Work out how much paint a room needs. Enter dimensions, deduct openings, and get gallons or litres for walls, ceiling and trim plus a cost estimate.
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Enter details and calculate to see breakdown.
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This paint calculator figures out how much paint a room needs. It handles walls, ceiling, and trim separately, subtracts doors and windows, and adds a waste factor. You’ll get gallons or litres for the main surfaces and quarts or small cans for trim, plus a full cost breakdown.
A gallon of paint covers about 350 square feet. That’s the number on the label. But the real amount you need depends on how many coats, how much waste, and whether you’re painting the ceiling or just the walls. This tool pulls all those variables together. Enter room dimensions, dedut openings, pick a paint type, and set the coverage. It returns the containers to buy and what they’ll cost.
Pick the unit system: imperial (ft, ft², gal) or metric (m, m², L).
Enter the number of identical rooms.
Type the room length, width, and wall height.
Choose which surfaces to paint: walls, ceiling, trim.
If painting trim, set the baseboard height. 0.33 ft (4 inches) is typical.
Enter the number of doors and windows, plus their dimensions.
Pick a paint type and number of coats. Two coats is standard.
Adjust coverage per gallon and waste factor, then add costs.
The Summary tab shows total paint needed and the cost. Detailed Breakdown lists every area and calculation.
The tool works in three stages: net area, area with coats and waste, then containers and cost.
Formula: Perimeter = 2 × (Length + Width)
Formula: Gross wall area = Perimeter × Wall height
Formula: Net wall area = Gross wall area − Door area − Window area
Formula: Ceiling area = Length × Width
Formula: Trim area = Perimeter × Trim height
Formula: Total net area = (Net wall + Ceiling + Trim) × Rooms
Formula: Area with coats = Total net area × Coats
Formula: Area with waste = Area with coats × (1 + Waste %)
Formula: Main containers = [ (Wall + Ceiling area with waste) ÷ Coverage per gallon ]
Formula: Trim containers = [ (Trim area with waste) ÷ (Coverage per gallon ÷ 4) ]
Trim uses quart cans, so its coverage is one quarter of a gallon. That’s why the tool divides coverage by four when calculating trim containers. If you’re working in metric, the same logic applies with litres and small cans.
Deductions are per room. If you have three identical rooms, the tool subtracts doors and windows from each room’s wall area, then multiplies by three. Waste and coats are applied after all areas are summed.
Costs are straightforward. Main paint cost is containers times price per gallon. Trim cost is trim containers times price per quart. Supplies, tools, labor, and additional costs are added on top. Cost per square foot is total cost divided by total net area.
Say you’re painting a 12 ft by 10 ft bedroom with 8 ft walls. One room. You’re painting walls and trim, not the ceiling. Two doors (3 ft × 7 ft) and two windows (3 ft × 4 ft). Two coats. Coverage is 350 ft² per gallon. Waste is 10%. Paint costs $30 per gallon, trim paint $12 per quart.
Perimeter: 2 × (12 + 10) = 44 ft
Gross wall area: 44 × 8 = 352 ft²
Door area: 2 × 3 × 7 = 42 ft²
Window area: 2 × 3 × 4 = 24 ft²
Net wall area: 352 − 42 − 24 = 286 ft²
Trim area: 44 × 0.33 = 14.5 ft²
Total net area: 286 + 14.5 = 300.5 ft²
Area with two coats: 300.5 × 2 = 601 ft²
With 10% waste: 601 × 1.10 = 661.1 ft²
Main area (walls only): 286 × 2 × 1.10 = 629.2 ft²
Main containers: ⌈629.2 ÷ 350⌉ = 2 gallons
Trim area with waste: 14.5 × 2 × 1.10 = 31.9 ft²
Trim coverage: 350 ÷ 4 = 87.5 ft² per quart
Trim containers: ⌈31.9 ÷ 87.5⌉ = 1 quart
Main paint cost: 2 × $30 = $60
Trim paint cost: 1 × $12 = $12
Material subtotal: $72
Cost per ft²: $72 ÷ 300.5 = $0.24
Two gallons of wall paint and one quart of trim paint. The trim area is small, so a single quart covers it. Without the trim, the job would still need two gallons because the wall area alone pushes past 600 ft² with two coats.
A 12×12 room with 8 ft walls has about 384 ft² of wall. Deduct two doors and two windows and you’re near 300 ft². Two coats at 350 ft² per gallon needs about 1.7 gallons, so buy 2.
Measure the room perimeter, multiply by wall height, then subtract doors and windows. Multiply by coats and waste, then divide by coverage per gallon. The result is gallons needed.
Yes. Ceiling area is length times width. It’s usually painted with a different product, often flat white. Add ceiling area to your total if you’re painting it.
Ten percent for brush and roller. Fifteen to twenty percent for sprayers or rough surfaces. Waste covers drips, tray residue, and touch-ups.
Two coats for a color change or new drywall. One coat for a refresh in the same color. Three coats when going from dark to light.
Trim area is perimeter times trim height. A 44 ft perimeter with 4-inch baseboard is 14.5 ft². Trim paint is sold in quarts, so coverage per quart is about 87.5 ft².
Most interior latex covers 350 to 400 ft² per gallon. Primer covers 200 to 300 ft². Exterior paint covers 250 to 350 ft². Check the label for the exact figure.
Forgetting to subtract doors and windows. Using one coat when two are needed. Underestimating waste on rough surfaces. Buying trim paint by the gallon instead of the quart.
The math works for any flat surface. But exterior paint coverage is lower, and you may need to account for siding texture. Set the coverage and waste to exterior values.
It’s a planning estimate. Actual paint use depends on surface porosity, application method, and how much you load the brush. Buy one extra quart if you’re unsure.
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