Paint Calculator
Walls, ceiling, doors and windows all get subtracted properly, so you buy three gallons instead of four — and still have enough for a second coat.
Reviewed · Method: measured area + industry waste allowance, rounded up to real package sizes · How the numbers are calculated
Enter your dimensions and hit Calculate my paint order. You will get the quantity, the buyable quantity, a shopping list, and a budget range.
Figure. Doors and windows come off the wall area before coats are applied. Primer is counted separately from finish paint.
Figure. Wall area is the room perimeter times ceiling height. Measure the perimeter, not two walls.
How the paint quantity is calculated
Wall area is the perimeter (length plus width, doubled) times wall height. Each door subtracts about 20 sq ft and each window about 15 sq ft, which is the standard allowance for a 3x7 door and a 3x5 window. Ceiling area is length times width, added only if you tick that option. Total area is multiplied by the number of coats and divided by 350 sq ft per gallon, the coverage most premium interior paints actually deliver on a smooth surface.
Buy nap and sheath in the same trip
The calculator also returns roller covers, brushes, tape, drop cloths and tray liners. Two roller covers is not overbuying — a loaded 3/8-inch nap cover starts matting after about two gallons, and a matted cover leaves texture. One inch of blue tape covers roughly 55 linear feet of trim, so a 14x12 room with baseboards needs two rolls.
Primer is not optional in three cases
Bare drywall, patched walls, and any color change of more than two shades. Primer on new drywall also seals the paper so the topcoat does not flash. If you are covering a dark red or a deep navy with white, use a tinted gray primer and you will save a full coat of finish paint — usually more than the primer costs.
One gallon, one room
A common misconception is that a gallon covers a room. On a standard 12x12 bedroom with 8 ft ceilings, two coats on walls alone is roughly 2.7 gallons. That is why the answer comes back as gallons plus a quart rather than a single bucket.
Ceilings, trim and the doors themselves
A ceiling is a flat rectangle and it drinks paint faster than a wall because you are rolling into a non-absorbent surface with no way to press. Add 10% to the ceiling figure and buy ceiling paint in white, which is cheaper and flatter than wall paint.
Trim and doors are measured in linear feet, not square feet. A standard door has about 18 linear ft of edge and two faces. If you are painting trim, add roughly one quart per three doors and one gallon per 100 linear ft of baseboard.
Why the same room can take 2 or 5 gallons
Colour change is the biggest variable. White over white takes two coats. Navy over white takes three, and a tinted primer plus two coats is usually faster and cheaper than four coats of finish. Sheen matters too: high-gloss shows every roller mark, so it needs a smoother surface and more careful application.
Spraying uses 20 to 30% more material than rolling because of overspray, but it is two to three times faster on trim and doors. If you are spraying, add a gallon to the estimate rather than discovering it at 80% complete.
Worked example: 14 x 12 ft room, 8 ft ceiling
| Wall area before openings | 2 x (14 + 12) x 8 = 416 sq ft |
| Minus 2 doors and 2 windows | 416 - 72 = 344 sq ft |
| Ceiling | 14 x 12 = 168 sq ft |
| Total paintable surface | 344 + 168 = 512 sq ft |
| Finish paint at 2 coats | 3 gallons |
| Primer | 2 gallons |
| Budget band | $177 low, $259 mid, $394 high |
Mistakes that cost real money
Using the floor area as the wall area
A 14 x 12 ft room has 168 sq ft of floor and roughly 416 sq ft of wall before openings. Painting by floor area underestimates the job by more than half, which is why so many projects finish one gallon short.
Counting doors and windows as full rectangles
A standard interior door is about 21 sq ft including the casing, and a double-hung window about 15. Subtract them, but not twice: the trim around an opening still gets painted and the calculator already leaves that in.
Assuming one gallon covers 400 sq ft
That figure is for a smooth, primed surface in a light colour. New drywall soaks up 20 to 30% more. A deep colour over a lighter one often needs three coats, and red or yellow pigment can need four.
Skipping primer to save money
Primer is cheaper per gallon than paint and it is what stops a patch from flashing through the finish. On bare drywall or a colour change of more than two shades, primer is not optional, it is the difference between two coats and four.
Buying gallons one at a time
Each gallon is a separate batch. Slight tint differences that are invisible in the can show up as a seam on a large wall. Buy the whole job at once, or have the store box the gallons together so they are tinted as one batch.
Frequently asked questions
How many square feet does a gallon of paint cover?
About 350 to 400 sq ft per coat on a smooth, primed interior wall. Rough or textured surfaces drop to 250 to 300 sq ft. This calculator uses 350 as a conservative default.
Do I need to subtract doors and windows?
Yes, or you will overbuy by roughly 10 percent. This tool subtracts 20 sq ft per door and 15 sq ft per window automatically.
How much primer do I need?
One coat of primer over the same area, so the same gallon count as a single coat of paint. Tinted primer is worth it when covering dark colors.
Is one coat of paint ever enough?
Only when repainting the same color on a sound, clean surface. Coverage and durability both suffer with a single coat on new drywall or a color change.
Should I buy a five gallon bucket instead of singles?
If the total is four gallons or more, yes. Five gallon buckets cost less per gallon and guarantee a single dye lot, which matters for large or connected spaces.