Standard ZDoom units
distance – mapunits (also called mappixels)
time – tics (or seconds in non-gameplay contexts)
angle – turns counterclockwise
Derived units
velocity – mapunits per tic
acceleration – mapunits per tic^2
gravity – as acceleration
Gravity is defined by free fall acceleration, and is measured using the same units as normal acceleration.
Non-standard units
These units should be converted to standard units before performing any operations with them.
Space
meters, feet, and other nonsense – Doom doesn't have an accurate scale
sv_gravity units (800:1) – converted to acceleration by dividing by 800
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Time
seconds (35:1)
octics (4.375:1)
Angles
degrees (360:1)
radians (tau:1)
byte angles (256:1)