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Original Prusa i3 MK3S Review 2021

[Editors’ Note: While PC Labs was reviewing the i3 MK3S, Prusa Research issued a minor upgrade to it, the Original Prusa i3 MK3S+. The changes should provide a modest performance boost, and more substantially improve durability. Although this review is written for the i3 MK3S that we tested, most everything in it should apply to the i3 MK3S+ as well. Differences between the two models are small, and are detailed toward the end of the review.]

The Original Prusa i3 MK3S is Prusa Research’s flagship 3D printer, descended through many iterations from the Prusa i3, which was sold by the company at its founding in 2012. As befitting a 3D printer that emerged from the RepRap tradition and that has been consistently improved over the years, the i3 MK3S is simple to use and consistently produced good-quality prints in our testing. It is an easy Editors’ Choice pick for a mid-priced 3D printer for hobbyists and makers.

The Anatomy of Prusa’s Flagship

Unlike the Original Prusa Mini, which ships exclusively as a kit, the i3 MK3S is available either as a kit ($749) or fully assembled ($999). Our review unit was the latter, shipped from the Czech Republic. (Note that on purchases of more than $800, U.S. customers may have to pay an import duty on receipt.)

This red-and-black printer measures 15 by 19.7 by 22 inches (HWD), excluding the spool and the spool holder, which sit atop the printer. It is considerably larger than the Original Prusa Mini, which measures 14.6 by 13 by 15 inches (HWD). The i3 MK3S also has a larger print volume, 9.8 by 8.3 by 7.9 inches, compared with the 7-by-7-by-7-inch print volume of the Prusa Mini. The i3 MK3S’s print volume is also larger (at least in two dimensions) than the 6.7 by 10 by 6 inches of the Editors’ Choice-winning Dremel DigiLab 3D45 (a feature-rich but pricier 3D printer), and considerably larger than the LulzBot Mini 2‘s printing space (7.1 by 6.3 by 6.3 inches).

The basic form of the i3 MK3S is a square arch, with the build plate below it in the middle. On each side, extending from the base to the top of the arch, are two vertical metal dowels. The inner dowels are threaded and support a carriage holding the extruder assembly, which can move either vertically (Z-axis) or from side to side (X-axis). The build plate rests on a carriage and can move either toward or away from the user (Y-axis).

In front of that carriage is a control panel with a monochrome LCD, from which the user can load and unload filament, select a file from the SD card and launch a print job, access settings, and run a calibration routine…

Original Prusa i3 MK3S

Behind the arch on the right is the power supply, which accepts an AC cord and has an on/off switch. Behind the arch on the left is an electronics box containing the motherboard and accepting a variety of cables. On top is the USB Type-B port.

The Setup’s a Cinch

As mentioned, our i3 MK3S test unit was of the prebuilt variety. The only thing that required any assembly was the spool holder, and even that was simply a matter of snapping three plastic pieces together and attaching the works to the top of the printer. The holder can fit two spools at once, but can feed from only one at a time, as the i3 MK3S is a single-extruder model. To switch to the second spool, you have to unload the filament already in place, load from the second spool, and extrude until the filament from the first spool has been flushed from the extruder and the new color appears.

Original Prusa i3 MK3S

Although our test unit was not a kit, and we can’t speak to it as such (other than that Prusa Research says that it should take about 8 hours to assemble), it did come with one kit-related item that I recognized from the Original Prusa Mini:  a package of Haribo Goldbären—better known in the U.S. as Gummi Bears. With the kit, you eat them as rewards for completing certain steps as specified in the assembly guide. No such dietary restrictions apply to the pre-assembled version.

The i3 MK3S comes with the 3D Printing Handbook, the user manual for the i3 MK3S. Unlike most 3D printer manuals, which tend to be spartan, the 3D Printing Handbook is a beautiful, professionally printed guide. The handbook is the user guide for both the preassembled version and for the kit. (To actually build the kit version, though, users are directed to the online manual.)

Our preassembled Prusa i3 MK3S arrived already tested and calibrated. There was even a completed test print—the Prusa plaque—on the print bed. Unlike the Original Prusa Mini kit, which took me a long time to calibrate before it would print successfully, the i3 MK3S worked fine right off the bat. Once I loaded the filament, selected a test object, and launched a print job, it was off and running, and never looked back. It printed all of our test objects without a misprint, and in good quality. 

Filament, Connectivity, and Safety

The i3 MK3S supports a variety of filament types, including but not limited to PLA (polylactic acid), PETG (polyethylene terephthalate enhanced with glycol), ABS (acrylonitrile butadiene styrene), ASA (acrylonitrile-styrene-acrylate, an alternative to ABS), and Flex. The i3 MK3S includes a 1-kilogram spool of PLA. Prusa sells its filament (which it dubs “Prusament”) for $24.99 per 1-kilo PLA spool and $29.99 per PETG or ASA spool. You can also use third-party filament, which should be more cost-effective than having Prusament shipped from the Czech Republic.

Connectivity is to a computer over a USB connection. The i3 MK3S has a USB 2.0 Type-B port, the kind commonly found on ordinary printers. It also supports printing from an SD card. Indeed, it comes with one that is filled with test files.

Original Prusa i3 MK3S

As the Prusa i3 MK3S is a totally open-frame printer, it’s important to keep children and pets away from it during printing, as well as any objects in materials such as plastic or paper that could conceivably melt or ignite should they come into contact with the hot extruder. Fortunately, once a print job is finished, the extruder and build plate automatically cool down. Another plus is that the Prusa i3 MK3S is relatively quiet, which isn’t always the case with open-frame printers.

The PrusaSlicer Software

The Prusa i3 MK3S uses PrusaSlicer 2.2 software. It is based on Slic3r, an open-source program that takes 3D object files in STL, OBJ, AMF, or 3MF formats and converts them into G-code files, which are essentially the coded instructions for outputting the object on a 3D printer. Slic3r is an alternative to Cura, another open-source platform, on which the slicing programs of numerous 3D printers we have reviewed are built, including models by Dremel, LulzBot, Monoprice, Ultimaker, and XYZprinting.

Original Prusa i3 MK3S

PrusaSlicer performs the same basic functions that Cura-based programs do, including loading objects (which are depicted to scale on a simulation of the printer’s build plate), editing them (resizing them, changing their position or orientation, adding multiple objects, and the like), setting the resolution and filament type, and adding supports or a brim. I had originally installed PrusaSlicer when testing the Original Prusa Mini, so I added the MK3S from the program’s list of printers. Then you slice the file, codifying the multiple thin layers that will be printed, one at a time, as the printer builds the object.

Original Prusa i3 MK3S

PrusaSlicer lets you select from a Simple, Advanced, or Expert interface, which each offers an increasingly wide range of settings to choose from. I liked the feel of the PrusaSlicer interface, and it let me do what I needed with it, with one exception.

Unsurprisingly, it wasn’t able to slice one object—a representation of the Apollo Command, Service, and Lunar Modules—that I have had no trouble slicing and printing with Cura-based programs. I say “unsurprisingly” because it also couldn’t slice with the Prusa Mini. (I received a message that there was a problem with the file, something about blank layers, and was not able to slice it into a Gcode file.)

You can send your Gcode files over the USB connection to the printer, or you can save them to the included SD card and print straight from that.

Consistently Good Print Quality

I completed 11 test prints with the Prusa i3 MK3S. These included many of our normal test objects, including several commonly described as “stress tests” or “torture tests,” which are primarily used in testing the printing ability of 3D printers. I also printed several of the files provided on the USB key that came with the Prusa Mini.  About half were printed at the default 150-micron (Quality) setting, and half at the 200-micron (Speed) setting. I did not notice any quality difference between the two resolutions.

Original Prusa i3 MK3SOriginal Prusa i3 MK3S

The i3 MK3S was able to print all the objects successfully. The only anomaly I experienced was when printing stopped in the middle of a print, and I was instructed to unload the filament and then to reload it, upon which printing resumed and the print job finished. I am not sure what caused the issue; sometimes with 3D printers, filament stops feeding because of a snag in the spooled filament, but that didn’t seem to be the case here.

The good news is that the printer was able to identify that there was a problem, and offered a solution that fixed it and allowed printing to resume. All too often with 3D printers, in the event of a snag, the print is ruined and has to be scuttled.

Original Prusa i3 MK3S

Even better, print quality for nearly all the objects was above average. The Prusa i3 MK3S did very well in rendering fine detail on all its prints, including on our stress-test objects. Output on our main geometric test object (the tall item pictured above) was right up there with the Prusa Mini in being among the best I have seen. None of the finished prints had any significant problems, and many showed no visible flaws.

Original Prusa i3 MK3SOriginal Prusa i3 MK3S

The i3 MK3S vs. the i3 MK3S+

As noted earlier, while we were reviewing the i3 MK3S, it was tweaked and replaced in Prusa’s line by the i3 MK3S+, a modest upgrade. If you do a shopping search on the i3 MK3S, you will come across a number of entries, mostly from, for the i3 MK3S or i3 MK3S clones. Actually, they are all clones—other companies duplicating the open-source Prusa i3 MK3S—and although many cost less than the i3 MK3S, they are of questionable quality. We would suggest that you stick to Prusa’s own printers, and go for the i3 MK3S+.

Prusa characterizes the changes between the i3 MK3S and its successor, the i3 MK3S+ as minor, offering improved durability with little change in performance. The MK3S+ has a different mesh bed leveling probe called SuperPINDA, which is temperature-independent. However, Prusa says that the previous probe was already precise, and the change was merely compensating for temperature drift. MK3S users will see only a small improvement in first-layer accuracy. This change is more significant for the Original Prusa Mini+, which replaces the Prusa Mini. (Prusa unified the mesh bed leveling probe on all its machines.)

Prusa made a few other hardware improvements to the i3 MK3S+. The Y-axis has metal clips instead of the old U-bolts to hold the bearings. Also, some new plastic parts hold the smooth rods. (Zip ties were used before.) The X-axis has a different belt-tensioning system. The extruder’s plastic parts are also slightly different, to allow for better cooling airflow.

While we can’t definitively evaluate a product we haven’t tested yet, the i3 MK3S+ seems like a positive, if incremental, update to the i3 MK3S that we have reviewed here, with no apparent downside. Also, in the event you own a MK3S or land one in early 2021, Prusa Research says that an upgrade kit, which will in effect convert the MK3S into the MK3S+, will be available sometime in 2021.

A Near-Perfect Open-Frame Printer

The Prusa i3 line is one of the most popular lines in 3D printer history: reasonably priced, dependable, with great print quality and comprehensive customer support. It evokes, and is the culmination of, the great maker tradition out of which it evolved. The preassembled version of the i3 MK3S is practically plug-and-play, only requiring piecing together the spool holder, adding a filament spool and loading the filament, and then selecting an object file from the included SD card and launching the print. Printing was smooth and near-flawless, with no misprints and high-quality prints. And the PrusaSlicer software is easy to learn and use.

Original Prusa i3 MK3S

Whether for safety or aesthetics, some people may prefer a closed-frame 3D printer. But for those who are comfortable with an open-frame design, the Prusa i3 MK3S offers a lot for its price. It is our new Editors’ Choice pick among mid-priced 3D printers for hobbyists and makers. We can’t wait to see how its tweaked successor, the Prusa i3 MK3S+, performs.

Original Prusa i3 MK3S Specs

3D-Printing Technology Fused Filament Fabrication (FFF)
Materials Supported PETG, ABS, PLA
Number of Print Colors 1
Number of Extruders 1
Frame Design Open
Maximum Build Area (HWD) 9.8 by 8.3 by 8.3 inches
Top Print Resolution 50 microns
LCD Screen Yes
Primary Interface(s) SD Card, USB 2.0
Built-In 3D Scanner? No
Dimensions (HWD) 15 by 19.7 by 22 inches
Weight 17 lbs
Warranty (Parts/Labor) 2 year(s)

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