Every fabrication package contains one file that is not like the others. The Gerber files describe the board’s printed layers — copper, mask, legend, paste — but the holes are specified somewhere else: in a separate drill file, most often in Excellon format. If Gerber files are pictures of layers, the drill file is a work order for a machine: a list of tools and the coordinates where each tool should plunge. Understanding that difference explains most drill problems, including the classic “the holes are the wrong size, or in the wrong place” support question. This article covers what the file contains, how it relates to the Gerber layers, and how to check it in Gerbv — the practical routine is in viewing drill files (Excellon).
Why drills live in their own file
The split is historical and practical. Drilling is a mechanical operation done by a different machine from the photoplotters Gerber was written for, and its data needs are different: not shapes, but diameters and positions. So while a Gerber layer says “draw this”, the Excellon file says “with tool such-and-such, drill here, here and here”. The fab reads your copper layers from the Gerbers and your holes from the drill file, and the two must agree — in units, in origin and in scale — because they were almost certainly exported in two separate steps of the same CAD session.
Anatomy: the tool table and the hole list
An Excellon file has two parts. First comes a tool table: a numbered list of tools, each with a diameter — T1 might be a small via drill, T2 a mounting-hole drill, and so on. The number of tools and their sizes are the design’s complete hole inventory in miniature. Then comes the body: blocks of coordinates grouped by tool, meaning “every coordinate in this block is drilled with this tool, at this diameter”. Open the file as text and you can read both parts directly — it is one of the most human-legible files in the package, and glancing at the tool table is a fast sanity check: if a diameter you expect is absent, or an absurd one is present, the export deserves a second look before it goes anywhere.
How drill data meets the Gerber layers
On their own, coordinates are just numbers. They become holes in your board only when the drill file is overlaid on the copper layers with matching units and a shared origin — which is why the standard check is visual and comparative. Load the drill file together with the top copper in a viewer and the truth is immediate: holes sit centred on their pads, or they visibly do not. Gerbv views Excellon drill files alongside Gerber layers for exactly this purpose; get it from the download page and follow the checking flow in drill holes the wrong size or offset when something looks off. The viewer also reports drill-code statistics, so the tool inventory in the file can be compared against the design’s expectations.
Why scale and offset errors happen
Drill files fail in families. Size errors usually trace back to the tool table or to units: a diameter interpreted in the wrong unit becomes a hole wildly too big or too small, even though every position is right. Position errors come in two shapes. A uniform offset — every hole shifted by the same amount — points to an origin mismatch between the drill export and the Gerber export. A scale pattern — holes that register at one corner and drift further out of place across the board — points to a format mismatch: drills written with a different number format or zero-suppression convention than the reader assumes, so coordinates are stretched rather than shifted. The diagnostic habit is to separate the two questions early: are the diameters right (a tool-table question) and are the positions right (a units/origin/format question)? Each has a different fix, and both fixes belong in the CAD export settings, not in the viewer.
The 60-second drill check
Before any order: open the drill file’s text and eyeball the tool table against your design’s hole sizes. Then overlay the drill file on the copper in Gerbv, zoom to a corner and to a mounting hole, and measure one known hole or one known board dimension. If sizes, positions and one measurement agree, the file is doing its job. The general context — how drill data fits the rest of the package — is covered in what is a Gerber file and the Gerber viewer hub, and the tools for this check are compared in the best free Gerber viewers roundup.
Frequently asked questions
Is an Excellon file a Gerber file?
No. Gerber files describe printed layers as images; an Excellon file is a separate drill program with a tool table of diameters and coordinate lists per tool. Gerbv views both, stacked together, so holes can be checked against the copper pads they belong to.
What is the tool table in a drill file?
It is the header list that assigns each tool number a diameter. The body of the file then gives coordinates per tool. Reading the tool table is a quick audit: every hole size in your design should appear there, and no unexpected diameter should.
Why are my drill holes scaled wrong when the Gerbers are fine?
Because the two were exported separately. A unit, origin or number-format mismatch in the drill export alone produces shifted or stretched holes while the Gerber layers stay correct. Diagnose size and position separately, then fix the drill export settings in your CAD tool.