IconVault's Paint Calculator estimates how much paint a room needs from real measurements. Add each wall's width and height, subtract doors and windows as openings, choose your number of coats and your paint's coverage per gallon, and it computes the paintable area plus the gallons and liters required, including a rounded-up buy quantity so you do not run short mid-wall.

It is free, needs no signup, and runs fully in your browser. The honest caveat: this is an estimate, not a guarantee. Real coverage varies with paint brand, color change, surface texture and application method, so buy a little extra and check the coverage figure printed on your paint can.