Letterbox bar pixel size calculator for video canvases

Enter the source ratio, the target canvas ratio, and the canvas height. The result identifies whether bars belong above and below the picture or on its left and right sides, then reports the size of each equal bar. Matching ratios correctly return zero because the source already fills the canvas without cropping or padding.

Start with the source and canvas ratios

An aspect ratio describes width relative to height, so 16:9 and 1.777777 are two ways to describe the same shape. Supply the source ratio for the picture you want to preserve and the target ratio for the finished canvas. The canvas height establishes the actual pixel scale; its width is derived from the target ratio. The calculator accepts ratios written as W:H, W/H, or a positive decimal, making it suitable for common labels such as 4:3, 16:9, 21:9, and 2.39:1. Use the true display aperture when you know it rather than assuming that a marketing label is exact. For example, some formats casually called 21:9 use slightly different numerical ratios. Small ratio differences can produce visible bars at high resolutions. All dimensions must be positive and finite, and the canvas height must be a whole number of pixels. If the source and target ratios are equal, no padding is necessary, so every bar dimension is returned as zero rather than treated as an error.

Understand top-and-bottom versus side bars

The source is fitted entirely inside the canvas without stretching and without removing any picture content. When the source is wider than the target canvas, its width touches the canvas edges first. Its fitted height is then smaller than the canvas height, leaving unused space split evenly above and below the image. The result calls this orientation top_bottom and reports the height of each bar. When the source is narrower than the target canvas, its height touches first and unused horizontal space remains. That space is divided equally between the left and right sides, producing a left_right orientation and a width for each side bar. This second case is often called pillarboxing, although both cases are handled by the same fit calculation. The bar_size_pixels field always contains the size of one bar, not the combined padding. Dedicated top, bottom, left, and right fields make the result unambiguous for layout code and avoid requiring callers to interpret the orientation before assigning padding.

Apply the result in editing and rendering workflows

Use the returned dimensions when preparing overlays, preview frames, encoded masters, projection layouts, or CSS and canvas compositions. The calculator reports fractional pixels when the geometry does not divide evenly, rounded deterministically to six decimal places. A rendering tool that requires integer coordinates must choose its own rounding policy because rounding both bars independently can otherwise make the final dimensions differ by one pixel. A practical raster workflow may floor one edge and assign the remaining pixel to the opposite edge, while a vector or browser layout can often keep the fractional value. The calculation assumes a centered contain fit: it preserves the complete source, does not crop, and places equal padding on opposing sides. It does not account for anamorphic pixel aspect ratios, rotation metadata, overscan, or safe-area guides; convert those into an effective source display ratio before using this tool. API automation costs $0.002 per request, while the same deterministic calculation can be exposed in a browser runner without sending media files anywhere.