Definition
F belongs to the move state, not just the line where it first appears. A block such as G1 F1800 sets 1800 mm/min—30 mm/s—for later moves until another F changes it. Unit mode matters: a controller using G20 may interpret the same number in inches per minute.
Commanded feed rate is not guaranteed physical speed. M220 can scale it, firmware caps it, and acceleration plus cornering limits determine whether short segments ever reach it. A useful viewer therefore separates the written F value from a motion-planned estimate.
What matters in practice
Usually per minute
F1800 is normally 1800 mm/min, not 1800 mm/s, in metric printer G-code.
Modal value
A later move can inherit the most recent F even when its own line contains no F word.
Not actual speed
Overrides, acceleration, axis limits and firmware planning can all lower the effective speed.
Example in G-code
Set once, use twice
G1 F1800
G1 X40 E1.2
G1 X80 E1.2Both spatial moves request the active 1800 mm/min feed rate unless another state or limit changes it.
What this viewer can show
The viewer converts F to mm/s, tracks M220 from the line where it appears, and also estimates acceleration-planned speed. It does not claim to reproduce every firmware or machine limit.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.