Free Online G-Code Simulators: Best Browser-Based CNC Tools for 2026

A G-code simulator is a software tool that reads a CNC program and shows a visual representation of the toolpath the machine will follow. The simulation runs on your computer without any machine movement. It lets you verify that the program is correct before sending it to the CNC machine.

Running a program in a simulator catches errors that would crash the machine or ruin a part. A missing decimal point, a wrong coordinate, or an incorrect feed rate are all visible in the simulation before any material is cut. The time spent simulating is always less than the time spent recovering from a crash.

This guide covers the best free online G-code simulators available in 2026, explains what to look for when choosing a simulator for your specific needs, and provides practical tips on how to use them effectively in your daily CNC workflow.

Why Use a G-Code Simulator

A G-code simulator prevents crashes and saves expensive material by catching errors before the machine starts cutting. Every CNC programmer has run a program that had an error that was not visible when reading the code listing line by line but was immediately obvious in the visual simulation. The simulator shows the tool moving through three-dimensional space on your screen and makes it easy to spot paths that go through clamps, cut past the material boundary, or plunge too deep into the workpiece.

Simulators also help with learning G-code programming from scratch. Beginners can write a short program by hand and see the resulting toolpath appear on screen immediately. The visual feedback on screen creates a direct connection between the G-code commands typed on the keyboard and the resulting machine motion shown on the simulation display. Writing your own G-code blocks by hand and seeing the tool move immediately in the simulator is one of the most effective and engaging ways to learn CNC programming fundamentals quickly.

Simulators are useful for optimizing programs to reduce cycle time. The visual display clearly shows where the tool moves slowly through the material, where it makes unnecessary rapid moves that waste time between cuts, and where the overall toolpath could be rearranged for greater machining efficiency. Optimizing the program in the simulator before running it on the real machine saves valuable cycle time on every single part that is produced afterward.

Common Errors Simulators Catch

Missing decimal points are the most common G-code error. A coordinate value written as X10 instead of X10.0 is interpreted as X0.0010 by many controllers. The simulator shows the tool moving to an unexpected position which alerts you to the missing decimal point.

Wrong sign on coordinates is another common error. A negative Y value when the part requires a positive Y sends the tool in the wrong direction. The simulator shows the toolpath in the wrong quadrant and makes the error obvious.

Feed rate errors are visible in the simulator through the displayed time estimate value. A program that should take about 10 minutes to run but shows an estimated machining time of 2 hours has a feed rate that is set much too slow and will waste time. A program that shows an estimated time of only 30 seconds has a feed rate that is far too fast and will likely break the tool on the first cut.

Best Free Online Simulators

Several free online G-code simulators are available. Each has different strengths and limitations. The best choice depends on what you need to simulate.

NC Viewer

NC Viewer is the most popular free online G-code simulator. It runs entirely in the browser with no installation. The interface shows the toolpath as colored lines with different colors for rapid moves, feed moves, and arcs.

NC Viewer supports standard G-code for milling and turning. It handles multiple tools and shows tool changes. The simulation can be rotated, panned, and zoomed for viewing from any angle. The step-through mode lets you advance one line at a time to see each movement.

The main limitation of NC Viewer is that it does not simulate material removal. The toolpath is shown as lines, not as a solid model. You can see where the tool moves but not what the finished part looks like.

NC Viewer is freely available at ncviewer.com and requires no registration, account creation, or payment to use all of its features.

G-Code Simulator by CAMotics

CAMotics is an open-source G-code simulator that runs on Windows, Mac, and Linux. The online version through the browser provides a simplified interface. The desktop version is more capable with full 3D simulation and material removal visualization.

The CAMotics simulator shows the toolpath as a 3D solid model that changes as material is removed. The simulation shows the finished part shape which makes it easy to verify that the program produces the intended geometry.

CAMotics is freely available at camotics.org and the full desktop version is free and open source software.

G-Code Simulator by Onefinity

Onefinity provides a free online G-code viewer designed for their CNC machines. The viewer is simple and fast. It shows the toolpath with color-coded moves and estimates the machining time.

The Onefinity viewer works well for basic verification of G-code programs. It handles standard G-code commands and shows the complete toolpath. The simulation is not as detailed as CAMotics but is faster to load and use.

G-Code QnA Viewer

G-Code QnA is a free browser-based G-code analyzer and viewer. It shows the toolpath and provides information about the program including the number of lines, estimated time, and rapid move distances.

The G-Code QnA viewer is useful for quick checks of short programs where you only need to verify the overall toolpath shape. The built-in analysis information helps identify programs with excessive rapid moves or inefficient toolpath patterns that waste cycle time. The viewer is best used as a quick verification step before running a program, not as a detailed debugging tool for complex programs.

What to Look For in a Simulator

A good simulator should show the complete toolpath in 3D with the ability to rotate and zoom. Color-coded lines for different move types help identify rapid moves that could cause crashes. The simulator should support the G-code commands your controller uses including G0, G1, G2, G3, canned cycles, and tool changes.

Material removal simulation is a valuable feature for verifying that the finished part matches the intended geometry. Simulators with material removal show the stock block and update the shape as the simulated tool moves through it.

Step-through mode allows advancing one line at a time through the program. This is essential for debugging. Watching the tool move line by line reveals errors that are not visible when running the full simulation at speed.

Simulator Feature Comparison

Feature	NC Viewer	CAMotics	Onefinity	G-Code QnA
Browser-based	Yes	Limited	Yes	Yes
Free	Yes	Yes	Yes	Yes
3D view	Yes	Yes	Yes	Yes
Material removal	No	Yes	No	No
Step-through	Yes	Yes	Limited	No
Tool changes	Yes	Yes	Yes	No
c Canned cycles	Yes	Yes	Partial	No
Time estimate	No	Yes	Yes	Yes
Open source	No	Yes	No	No
Offline use	No	Yes	No	No

How to Use a Simulator Effectively

Load the G-code file into the simulator by dragging and dropping the file onto the browser window or using the file selection dialog. Most simulators accept standard .nc, .gcode, or .txt files. Some allow pasting the code directly from the clipboard into a text box on the page.

Run the full simulation first at high speed to see the overall toolpath geometry clearly. Look for tool movements that extend beyond the material boundary or travel outside the expected work area. Check that the tool retracts to a safe height above the workpiece between different operations. Verify that the toolpath does not pass through any clamps or fixtures that would cause a crash.

Use step-through mode to examine any suspicious tool movements in detail at your own pace. A toolpath that looks wrong in the full speed simulation can be investigated line by line to find the exact line number where the problem occurs. The line number from the simulator points directly to the problematic code in the program.

Always simulate programs that use cutter compensation with G41 or G42, or canned cycles like G81 for drilling. These features are easy to configure incorrectly in the G-code. The simulator shows the actual compensated toolpath and reveals errors in the compensation direction or the canned cycle parameters that are not obvious when reading the raw code.

Simulate the program again after making any changes to the code. A single edited line or changed coordinate can introduce a new error that was not present in the original program. Re-simulating after every change catches the new error before the program reaches the machine and causes damage.

Integration with Workflow

Integrate simulation into your standard workflow by simulating every program before it runs on the machine. Make simulation a required step between CAM post-processing and loading the program into the controller. A consistent simulation habit prevents crashes and saves material over time.

Keep the simulator open in a dedicated browser tab on your computer while you edit the program in your text editor or CAM software. Make a change, reload the file in the simulator, and check the result immediately. The rapid edit-simulate loop speeds up program development and catches errors as they are introduced.

For programs that will run multiple times on the machine in production, save a simulation screenshot or short video recording along with the program file in the same folder. The saved simulation record provides a very useful visual reference for future production runs of the same program and helps quickly identify what changed if the program is edited later and needs re-verification.

Simulators for Specific Machine Types

Different CNC machine types generate G-code with different structures and commands. The simulator must support the specific G-code dialect your machine uses.

For GRBL-based routers and mills, the G-code uses a simplified format without program numbers or certain advanced commands. Most online simulators handle GRBL code very well because the command set is small and standard. NC Viewer and CAMotics both work well with GRBL output.

For Mach4 and Centroid Acorn machines, the G-code includes additional commands for probing cycles and tool change macros. Some online simulators may not support these advanced commands. Test the simulator with a short program before relying on it for full program verification.

For industrial controls like Fanuc, Haas, and Siemens, the G-code includes canned cycles, subprograms, and macro variables. Browser-based simulators often handle standard G-code well but may not support all variations of canned cycles. CAMotics on the desktop handles the widest range of G-code commands.

For LinuxCNC machines, the G-code supports advanced features like axis transformation and custom M-codes. Desktop simulators like CAMotics handle LinuxCNC code better than browser-based tools.

Before Simulation Checklist

Before loading any program into the simulator for the first time, verify that the file format is correct for the simulator. The program should have .nc, .gcode, or .txt extension. Remove any comments or formatting that the simulator may not recognize. Ensure the program starts with a safe setup block that includes G90 G94 G17 and the correct unit selection.

Check the program for matching M-codes that open and close correctly. Every M03 spindle start command should have a corresponding M05 spindle stop command later in the program. Every M06 tool change command should have the correct tool number assigned. Matching M-codes throughout the program confirm the program is complete and will run successfully from start to end without leaving the spindle running after the program finishes.

Verify the Z-axis movements are safe. The first Z movement in the program should be a rapid move to a safe retract height above the workpiece. A program that starts with a G01 Z feed move from an unknown position is dangerous.

Limitations of Simulators

Free online simulators do not simulate machine-specific behavior accurately. They do not account for the machine acceleration, deceleration, or corner rounding settings that affect the actual toolpath. The actual path on the machine may differ slightly from the simulated path especially in corners and tight curves.

Simulators do not detect logical errors in the program. A program with the wrong work offset value entered in the machine controller will simulate correctly on screen because the simulator has no way of knowing where the part zero reference is physically located on the machine. The simulator can only show the relative tool movements from the G-code, not the absolute position relative to the machine table or workpiece.

Material removal simulation in free browser tools is approximate at best. The voxel resolution of the simulation affects how accurately the finished shape is represented. Low-resolution simulations may miss small features like chamfers or fillets. The simulation is adequate for verifying the overall toolpath shape and detecting major errors but not for checking fine surface details or small feature geometry accurately.

Desktop simulators like CAMotics provide more accurate material removal simulation than browser-based tools. For critical parts with tight tolerances where material waste is expensive, use a desktop simulator or the simulation built directly into your CAM software for the most accurate and reliable results.

A G-code simulator is one of the most useful and practical free tools available to every CNC programmer regardless of experience level. It catches errors before they damage the machine, helps beginners learn G-code through visual feedback, and reveals optimization opportunities that reduce cycle time. Every CNC programmer should have at least one simulator in their workflow and use it for every program before cutting material.

The best free simulators available in 2026 are NC Viewer for quick browser-based verification and CAMotics for more detailed material removal simulation. Both tools are completely free and support the standard G-code commands used by most hobby and professional CNC machines. Start with NC Viewer for daily use and add CAMotics when you need material removal verification.

Developing the consistent habit of simulating every single program before running it on the machine is one of the clearest marks of an experienced and careful CNC programmer. The few seconds or minutes you spend on simulation before every job save hours of crash recovery time and prevent expensive damage to cutting tools, workpieces, and the machine’s spindle and axis components.

For more information on G-code programming, see our [G-Code for Beginners Guide](

G90 G54 G00 X0 Y0
G01 Z-0.1 F10
G01 X1.0 F20
M30

/posts/g-code-for-beginners/) and Complete G-Code List Reference. For CAM software, see our CNC Router Software Comparison.