What is the best free G-code viewer?

It depends on the job. For a quick visual check of a toolpath in the browser, NCViewer is fast and convenient. For simulating material removal on the desktop, CAMotics is the open-source standard. For knowing how long a program will actually take to run, with acceleration, cornering, canned cycles, subprograms and tool changes on the clock, UltraNC is built for exactly that, free and in the browser, and it now carves a virtual stock as the program runs.

Why do different viewers report different cycle times?

Because they model motion differently. Most tools estimate time as path length divided by feed rate, which assumes the machine reaches the programmed feed instantly and never slows for corners, a best case no machine achieves. A motion planned simulation with your machine's rapid, acceleration and jerk limits accounts for the ramps and corner slow-downs that dominate real cycle time.

Are these comparisons fair?

That is the intent. Every competitor claim is sourced to that product's own documentation or independent published testing, each page carries a last-verified date, and there is a standing invitation to correct anything that is wrong or has gone stale. Products change, and a stale claim helps nobody.

Compare

How UltraNC compares to other G-code viewers

Different tools are built for different jobs. These pages compare UltraNC honestly with the three most popular alternatives, including where each one genuinely wins. Every claim sourced and dated.

UltraNC does one job unusually well: telling you how long a program will actually take to run. It simulates real machine motion (acceleration, cornering, jerk limiting if required) and expands the things that eat cycle time: canned drilling and boring cycles, rigid tapping, dwells, tool changes and subprogram calls. It also carves a virtual stock as the program runs, so a rapid that ploughs through material is visible in the carved stock. If your question is “can I edit this file quickly?”, one of the tools below may serve you better. The comparison pages say so plainly.

UltraNC vs NCViewer

The most popular free browser viewer. Fast visual checks and a built-in editor, but its cycle time is path ÷ feed. When the number matters, the difference is large.

UltraNC vs CAMotics

The open-source desktop simulator for material removal preview. Its own docs list the cycles and feed modes it doesn’t run. Most of them are timing critical.

UltraNC vs CIMCO Edit

The most widely used paid CNC editor with deep dialect and macro support. UltraNC isn’t a replacement for it. It’s the free browser complement for cycle time questions.

How these comparisons are made

Every claim about another product is sourced to that product’s own documentation or to independent published testing, and each page states when its claims were last verified. Products improve, so if you spot anything out of date or plain wrong, tell us and we’ll correct it. A comparison you can’t trust isn’t worth publishing.

The background reading

The reason estimates differ so much between tools is motion physics. The physics is in feed rate vs. cycle time, the missing overhead in non-cutting time, and the practical method in how to calculate run time from G-code.

Try UltraNC with your own file →