Most people work out paint the same way: multiply the wall length by the height, add up the four walls, divide by whatever the tin claims to cover. It is quick, and it is usually wrong in both directions at once.
Take the openings off
A standard door is roughly 20 square feet. A window is somewhere between 12 and 15. In a room with two doors and three windows you have just counted about 80 square feet of wall that does not exist. On a small job that is most of a tin.
Coverage depends on the finish and on what is underneath
The number printed on the tin assumes a decent surface and a middling finish. Change either and it moves a long way. Semi-gloss covers around 350 square feet to the gallon. Bare brick can drop you to 175, because the surface drinks the first coat. Same room, twice the paint, for reasons that have nothing to do with the arithmetic.
Two coats is the default, not the exception
Colour changes, patched walls and anything porous need two. Buying for one coat and hoping is how a job stops halfway through on a Sunday afternoon with the shops shut.
Round to tins, not to decimals
You cannot buy 1.4 gallons. Work out the real figure, add a margin so you have something left for touch-ups in two years, then round up to what the shop actually sells.
The short version
Wall area, minus doors and windows, times the number of coats, divided by the coverage rate for that finish on that surface, plus a touch-up margin, rounded up to whole tins.
If you would rather not run that on paper every time, PaintCalc is an Android app that works through exactly those steps for a room. It takes the openings back off, applies the coverage rate for the finish and surface you are actually painting, adds the touch-up margin and rounds to tins you can carry out of the shop.