According to Merriam-Webster, “overall” is an adverb that means generally or as a whole. As a noun, overalls are pants with a bib and houlder straps, or loose, protective trousers worn over your normal clothes. As an adjective, “overall” means general, including everything, or viewed as a whole.
Overall, I think the drilling went well. (adverb)
Put on overalls if you are going to work with acids. (noun)
Unfortunately, the acid stimulation was an overall failure. (adjective)
“Over all” is an expression that contains a preposition (over) and a pronoun (all). It means “above the whole thing” or “on top of everything.”
Make sure you put tarps over all the equipment to keep the rain off.
“I am Yertle the Turtle, oh marvelous me, for I am king over all that I see.” – Dr. Seuss
If in doubt, substitute the word “general” or “generally” for “overall” in the sentence, and if it fits, use the single word. If it doesn’t fit, use “over all.”
Generally, I think the drilling went well. (OK to use overall as adverb)
Unfortunately, the acid stimulation was a general failure. (OK to use overall as adjective)
Make sure you put tarps generally the equipment to keep the rain off. (Not OK to use overall; use over all)
Profound Quote of the Day:
“Opportunity is missed by most people because it is dressed in overalls and looks like work.”
– Thomas Alva Edison, American inventor (light bulb, phonograph, motion picture camera), 1847-1931