How to Calculate Paint Cost — Step by Step

The short answer

To work out what painting a room costs, measure the room, work out the wall area, subtract the doors and windows, and price the gallons and the supplies. For a 12 by 10 foot room with 8 foot ceilings that gives 352 square feet of wall and 316 square feet paintable, which needs 2 gallons and costs $125.00 in total.

The three numbers that matter

The first number is the wall area, which is twice the length plus the width, times the ceiling height, so the wall area is 352 square feet. The second number is the paintable area, which is the wall area minus the doors and the windows. A door takes 21 square feet out and a window takes 15, so 352 minus 21 minus 15 is 316 square feet.

The third number is the gallons, which is the paintable area times the coats, divided by the coverage, rounded up to whole gallons. At 350 square feet a gallon, 316 times 2 divided by 350 is 1.81, rounded up to 2. The paint costs 2 times $45, which is $90.00, and the supplies are $35, so the total is $125.00. The rounding is the part that changes with the room: a room that needs 1.81 gallons and one that needs 2.01 both buy 2 gallons, but one that needs 2.57 buys 3.

The method

The method is three multiplications, a subtraction, and an addition. One: multiply twice the length plus the width by the ceiling height to get the wall area. Two: subtract 21 square feet a door and 15 a window to get the paintable area. Three: multiply the paintable area by the coats, divide by the coverage, and round up to whole gallons. Four: multiply the gallons by the price per gallon. Five: add $35 of supplies. Check it with the cost per paintable square foot.

Step by step

Step 1: find the wall area. Measure the room wall to wall at the longest points, in feet, and measure the ceiling height. For a 12 by 10 foot room with 8 foot ceilings that is 2 times 12 plus 10, times 8, which is 352 square feet.

Step 2: find the paintable area. Subtract 21 square feet for each door and 15 for each window. For this room that is 352 minus 21 minus 15, which is 316 square feet. The doors and windows take 36 square feet out of the wall, about a tenth of the room, so skipping this step overbuys paint.

Step 3: work out the gallons. Multiply the paintable area by the coats and divide by the coverage. For this room that is 316 times 2 divided by 350, which is 1.81 gallons, rounded up to 2 whole gallons. The shop does not sell part of a gallon, so the rounding up is real money, not a rounding error.

Step 4: price the paint. Multiply the gallons by the price per gallon. At $45 a gallon that is 2 times $45, which is $90.00. This is the biggest of the two costs, and it moves the most when you change the paint.

Step 5: add the supplies and check. The supplies are $35, for rollers, brushes, tape, drop cloths and a tray, so $90.00 plus $35 is $125.00. The check is the cost per paintable square foot: $125.00 divided by 316 is $0.40, the number to compare between rooms and between paints.

The worked example

A homeowner is painting a 12 by 10 foot bedroom with 8 foot ceilings, and the shop charges $45 a gallon for paint that covers 350 square feet a gallon. The wall area is 2 times 12 plus 10, times 8, which is 352 square feet. The paintable area is 352 minus 21 minus 15, which is 316. The gallons are 316 times 2 divided by 350, which is 1.81, rounded up to 2. The paint costs 2 times $45, which is $90.00. The supplies are $35. The total is $125.00, or $0.40 per paintable square foot.

A bigger room, and other round numbers

Some rooms are worth knowing by heart. A 15 by 12 foot room with 9 foot ceilings is 486 square feet of wall, 450 paintable with one door and one window, which needs 3 gallons at 2 coats and costs $170.00. One coat instead of two on the 12 by 10 room needs only 1 gallon, and the total falls to $80.00. Paint at $60 a gallon instead of $45 costs $120.00 for the paint alone, and the total rises to $155.00. Three coats instead of two needs 3 gallons, and the total is $170.00, the same as the bigger room.

Working backwards from a budget

To find how much wall a budget covers, subtract the supplies, divide by the price per gallon, round down to whole gallons, and multiply by the coverage over the coats. A $200 budget leaves $165 for paint, which buys $165 divided by $45, or 3.67 gallons, rounded down to 3. Three gallons at 350 square feet a gallon over 2 coats covers 525 square feet of wall, more than the 352 of this room, so $200 covers the room with a gallon to spare.

Common mistakes

The most common mistake is pricing the floor area, not the wall area. A 12 by 10 foot room is 120 square feet of floor but 352 of wall, so pricing the floor buys a third of the paint you need. The second mistake is forgetting the doors and windows, 36 square feet between them here. The third is forgetting the supplies, $35, more than a quarter of the total. The fourth is buying part gallons in your head: the shop sells whole gallons, so 1.81 gallons is 2, and 2.57 is 3.

The verdict

A 12 by 10 foot room costs $125.00 to paint: 352 square feet of wall, 316 square feet paintable, 2 gallons of paint, $90.00 of paint and $35 of supplies. Measure the room, subtract the doors and windows, and price the gallons and the supplies. Check the numbers yourself on the paint cost calculator, work out the carpet for the same room with the carpet cost calculator, or check the floor over 25 years with the hardwood vs laminate flooring calculator.