PLT and HP-GL files

A .plt file is almost always an HP-GL or HP-GL/2 plot: the pen moves a CAD program would send to a plotter, saved to a file instead. Drop one into the converter above and it comes back as a vector PDF or SVG, at the size it would have been plotted. The file is read in this browser tab and is never uploaded.

Updated 19 October 2026 · How we test this format

Which conversion do you need?

• PLT to PDF to view, print, share or archive a plot. The page is the plot's own size by default, or A4, A3 or Letter, at 1:1 or fitted.
• PLT to SVG to edit a plot in an illustration app, put it on a web page or send it to a cutter, at real-world size, with a hairline option.

The converter above does both. Pick the format under To before you add your files.

What a PLT file is

HP-GL, the Hewlett-Packard Graphics Language, was created by HP in the late 1970s to drive its pen plotters. A plot file is a list of short instructions, each two letters and a few numbers: select a pen, lift it, move it, put it down and draw to a point. Coordinates count plotter units of 0.025 mm, so 40 units make a millimetre and 1,016 make an inch. Plotters and cutters from other makers learned the language too, and it became the usual way to talk to a plotter.

Here is a complete plot. It picks pen 1 and draws a rectangle 100 mm wide and 50 mm tall:

IN;SP1;PA0,0;PD4000,0,4000,2000,0,2000,0,0;PU;

HP-GL/2, the later revision, kept the same instructions and added what large-format printing needed: line widths, colour palettes, Bézier curves and a compact encoding that packs each coordinate into a few printable characters. It's built into PCL 5, the printer language of HP's LaserJets, and HP's DesignJet large-format printers read it alongside HP RTL, a language for raster images.

The .plt extension names the job, not the language. Many .plt files come from a CAD program's plot-to-file option with an HP plotter driver selected; AutoCAD, MicroStation and Allplan can all do it. Sign-making and vinyl-cutting software writes HP-GL for cutters, and pattern software in the garment trade plots markers with it. Engineering and architecture archives hold plot files too, sometimes long after the program that made them has gone. Files named .hpgl, .hgl, .gl2 and .hpg hold the same kind of content.

Versions and variants we read

• HP-GL. The original language of HP's pen plotters. Old device-control sequences, which set up the plotter rather than draw, are skipped.
• HP-GL/2. Including encoded polylines in their 7-bit and 8-bit forms, pen widths, palettes, polygon fills and Bézier curves.
• HP-GL/2 inside PCL 5. The drawing in a printer job converts; PCL text next to it is left out, and What changed says so.
• PJL print jobs. Jobs that switch to HP-GL/2 with a PJL language command convert like a plain plot.
• HP RTL jobs. DesignJet jobs convert when they carry only HP-GL/2 vectors. A job with raster images in it is refused, even when it has vectors too.

Other programs write other things under the same name, so we look at what's inside rather than trusting the extension. A .plt that holds PostScript, PDF, PCL XL or DM/PL, another plotter language, is refused with a message, as is a print job with nothing but PCL text in it. Tecplot, the engineering data program, saves its data files as .plt too. Those hold numbers rather than a drawing, and this converter doesn't read them.

What we draw

We follow the instructions the way a plotter would: pen moves and encoded polylines, circles and arcs drawn in short straight chords as the plotter draws them, Bézier curves, rectangles, wedges and filled polygons, hatching, line types, clipping windows, scaling and rotation. A plot with several pages, split by page-advance instructions or form feeds, gives one file per page.

Pens become colours. When a plot doesn't set its own, the HP-GL/2 defaults apply: pen 1 is black, then red, green, yellow, blue, magenta and cyan for pens 2 to 7. Pen 0 is white, which doesn't show on a white page unless the plot gives it another colour. A plot that never selects a pen is drawn with pen 1; a real plotter would draw nothing, which isn't much help. Choose All black under Pens for a monochrome copy.

Line widths are where a plot loses most today: every line is drawn with one line weight, and What changed says how many widths the plot asked for. Labels are set in Helvetica in a PDF and in Arial or a similar sans-serif in an SVG, instead of the plotter's stick font, so they take a little more or less room. Many CAD programs turn text into lines or filled shapes before they plot, and that kind of text comes through exactly.

Some settings aren't reproduced: vertical text, line ends and joins, screened vectors, character fill modes, the hatch anchor, merge control and the old HP-GL character sets. When a plot uses one of them, What changed names it. Raster and PCL fill patterns become a light solid fill. Polygons filled with the non-zero winding rule are filled with the even-odd rule instead, which can turn overlapping parts into holes.

A plot can also scale its drawing to the corners of the paper without saying how big the paper is. Then we assume an A4 sheet, What changed says so, and the size may differ from the original print.

How we test

The table above is for PDF output; SVG differs only in how the real size is kept. It comes from running the real converter on 26 licensed test files. Of those, 19 are plots, and every one converts to both PDF and SVG. The other 7 are there to test the refusals: 6 hold PostScript, PCL text, DM/PL or raster images, and 1 isn't a plot at all.

Among the plots are 4 real HP-GL/2 files from the test suite of ezdxf, an open-source CAD library with its own HP-GL/2 reader, and we compare our drawing with that reader's point by point. Between them they hold 41,307 lines and 3,093 filled areas, and 100% of their points match within 0.05 mm in both directions; every plot covers exactly the same area. The other 15 are small plots written for this site, each testing a few features: arcs, fills, line types, labels, scaling, rotation, clipping, several pages, and plots wrapped in printer jobs.

These are checks on geometry and completeness, not a pixel-by-pixel comparison. Our tests convert the whole set again on every change to the code, recount the lines and fills of the real plots, and fail if a number moves.

When a PLT won't convert

The file's row tells you when something stops a conversion. Here's what each message means and what to do.

• "This format isn't on our list yet" This page takes .plt, .hpgl, .hgl, .gl2, .hpg. A plot saved under another name, such as a .prn file from a printer driver, may convert once you rename it to .plt.
• "This file isn't one we can convert with this tool." The file isn't an HP-GL drawing. It holds PostScript, PDF, PCL XL, DM/PL or only PCL text, or it's an HP RTL job with raster images in it. Plot it to file again with an HP-GL/2 driver, or use your CAD program's PDF plotter if a PDF is all you need.
• "This file looks damaged, or it isn't the format its name suggests." There are no HP-GL instructions in the file. It may be a data file that only shares the .plt name, or a file in another format that was renamed.
• "This file is too large to convert on this device. A smaller file or more memory may help." For a plot, this means more than 500 pages, or more detail than one tab can draw safely. Every line, point, curve and fill counts toward a budget of 20,000,000, which stops the conversion before the tab runs out of memory, so more memory won't help with it. Splitting the job into smaller plots in the program that made it will. Files over 1 GB get the same message before they start.
• "This file is on the large side" Plots over 20 MB ask before they start, and you can always continue. Dense plots use a lot of memory on phones and older computers; if the whole tab crashes, it ran out. Closing other tabs gives it more room.
• "Something unexpected stopped this conversion. If trying again doesn't help, reload the page." Reload the page and convert the file once more. If the same plot keeps failing, plot it to file again and try the new copy.

What isn't here, and why

• One PDF for a multi-page plot. Each page is its own file for now, so a plot with several pages arrives as a ZIP.
• Pen widths. The widths are read from the plot and counted in What changed, but our PDF and SVG writers draw every line with the same weight for now.
• Raster images. HP RTL images are rows of pixels, not lines. A job that carries them is refused rather than converted with parts missing.
• PLT to DXF, or to an image. A plot keeps lines, fills and labels, but not the layers, blocks and dimensions of the drawing it came from, so a DXF made from it would be a poor stand-in for the original. If you still have that drawing, export a DXF from your CAD program, and our DXF converter turns it into a PDF or SVG. For a PNG or JPEG, convert to PDF first; Preview on a Mac, for one, exports a page as an image.

Try it with a sample file

No plot of your own to hand? Download sample-plot.plt (16 KB), an A2 test sheet in HP-GL/2 from the open-source ezdxf library, made with Allplan 2022. It has shapes in three pen colours, a hatch swatch, a solid fill and bold text built from filled shapes. It's MIT-licensed; the sample notes say where it comes from.

Tested on 14 October 2026: the sample, converted to PDF, opened in Preview 11.0 on macOS 26.2 as one page at the drawing's size, with every shape, colour, the hatch, the fill and the text in place.

Checked in other software

Last checked on 14 October 2026, with every plot in our test set, the pages of the multi-page plot included:

• Every PDF opened in pdf.js 6.4, the open-source PDF engine behind Firefox's viewer, and in Ghostscript 10.08.
• Every SVG rendered in librsvg 2.63, the SVG library many Linux desktop apps use.