Hello lemon613. Welcome to the Community!
The Home Depot has a paint calculator on-line which makes it easy to estimate how much paint to buy
You can find it here: LINK
It looks like this:
As you can see above, you measure the room size, and then window sizes, door size and trim.
The calculator then will tell you how much ceiling, wall and trim paint you will need.
Is this what you are looking for?
Chris.
.
Hello lemon613!
Calculating how much paint to purchase is only an approximation ... so make it easy like this:
1) Walk around the room along the walls and count the strides it takes to return to your starting point;
2) My average stride is approximately two-feet, so I count two, four, six, eight ... as I walk;
3) Multiply your count by the height of the walls ... most walls are either eight- or nine-feet;
4) The number you get will be the approximate square-feet of wall to be painted.
FOR EXAMPLE:
As I stride around the room counting by two, I return to the starting point just as I say 48.
My ceilings are eight-feet, so I multiply 48 times 8 and get 384-square feet.
HOW MUCH PAINT?
The average gallon of paint covers between 250- and 400-square feet.
In the room I just measured, I expect to use approximately one-gallon of paint per coat ... two gallons for two coats.
FINALLY:
Following the 384-square foot example, I would purchase no more than two-gallons ... and might only purchase one if the old and new colors were very similar.
Proper painting technique suggests that you take a break between coats to allow the first coat to dry before applying the second, so always purchase enough to complete the first coat.
While the first coat dries, you can go back to The Paint Pit if you need another gallon.