What Is a PLT File? An Extension, Not One Format
Three letters, twelve owners. Jump to your question below, or work through it in order.
Three letters, twelve owners. Jump to your question below, or work through it in order.
PLT is a file extension, not one file format. It is short for plot. Most PLT files hold plotter instructions written in Hewlett-Packard Graphics Language, and that part is plain text you can read in Notepad.
Some hold PostScript or PCL under the same name, because the plotter driver chose the language. At least twelve unrelated programs use .plt for something else entirely.
So the first question is never how to open it. The first question is what you have, and the first three bytes answer that in seconds.
CAD software and drawing programs write one when you choose Plot to file rather than straight to the printer, and the name stuck to the result. Nothing about that name says what went inside.
A plotter drew by moving a pen, printing using lines instead of dots, which is why technical drawings went to one and photographs did not. Large-format printing inherited the same plotting language and never gave it up.
In most cases what went inside is Hewlett-Packard Graphics Language, the two-letter command language HP named in 1977 for its pen plotters. One line of it looks like PA5000,1000; and means plot absolute, to those coordinates. It is ordinary text, so any editor opens it.
Directories call it a vector-based plotter file, which is right about the common case and silent about every other one. Several also say the format is based on the HPGL file format, which inverts the relationship: HP-GL is what goes inside, not the thing PLT was built on.
In plenty of other cases it is not HP-GL at all. Your file may hold PostScript instead. It may hold printer data, binary data from a science program or game artwork. The extension is identical every time.
One competitor says this out loud and then stops. Software Companions writes that the PLT extension does not define one single internal format. True, and the useful half is missing: how to find out what yours is.
.plt, short for plotapplication/vnd.hp-HPGL, registered with IANAx-fmt/83, an outline record from 2005Here for the language rather than the file? Our HPGL file guide covers the commands, plotter units and HP-GL/2 in full. This page is about the file on your disk and what is in it.
Right click, open with Notepad, read the first line. Three bytes separate plotter files from PostScript from Tecplot datasets, and you do not have to install anything to see them.
This is the step everybody skips, and it is the one that saves the afternoon. PLT files carry no magic number and no header, so no viewer can tell you what one is. The opening bytes can.
On Windows, right click the file, choose Open with and select Notepad. On macOS, drag it onto TextEdit. Nothing is modified by looking. Then match what you see against the table below.
| The file starts with | What you have | What to use |
|---|---|---|
IN; DF; SP1; PA IP | HP-GL or HP-GL/2 plotter commands | hp2xx, Inkscape, ideaMK PLT Viewer |
%!PS | PostScript wearing the wrong extension | Ghostscript, or rename it to .ps |
An escape character, then E | PCL printer data, often with HP-GL/2 inside | GhostPCL, or send it to the printer |
#!TDV | A Tecplot binary dataset, not a drawing | Tecplot 360 |
| Two capitals and digits, no semicolons | A cutting plotter dialect | The cutter's own software |
| Unreadable bytes, none of the above | One of the other programs that claim .plt | Section 6 lists them |
Readable text in the window means a plotter file nine times out of ten, and it also means the file is intact. Walls of symbols mean binary, and the first few characters still name the owner more often than not.
Long single lines are normal. Many plotter files have no line breaks at all, so Notepad shows one line running off the screen. Turn on word wrap and read it anyway. That is a writing habit, not damage.
For the plotter case, six free programs cover every system and you already have one of them. Windows and macOS ship nothing that reads one, which is the entire reason this looks hard.
The page this one replaces named three programs and every one cost money. Start with the text editor you already have, then take one of the free converters below. These are the programs that open PLT files for nothing, on Windows, macOS and Linux.
| Software | Runs on | Why choose it |
|---|---|---|
| Any text editor | Everything | Tells you what the file is. Notepad, TextEdit, VS Code. Start here |
| hp2xx | Windows, Mac, Linux | The free converter. Reads HP-GL, writes SVG, PDF, DXF, PNG and more |
| Inkscape | Windows, Mac, Linux | Imports the line work and lets you edit it. See the warning below |
| ideaMK PLT Viewer | Windows | Free viewer for HP-GL, HP-GL/2 and PLT, with image export |
| IrfanView | Windows | Free, fast and already on plenty of machines |
| Ghostscript | Windows, Mac, Linux | For the PLT that turns out to hold PostScript |
Paid programs read PLT too. AutoCAD writes them far more readily than it reads them. CorelDRAW Graphics Suite imports and exports PLT for cutting plotters, and ViewCompanion Standard, Pro and Premium all open one and batch convert. None of them does anything the six free ones cannot.
Open it in Notepad. Free, instant and it answers the real question.
Run hp2xx. One line turns plots into PDF or SVG.
Inkscape skips circles and arcs. Its HP-GL import extension handles lines and misses the CI command, which is an open issue on Inkscape's own tracker. Good for a drawing of straight runs, wrong for anything curved. Convert with hp2xx instead and open the result.
Trouble with the computer rather than the PLT? Our Windows error guides and macOS fixes cover the file association problems behind most double-click failures.
Four causes, and the first two are about software rather than the file. Nothing you already own knows what a plot file is.
Double clicking PLT files on a clean Windows install does nothing useful, because no part of Windows knows what one is. Nothing is wrong with your file.
Notice what is absent from that list. There is no virus explanation, no registry explanation and no disk-error explanation, which is what the page this one replaces offered.
Do not upload the file to an online converter to find out what it is. Plot files are somebody's engineering drawing or somebody's cutting path. Notepad answers the identification question on your own computer in less time than the upload takes.
A plotter PLT has no header, no index, no checksum and no compression. It is a list of instructions read from the top, and that explains almost everything people find strange about it.
Open any plotter PLT and you get two capital letters, some numbers and a semicolon, over and over. PU lifts the pen, PD puts it down, PA moves it to a coordinate and SP1 selects pen 1. That is the shape of the entire file.
Nothing describes the drawing overall. There is no page size, no object list, no layer table and no thumbnail, because the file was never meant to be a document. It was one conversation with one machine, and somebody saved it.
| A plotter PLT | A DXF or PDF | |
|---|---|---|
| Header | None | Yes, naming the format and version |
| Byte signature | None | Yes |
| Page size | Not stored | Stored |
| Layers and objects | No. Only pen movements | Yes |
| Checksum | None | Varies |
| Readable in Notepad | Yes | DXF yes, PDF no |
| Can be cut in half and still draw | Yes, up to the cut | No |
That last row is the useful one. Lose the end of your PLT and the first half still plots perfectly, because every command stands on its own. Lose the end of a PDF and you have nothing.
The commands themselves belong to the language rather than the file. So do the plotter units behind wrong-size output and the encoded coordinate form that makes part of some files unreadable. The HPGL file format guide covers every one.
HP-GL/2 can carry a binary section. The encoded polyline command packs coordinates into characters that look like noise in a text editor. Your file is not damaged. That run of symbols in the middle of readable commands is compression, and every proper reader handles it.
At least twelve unrelated programs write something called PLT. Windows Update is one of them. Nintendo DS color palettes are another.
File extensions were never allocated by anybody. Three characters and first come, so nothing stops two programs choosing the same three. PLT was an obvious choice for anything that plots, and then for anything that shortens to P, L and T.
Three file types use the PLT file extension on FileInfo alone. Nine of them are listed on file-extensions.org, and the two directories do not agree on which nine.
There is no registry of extensions and never was. Media types have IANA and domain names have ICANN. The three letters after the dot in a filename have nobody at all.
| What writes it | What the file actually is |
|---|---|
| CAD plot to file | HP-GL, HP-GL/2, PostScript or PCL. The common case |
| Cutter software | HP-GL cutting paths for vinyl and laser cutters |
| Tecplot | A binary data file holding multiple frames and datasets. Opens #!TDV |
| BioWare Infinity engine | Bitmap graphics from Baldur's Gate and Icewind Dale |
| Microsoft Windows Update | Update and security fix data |
| MicroStation | A plot driver configuration. Settings, never a drawing |
| Nintendo DS games | A color palette |
| Modtran | Atmospheric modeling output |
| dnaLIMS | A DNA sequencing sample sheet |
| CA-Clipper 5 | Pre-linked transfer data, obsolete |
| Psion EPOC | System data from the ancestor of Symbian |
| X-Wing vs TIE Fighter | A saved pilot, from 1997 |
The page this one replaces printed a shorter version of that list and said nothing about it. Seven program names sat under the heading “PLT file associated with Several software”, with no hint of why one extension had seven owners or what readers should do about it.
What you should do about it is read the first bytes. Tecplot datasets and AutoCAD plots have nothing in common except three letters, and no amount of trying different viewers will change that.
Plotter commands, almost certainly. Go to section 7 and convert it.
Find the program. Where the file came from names its owner faster than any tool.
PLT files under a Windows system folder are not yours to open. Windows Update writes files with this extension. If you found one under Windows rather than in your own documents, leave it alone and close the folder.
hp2xx is free, runs everywhere and writes PDF, SVG, DXF and PNG from one command. Choose your output by what the drawing has to do next, not by what is familiar.
Converting PLT files is lossless in the direction that matters. They hold coordinates, so vector output keeps every one of them. You only lose something by converting to pictures.
| You need to | Convert to | Why |
|---|---|---|
| Send it to somebody | Opens everywhere, prints at the right size, nothing to install | |
| Edit the drawing | SVG | Stays vector. Inkscape and Illustrator both open it |
| Put it back into CAD | DXF | AutoCAD, LibreCAD and FreeCAD all import it |
| Put it in a document | PNG or TIFF | Simple, and the only option that throws detail away |
| Keep it for the archive | Keep the PLT too | It is small, it is text, and it is the original |
hp2xx is GNU software, free on Windows, macOS and Linux. It reads HP-GL, works out the drawing extent for itself and writes the format you name. One file:
Swap pdf for svg, eps, png or tif and the rest stays the same. For an entire folder, let the shell do the repeating:
On Windows that line runs in Git Bash or WSL. The sources are untouched either way, which matters when the plot file is the only surviving copy of a drawing.
Nothing is lost. Coordinates stay coordinates.
One way only. Lines become pixels and will not scale again.
If hp2xx refuses the file, you have learned something. It reads HP-GL and nothing else, so a clean refusal means your PLT holds PostScript, PCL or data. Go back to section 2 and read the first bytes.
Comparing the destinations? The PDF file format is where a finished drawing goes to be read, and EPS files hold vector artwork for print. PostScript files are what a PLT turns out to be more often than people expect.
Your drawing did not decide what went in the file. The plotter driver you selected did. That one fact explains why two PLT files from the same program can be completely different inside.
In AutoCAD you open the Plot dialog, choose a plotter, tick Plot to file and press OK. The dialog saves something with a .plt name and the job looks finished. What it saved depends entirely on the plotter you chose two steps earlier.
The third case is the one that catches people. Choose a Windows system printer and the file holds that printer's own data stream, which nothing else reads reliably. It still lands with a .plt name.
Autodesk still ships a nonsystem HP-GL/2 driver precisely so that the first row stays possible. Producing a plot file for somebody else to read? That is the row to aim for, because it is the only one with free readers on every system.
The reverse case is worth saying too. AutoCAD writes plot files far more readily than it reads them. A PLT arriving in your inbox is often easier to open in hp2xx than in the program that made it.
Select an HP-GL/2 plotter. The result opens free on any system.
Plot to PDF instead. No identification problem for anybody.
An empty PLT is usually an empty selection. Plot to file writes something even when the plot area contains no geometry, so you get 300 bytes of setup commands and nothing to draw. Check the file size before blaming the driver.
PLT is not a dead format. Vinyl cutters and most laser cutters take HP-GL as their cutting path, so sign shops write new PLT files every working day.
Cutting plotters are pen plotters with blades where the pen was. They take the same instructions, move the same way and read the same two-letter commands. Pen down became blade down, and nothing else had to change.
That is why CorelDRAW and Inkscape still export PLT, and why the cutter in your local sign shop speaks the language named in 1977. Cutting a shape out of vinyl is exactly the problem HP-GL was designed for.
SP to tool choice or blade force rather than color.If your PLT came from a sign shop and will not open in a CAD viewer, that is the explanation. It is a cutting path rather than a drawing, and the machine it was written for is the thing that reads it best.
A cutter PLT is still readable text. Open it and you will see the same pen up and pen down commands. That is the quickest way to confirm the file reached you intact, before you load expensive material.
Nothing in a plotter PLT can corrupt the way people mean. There is no header to lose, no index to go stale and no checksum to fail. Open it in a text editor and you have your answer.
The page this one replaces listed three reasons PLT files become corrupt: hard disk errors, viruses and shutting the computer down with the file open. The same three sentences sat on our HPGL page too, and on a hundred other sites, because they are filler rather than diagnosis.
Here is what actually happens to PLT files. They are lists of self-contained commands with no structure above them, so there is no table to break and no pointer to go stale. Damage can only take one of two shapes.
Both are visible in thirty seconds. Open the file, scroll to the end and look. Plotter files normally finish with a pen-up and page-advance command, something like PU;PA0,0;SP;PG1;. Stopping mid-number means truncation.
Your file is fine. The viewer is the problem, not the bytes.
Truncated. Fetch it again from the source. No repair tool can invent the rest.
No product can repair a truncated plot file. The missing commands are simply absent, and nothing in the format records what they were. Anything advertising PLT repair is selling you a viewer with a different label on it.
Keep the PLT, add a PDF beside it. The plot file is small, readable and original. The PDF is what anybody else will actually be able to open.
For an archive, keep them. PLT files are tiny and they are plain text. For plenty of old drawings yours is the only record that survived the CAD program it came from. Converting costs you nothing and deleting the original costs you the evidence.
For sending anything to anybody, convert first. The person at the other end has the same problem you had at the top of this page, and a PDF spares them all of it.
Keep both. The PLT is the original. The PDF is the copy anybody can read.
Send a PDF. Attach the PLT only when they asked for the plot file.
Everything on this page in one block: what the extension covers, what the registries say and where every fact came from.
Every directory files this under Vector Image Files, which is right for the common case and wrong for several of the others. The PLT file extension your file manager shows names a container habit rather than a format, and that is the honest entry for it.
| Item | Value |
|---|---|
| Extension | .plt, short for plot |
| Kind | An extension, not one format |
| Usual contents | HP-GL or HP-GL/2 plotter commands, plain ASCII |
| Also found holding | PostScript, PCL, Tecplot binary data, game assets, settings |
| Known owners | At least twelve unrelated programs |
| Developer | None. An extension has no developer, and that is the point |
| Language developed by | Hewlett-Packard, HP-GL named 1977, HP-GL/2 the later standard |
| Written by | AutoCAD and other CAD, cutter software, CorelDRAW, Inkscape |
| Byte signature | None. There is nothing to match on |
| Common openings | IN; DF; SP1; PA IP %!PS #!TDV |
| Media type | application/vnd.hp-HPGL, registered with IANA |
| Often served as | application/octet-stream, which means nothing |
| Format registry | PRONOM x-fmt/83, Hewlett Packard Vector Graphic Plotter File |
| Registry status | An outline record. Source date 2 August 2005, never updated |
| Related records | x-fmt/293 and fmt/1174 for the language itself |
| Compression | None, except the encoded polyline form in HP-GL/2 |
| Free software | Any text editor, hp2xx, Inkscape, ideaMK PLT Viewer, IrfanView |
Formats are recognized by their first bytes. A PDF opens %PDF, a PNG opens with eight fixed bytes, a ZIP opens PK. A plotter file opens with whatever command the writing program felt like sending, and a dozen commands are all reasonable openings.
The file utility recognizes thousands of formats by their opening bytes. It could not identify this one at all until a patch landed in December 2022. The person who wrote that patch said plainly that the Hewlett-Packard graphics files have no unique magic pattern. The patch guesses from the first command and nothing more.
PRONOM, the national registry that archivists use, has an entry for PLT and it is a placeholder. The record describes itself as an outline record only, requiring further details, research or authentication. It is dated 2 August 2005 and has not been touched since.
application/vnd.hp-HPGL.The count of owners comes from two directories. FileInfo lists three file types under this extension and file-extensions.org lists nine. The sentence that the extension does not define one internal format comes from Software Companions, and it is the only competitor to say so.
The registry record and its 2005 date come from PRONOM at The National Archives. The media type registration comes from IANA. The missing magic number comes from the file utility mailing list thread behind its December 2022 patch. The driver behavior comes from Autodesk's own plotter driver documentation, and the cutter case from vendor documentation for sign-cutting software.
Read the first bytes. Section 2 turns that into an answer.
It is probably intact. Section 10 shows how to check.
Want the language rather than the file? Our guide to the HPGL plotter language covers the commands, the plotter units behind wrong-size output and the two versions of the language in full.
Turned out to be something else? The PostScript format and the EPS format cover the two files a plotter driver most often writes under a PLT name.
Moving a drawing between CAD programs? IGES files and STEP files were built for that job, and CorelDRAW CDR files are where plenty of cutter artwork starts.
Stuck on your computer rather than your drawing? Our Windows fixes and our macOS guides cover file associations and the double-click failures behind them.
New to all of this? What a file format is covers the ground underneath, and our file format guides list the rest.
Got a PLT whose first bytes match nothing here? Send us the opening line and we will tell you what wrote it.