depend on their own properties and behavior.

The CSS transform reference box supplies the geometrical reference used to interpret a CSS transform and its CSS transform origin. The transform-box property selects that reference. Its size supplies percentage bases, and its position can add an offset to the specified origin.^[ref]

For an own ordinary-box example, use a $100$-by-$60$ CSS pixel content box, zero borders, twenty pixels of left padding, ten of top padding and zero padding on the other sides. Assume the content dimensions are definite and unaffected by competing constraints. The CSS border box is $120$ by $70$, while the CSS content box begins at border-box coordinates $(20,10)$.

With transform-box: border-box, a transform-origin: 50% 50% point is $(60,35)$ in those coordinates. With transform-box: content-box, the same percentage declaration instead givesThe difference comes from both the reference size and its offset. Asymmetric padding makes that distinction visible; two centered symmetric boxes could conceal it.

The function translateX(50%) likewise uses the selected reference width. Its translation component is sixty pixels for the border-box choice and fifty for the content-box choice. Percentage translation and percentage origin therefore require a stated reference rectangle before they can be converted into lengths.^[ref]

The property has additional reference choices for SVG, including object bounds and viewport-based geometry. Their used-value mappings differ depending on whether an SVG element has an associated CSS layout box. The ordinary-box example does not establish a universal default for every SVG element.^[ref]

Reference-box selection does not itself assign a new content width or padding. It tells the transformation calculation which existing rectangle to use. Keeping it separate from layout sizing and from the chosen origin prevents two distinct percentage bases from being conflated.