Sketchup limits

how do i measure less than one inch? like .75 or .25?

Set the units to something reasonable for what you are modeling. Also set Display Precision sensibly.

What version of SketchUp are you using? Please complete your profile. That information helps us help you.

I am also interested in this topic. Eg if I work in mm what level of accuracy do I set.

Hope I am not butting in. If so delete this post.

Sketchup’s accuracy never changes, only the display precision. It can display to 6 decimal places of a mm if you want.
How you set the display precision is up to you and how you are working.
Too course and it will round up and not show accurate dimensions, too fine and you have too many decimals to deal with.

With inches for example, if you have the display precision set to 0" it will only show whole inches, but as you increase the precision it will show more accurate dimensions.