First to confuse matters;
In the US paint is sold in a 5 gallon bucket, a 1 gallon can, and 1 quart can.
Canada uses the same sizes except it shows the metric equivalent for the US gallons and quarts on the label. But when you go to the store in Canada you can still ask for a gallon of paint.
5 gallon bucket = 18.9 liter 1 gallon can = 3.78 liter 1 quart can = 946 milliliters
On the paint can the manufacture will indicate what the sq. footage is that the can of paint will cover. Depending on the brand and type of paint you can get anywhere from 250 to 400 square feet of coverage per gallon. The actual coverage will depend on the texture of the wall and the colour you are using to paint. A smooth wall will take less paint than a rough textured wall. Covering a lighter colour wall with a darker colour paint will take less paint than painting a lighter colour over a dark colour wall. In addition to all that, the way you apply the paint, equipment you use to paint with and how much paint ends up on you or the floor versus the wall, will have an impact on how much paint you need. Take the indicated sq. footage of coverage with a grain of salt and make sure you have enough paint on site to complete the job. If the calculated result of the paint = 0.75 gallons you could buy 3 quarts, but these 3 quarts most likely will cost you more than if you purchased the same paint in the gallon size. Keep in mind that it is always good to have a bit of paint left over for touch-ups at a later time.