Progress Bar vs Spinner

One question decides it: can you measure completion? If you can, a bar turns an open-ended wait into a bounded one and makes a stall visible — a bar that stops moving says something a spinner never can. If you cannot, a bar has nothing honest to draw, and faking the fill is worse than a spinner, because it promises an estimate that does not exist. Long operations often need both in sequence: measurable while bytes are moving, unmeasurable once the server takes over.

If you can measure how much is done, draw a bar. If you can only tell that something is running, spin.

Progress Bar

An indicator showing how much of a known-length operation is complete.

Reach for it when

You can measure completion — bytes uploaded, files processed, steps finished out of a known total.
The wait is long enough that “is this stuck?” becomes a real question, roughly past a couple of seconds.
The user has to decide whether to wait or leave, and the remaining time is what they are deciding on.

Avoid it when

You cannot measure it. An indeterminate bar and a spinner say the same thing, and the spinner is not pretending to be a measurement.
The wait is under a second. The bar appears and vanishes, which reads as a flash of broken layout.
You would have to fake it. A bar that crawls to 90% and waits there teaches people to distrust every bar you draw.

Spinner

A small looping animation shown while an operation is in flight, indicating activity but not progress.

Reach for it when

The wait follows a direct action — submitting a form, saving, deleting — and the spinner can sit on the control that was pressed.
The area is too small for a meaningful skeleton, like inside a button or next to an input.
You genuinely cannot predict the resulting layout, so a skeleton would guess wrong.
The wait is short and roughly bounded — under a couple of seconds.

Avoid it when

Initial page or list loads where the layout is known. A skeleton communicates more and prevents layout shift.
Operations that take more than a few seconds. Without progress information a spinner is indistinguishable from a hang — use a progress bar and say what is happening.
Several spinners on one screen. A page of rotating circles reads as chaos; prefer one region-level indicator.
Blocking the whole screen for something that only affects one part of it.