What is a Gerber File? — RS-274X, Excellon & X2 Explained

The Handoff Format Every Fabricator Speaks

If you've ordered a PCB, you've zipped a folder full of cryptic extensions — .gtl, .gbl, .gto, .drl — and emailed it to a fab house. Those are Gerber files. They're the universal language between PCB design tools (KiCad, Altium, Eagle, EasyEDA) and manufacturing machines (photoplotters, drill presses, pick-and-place heads). No Gerber, no board. Here's what's actually inside them and why the format has survived for 40+ years.

What's Actually Inside a Gerber File

Each Gerber file is a plain-text stream of 2D drawing commands (RS-274X format) describing a single PCB layer — top copper, bottom copper, top silkscreen, solder mask, and so on. Think of it as the manufacturing equivalent of a vector PDF: aperture definitions (D-codes) say what shape to stamp (round pad? square? rectangle?), and coordinate commands (G/M codes) say where to stamp them. Open one in Notepad and you'll see lines like G01X1000Y2000D01 — draw a line from the current position to (1,2) mm. That's it. The whole file is just a list of stamps and paths.

What the Format Guarantees

• One file per layer — top copper, bottom copper, each inner copper, top/bottom silkscreen, top/bottom solder mask, board outline. A 2-layer board ships 8–10 files. A 12-layer HDI board can push past 20.
• Excellon for drills — a separate plain-text format (not technically Gerber) for drill-hit coordinates and tool sizes. Plated and non-plated holes usually split into two files.
• RS-274X vs X2 — RS-274X (1998) is the baseline every fab reads. Gerber X2 (2014) is a superset that embeds layer function, net names, and component data so CAM tools do automatic DFM. Modern KiCad/Altium default to X2.
• Plain text and vendor-neutral — any editor can read a Gerber, any CAM tool can consume one. There's no proprietary binary format, no vendor lock-in, no licensing fee.
• Metric or imperial — the format supports both via the %MO directive. Most Western fabs use imperial (mils), most Asian fabs accept either. Mixing up units is the #1 cause of scale errors.
• Pick-and-place (CPL) is separate — X/Y rotation and component reference travel in a centroid file alongside the Gerber, not inside it. Assembly houses need both.

How to Work With Gerbers

1. Open pcbviewer.app — drop your Gerber zip (no extraction needed) onto the viewer.
2. Check the layer stack — top copper, silkscreen, solder mask should render crisply. If a layer is flipped or mirrored, your export settings are wrong.
3. Verify drill hits land on pads — misaligned drills come back as holes punched through traces. The 3D view makes this impossible to miss.
4. Check the board outline — an unclosed outline polygon forces the fab to guess the board edge, usually resulting in a rectangular fallback.
5. Zip and send — most fabs accept a zipped folder with all layer files plus Excellon drill files. Include a README listing stackup and net count for complex boards.

Why Gerber Still Wins After 40 Years

Gerber is plain text, vendor-neutral, human-readable, and every PCB fab on Earth supports it. Attempts to replace it — IPC-2581, ODB++ — have richer metadata but require licensed tooling or vendor buy-in, so they've stayed niche. Meanwhile RS-274X export is a checkbox in every EDA tool, and the file you export today will open in a CAM station 20 years from now. The format's boring reliability is exactly why it won.

When You'll Meet Gerber Files

• Ordering prototypes from JLCPCB, PCBWay, OSH Park — every fab wants a Gerber zip upload
• DFM review before committing to volume — CAM engineers run your Gerbers through DRC to flag tolerance issues
• Stencil manufacturing — laser-cut SMD stencils are driven by the paste-layer Gerber
• Archiving a design you no longer have source for — Gerbers reverse-engineer back to a rough board with any EDA tool
• Cross-checking a third-party layout — even without the source project, you can load their Gerbers and verify before paying the fab

Why a Browser Viewer for Gerbers

Your Gerber zip contains your board's full IP — every trace, every component position, every net name. Uploading to a cloud viewer is a red flag for anything pre-launch or under NDA. MakerSuite 3D parses RS-274X and Excellon entirely in your browser via pure JavaScript — no server, no upload, no queue. The file you drop never leaves the tab. Same parsing logic fabricators use internally, just ported to run client-side.