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Gerber file extensions: identify, then verify
Translate familiar filename conventions into review candidates. Check the actual content, layer order and export manifest before assigning a manufacturing function.
Original engineering guidance 路 References checked September 28, 2026
What do Gerber file extensions mean?
Gerber file extensions are naming conventions used by exporters and organizations to suggest the function of a file. Common examples include GTL for top copper and GBL for bottom copper. A suffix is a useful hint, but it is not a reliable substitute for reading file content, checking attributes and matching the image to the released stackup. Changing the suffix does not change the underlying format.
The same extension can be used for different roles, and different extensions can carry the same format. A set of descriptively named GBR files may be clearer than a mixed collection of historical suffixes. Generic TXT files are particularly ambiguous: an archive can include ASCII drill data, drill reports and ordinary instructions with the same extension. The correct question is 鈥渨hat does this file contain and represent?鈥�
This page owns the naming and mapping task. Start with the Gerber file fundamentals if the artwork format itself is unfamiliar. For all documents needed to request fabrication, use the PCB file-format handoff guide.
Common conventions: candidates, not automatic assignments
| Suffix or pattern | Common interpretation | What still needs verification |
|---|---|---|
| .GTL / .GBL | Top / bottom copper | Actual content, physical side and revision |
| .GTS / .GBS | Top / bottom solder mask | Correct side and intended opening geometry |
| .GTO / .GBO | Top / bottom legend | Orientation, visibility and print exclusions |
| .GTP / .GBP | Top / bottom solder paste | Assembly side, stencil scope and revision |
| .GBR / .GER | Gerber image | Function from attributes, name and manifest |
| .GKO / .GM1 | Mechanical or outline candidate | Whether this is the authoritative finished profile |
| .G1, .G2 and similar | Numbered-layer convention | Exporter-specific ordering and physical function |
| .DRL / .TXT | Possible drill data | Actual encoding, units and whether it is a report |
| .GBRJOB | Gerber job metadata | Associated image set and compatible reader |
The table intentionally leaves uncertainty visible. In particular, a mechanical layer can contain assembly information, dimensions or multiple boundaries rather than the final routed outline. Inner-layer numbering can represent a software naming convention rather than a direct top-to-bottom manufacturing order. Ask the release owner for a mapping rather than silently making a choice.
Map filenames without uploading design files
Paste one filename per line. The mapper recognizes a small documented set of common suffixes, flags repeated names without regard to letter case, and identifies unknown or numbered conventions for manual review. It accepts filenames only, not paths, file contents or archives. After inspecting the files yourself, record a manual mapping as filename | agreed role, one per line. The worksheet checks that each recorded filename exists in your list and is assigned only once. It counts files still lacking a recorded role; the meaning of your entries remains your responsibility. This detailed mapping record complements the file-format hub鈥檚 broader package classifier.
Gerber Filename Mapper
This distinction matters when several PCB jobs are placed in the same folder. A plausible GTL file could belong to a different project, while a correctly named inner layer could be an obsolete revision. The tool does not read artwork, inspect embedded attributes, verify checksums, count copper layers or decide whether a package is complete. Use its result to organize your next checks, not to authorize manufacturing.
When you export the text brief, the entered filenames and candidate roles are included. The tool does not send them to a server. Change any filename and the old result becomes unavailable until you run the mapping again. Use 鈥淪tart my project鈥� to clear the illustrative example before recording real release information.
A stronger mapping uses three independent clues
First examine the export source and its manifest: board revision, expected stackup and named output function. Second inspect meaningful content such as a recognized file-function attribute. Third open the image in a suitable viewer and compare it with neighboring files and the drawing. These clues should agree. A disagreement is a reason to investigate, not a reason to average the answers.
A four-layer job illustrates the method. The stackup defines L1 through L4. The manifest assigns one image to each copper position. In the viewer, outer-layer connector lands and expected internal plane clearances are consistent with those assignments. If an image is labeled 鈥淟2鈥� but the source record calls it bottom copper, stop and resolve the conflict at the exporter rather than relying on whichever label looks more familiar.
The provides authoritative format documentation. Its role is different from an application-specific filename table. A syntax specification describes what commands mean; a naming convention describes how an exporter or organization labels files. A trustworthy manufacturing handoff preserves both relationships without pretending they are interchangeable.
Naming mistakes that create expensive ambiguity
Hidden extensions: an operating system may hide the final suffix. A file displayed as 鈥渢op.GTL鈥� could actually be named 鈥渢op.GTL.txt.鈥� Inspect the complete filename before converting, renaming or uploading it. A upload portal rejecting a file type is not evidence that the underlying image is corrupt.
Case-only duplicates: 鈥淏oard.GTL鈥� and 鈥渂oard.gtl鈥� can behave differently across filesystems and archive tools. Avoid relying on case to distinguish separate outputs. The mapper flags these names together because a portable release should remain understandable after extraction on a different system.
Old and new exports mixed together: a renamed replacement can leave the previous file beside it. The receiving team then sees two candidates for the same physical layer. Build the release in a clean output directory, preserve the prior archive separately and make one set authoritative. Do not ask the manufacturer to infer revision priority from sorting order.
Mechanical output reused as profile: a familiar GKO or GM1 name may contain keepouts, drawing frames or dimensional notes. The manufacturing profile must represent the intended finished boundary and relevant cutouts. Review the actual geometry and written tolerances rather than treating a suffix as an instruction to route every visible line.
A practical manifest for a mixed-name archive
Imagine receiving 鈥渃ontrol.GTL,鈥� 鈥渃ontrol.GBL,鈥� 鈥渋nnerA.gbr,鈥� 鈥渋nnerB.gbr,鈥� 鈥渙utline.gm1鈥� and 鈥渃ontrol.txt.鈥� The first two names suggest outer copper, but nothing yet proves the inner order, the profile owner or the meaning of the text file. A useful manifest explicitly assigns innerA and innerB to approved stackup positions and identifies whether control.txt is drill data or a report.
Add the board identifier, revision and a short statement of the coordinate origin. Where holes are split into plated and non-plated sets, assign each population to its own named data and drawing requirements. If the construction includes blind or buried holes, preserve the layer-span relationship as well. A single 鈥渄rill鈥� label does not convey that information.
The result should be understandable without the original designer sitting beside the CAM operator. That does not mean writing a long document for every small board. A compact table with filename, function, side or layer span, and revision can eliminate several rounds of clarification. The complete package also needs construction requirements covered by the file-format hub.
Match the naming convention to the exporter
Software options can change filenames without changing the intended image. For example, KiCad offers a Protel-style extension option as well as descriptive GBR naming. The KiCad fabrication export guide keeps that choice separate from X2 attributes, copper selection and drill generation. Do not use the naming option as a proxy for format compatibility.
Altium output settings and different output routes also deserve an explicit record. A direct export and a saved Output Job should not be assumed to use the same configuration. Follow the Altium release checklist to identify the actual source and output route before changing names. Application-generated reports can help, but they still need to match the archive you send.
If a fabricator requests a particular convention, ask whether the requirement concerns filename recognition or actual file syntax. Those lead to different remedies. A descriptive manifest may resolve a naming ambiguity; an unsupported format feature requires a controlled compatibility decision and re-export. Randomly changing every file to GBR cannot solve the latter.
Give the next reviewer a clear question
When asking for help, provide the suspicious filename, intended function, exporter version and what you observed in a viewer. State whether the uncertainty concerns copper ordering, profile ownership or drill interpretation. A narrowly described ambiguity can be answered without guessing at the rest of the project.
For drilling-specific uncertainty, use the NC drill file guide to separate coordinate interpretation from hole classification. An extension cannot tell you whether a hole should be plated, whether its diameter is a finished requirement, or which layers a blind feature connects. Those decisions must come from the released design and agreed manufacturing definition.
Gerber extension questions
Is GBR better than GTL or GBL?
Neither suffix alone determines data quality. Descriptive GBR filenames combined with verified attributes and a manifest can work well. Historical suffixes can also be understandable when the exporter and receiving team agree on their meaning. Choose clarity and compatibility, then inspect the actual outputs.
Can I identify a drill file from TXT alone?
No. TXT is a generic text suffix. Inspect whether the content is coordinate and tool data, a summary report or ordinary notes. Preserve any declared units and format, then check the drilling geometry in a suitable viewer. A drill report listing hole counts is useful but does not necessarily contain machine-readable locations.
Should all unknown names be rejected?
No. An unfamiliar filename can contain valid, well-documented data. Mark the role unresolved and reconcile it with the source manifest, attributes and image. Rejecting unknown suffixes without review can discard required information; automatically accepting them as copper can create a different error.
Bring the files and the release requirements together
Send the board revision, construction, quantity and the file or engineering question you need reviewed. Identify unresolved requirements so the response can address them.
Send requirements for reviewOptional: attach the downloaded worksheet manually. This guidance does not certify a design, approve a shipment or replace the applicable authorized standards.