What Is a Gerber File?
A Gerber file is the standard manufacturing description of one layer of a printed circuit board. This explainer covers where the format comes from, what a file contains, how the layers fit together, and why you should always inspect the files before ordering boards.
The short answer
A Gerber file is a plain, structured data file that describes the artwork for a single PCB layer as a set of shapes: traces, pads, pours and text, positioned with coordinates. It is not a picture, and it is not your design project — it is the flattened, manufacturing-ready output your EDA tool creates when you export fabrication data. The factory’s photoplotters and imaging systems read these shapes to pattern the copper and other layers of the physical board. Because every board house accepts the format, the Gerber file is the common language between designers and manufacturers.
Where the format comes from: RS-274X and Ucamco
The Gerber format is standardised as RS-274X, often called Extended Gerber. The older RS-274D variant described shapes but left important information — such as the definitions of the drawing tools (apertures) — in separate files, which made exchanges error-prone. RS-274X embeds that information in the file itself, so a single file fully describes its layer. The format is maintained by Ucamco, which publishes the specification and provides a reference viewer. Modern tools export RS-274X by default; viewers such as Gerbv support RS-274X and explicitly do not support the legacy RS-274D variant.
One board, many files: the layer set
A single Gerber file describes exactly one layer, so a finished board arrives as a set of files that only make sense together:
- Copper layers — the traces, pads and copper pours for the top, bottom and any inner layers of the board.
- Solder mask — where the protective mask coating is opened, typically over pads, and where it covers everything else.
- Silkscreen (legend) — the printed reference designators, outlines and markings that appear on the finished board.
- Solder paste — the openings used to make the stencil for applying paste to surface-mount pads during assembly.
- Board outline and mechanical details — the board edge, cutouts and related fabrication geometry, exported either as its own layer or as part of another layer depending on the tool.
Drill data is normally not a Gerber file. Holes are described by a separate Excellon (NC drill) file, which lists drill sizes and hit positions. Assembly data is separate again: pick-and-place positions are commonly exchanged as a CSV centroid file. A viewer like Gerbv opens all three kinds together — Gerber layers, the Excellon drill file and a CSV pick-and-place file — and stacks them so you see the complete board. See how that works on our Gerber viewer page.
File extensions and naming
There is no single enforced extension for a Gerber file. The generic extensions .gbr and .ger are common, and some toolchains have historically used .art. Many EDA tools instead name each file after its layer — using suffixes or filename patterns that mark a file as top copper, bottom copper, top mask, top silkscreen, and so on — and the exact convention differs between tools and board houses. The practical lessons are two: never judge a file by its extension alone (viewers detect the content, not just the name), and when you send a set to a factory, keep the names consistent and include a short note or a layer list so each file’s role is unambiguous.
How a Gerber file describes a layer
Inside the file, a layer is built from a small vocabulary of ideas. The file defines a set of apertures — named shapes such as circles, rectangles and more complex pad shapes — and then draws with them: moving to a coordinate and flashing an aperture creates a pad, while drawing a line with an aperture creates a track of that width. Regions can be filled to form copper pours and other large areas. Header information in the file sets the units and coordinate precision the numbers use. That is why RS-274X is sometimes described as a vector description rather than an image: nothing in the file is pixels, so a viewer can zoom indefinitely and measure precisely, and the same file drives imaging equipment at manufacturing resolution. It is also why the format is compact — a board with thousands of identical pads stores the pad shape once and references it thousands of times.
Gerber X2: attributes that describe the files themselves
The Gerber X2 extension adds attributes — structured metadata inside the file. At a high level, attributes can state what a file is for (for example, which layer position and function it represents), identify features such as pads by their role, and carry net information. This matters because it lets software and factories interpret a file set with less guessing from filenames, and it reduces the classic error of a layer being used for the wrong purpose. X2 builds on RS-274X rather than replacing it: the shapes are still described the same way, with the attributes adding meaning on top.
Why you inspect Gerber files before ordering
Your EDA tool shows you the design as you intended it. The Gerber set shows you what will actually be built — and those two views can differ. An export can be mirrored, use the wrong units, omit a layer, keep an old revision of one file in the set, or produce mask and silkscreen results you did not expect. Viewing the exported files in an independent viewer, rather than trusting the design view, catches those problems while they are still free to fix. Stack the layers, overlay the drill file, measure critical features, and confirm the board outline — a few minutes of inspection is the cheapest quality control in the whole manufacturing process.
Gerbv was built for exactly this check: it is free, open source (GPL-2.0-or-later), runs offline on Windows, Mac and Linux, and exports views to PNG, PDF, PS, SVG and DXF for review records. Get it from the download page, follow the install guide for Windows, macOS or Linux, and read the version history if you need an older release.