TOOL_01 · Interior paint
Paint calculator
Gallons for the job — doors and windows subtracted, coats counted, primer optional. The math is shown, not hidden.
① Room dimensions
Paint needed
0.0gal
≈ 2 gal + 3 quarts
Total area
0sq ft
453 walls × 2 coats
Material Cost Estimator
Estimated Total for Interior Paint
$144.00
3 units × $48.00 per gallon
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How to measure it on site
Paint math starts with wall area, not floor area. Walk the room with a tape and write down four numbers per wall — length, height, and the openings you are not painting — before you touch the calculator.
Measure the perimeter, not the floor. Add the length of every wall you are painting. A 12×12 room is 48 ft around; an open floor plan may need two separate runs measured as separate rooms.
Measure height at the tallest point. Go floor to ceiling at the highest corner. Sloped ceilings and stairwells need the tallest measurement, or the gallon count runs short on the tall wall.
Count openings, don't subtract them twice. A standard interior door is 21 sq ft (3×7 ft) and a window is 15 sq ft (3×5 ft). Count only the openings you are not painting; the calculator deducts them automatically.
Decide on the ceiling before you buy. Ceiling area is not floor area in an L-shaped room. If the room is not square, measure the ceiling length and width separately and add it to your wall total.
Measure trim separately. Baseboard, casing, and door slabs are usually a quart of trim enamel, measured in linear feet. This calculator does not include them in the gallon count.
Common mistakes
These five errors account for most of the short-gallon trips back to the store.
- Using floor square footage as wall area. A 12×12 floor is 144 sq ft; the walls are 333 sq ft after openings. Floor area underestimates wall paint by more than half.
- Buying for one coat. Two coats is the default for a color change. One coat only holds on a same-color repaint over a sound, light surface.
- Assuming 400 sq ft per gallon. 350 is the conservative planning number. Texture, bare drywall, and deep bases can drop real coverage to 250–300 sq ft per gallon.
- Skipping the quart buffer. Rounding to the nearest gallon is not enough when every wall is cut in twice. The calculator shows quarts over whole gallons so you can buy the odd quart instead of a fourth gallon.
- Painting the ceiling by accident. Checking the ceiling box adds roughly a third of a gallon per coat in a 12×12 room. Leave it unchecked unless the ceiling is actually in scope.
Worked example
A 12×12 bedroom, two coats, one door, two windows
Walls are 8 ft high and the ceiling is out of scope. This is the same input the calculator's default example uses.
- Perimeter
- 12 + 12 + 12 + 12 = 48 ft
- Gross wall area
- 48 ft × 8 ft = 384 sq ft
- Subtract openings
- 384 − (1 × 21) − (2 × 15) = 333 sq ft
- Two coats
- 333 × 2 = 666 sq ft
- Divide by coverage
- 666 ÷ 350 = 1.90 gallons
Result: 2.0 gallons — buy two gallon cans and keep the leftover for touch-ups. The wall area is 333 sq ft.
What this calculator doesn't cover
This calculator sizes wall paint and an optional ceiling. It does not replace a site visit for these cases:
- Trim, doors, window frames, and cabinets — measured in linear feet and usually painted with a different product.
- Exterior siding, stucco, brick, and decks — exterior coverage and surface preparation follow different rules.
- Lead paint, mold, or failing surfaces — remediation is a separate scope and belongs with a qualified contractor.
- Color matching across rooms — buy every gallon for one color at once; tinting later rarely matches.
Reviewed . Sources: Interior latex coverage rates (350–400 sq ft per gallon) (Major paint manufacturers' technical data sheets); Primer coverage rates (about 200 sq ft per gallon) (Primer technical data sheets). Full methodology.
How the math works
Walls: perimeter × height, minus 21 sq ft per door and 15 sq ft per window. Ceiling: room area (estimated from your perimeter), added only if you check the box. Total: area × coats, divided by 350 sq ft per gallon, rounded up — because the store doesn't sell 0.7 gallons.
Primer math is separate: primer covers about 200 sq ft per gallon since it soaks in more. One coat of primer is included when you switch the calculator to “Paint + primer.”
Adjust for reality: rough texture, bare drywall, or deep color changes can eat 20–30% more paint. Buying a quart of buffer beats an emergency second trip with half a wall cut in.
Common questions
How many square feet does a gallon of paint cover?
One gallon of typical interior latex paint covers about 350–400 square feet per coat on a smooth, primed wall. This calculator uses 350 sq ft per gallon — the conservative end — so you don't run short mid-project. Rough or heavily textured walls can drop coverage to 250–300 sq ft per gallon.
Do I really need two coats?
For most repaints over a similar color, two coats is the standard for even color and durability. If you're making a dramatic color change (light over dark or vice versa), plan on two coats plus primer. New drywall always needs primer plus two coats.
How much paint does a 12x12 room need?
A 12×12 room with 8-ft ceilings has 384 sq ft of wall (48 ft perimeter × 8 ft). With two coats and two doors plus one window subtracted, you'll need about 2 gallons. Add a third gallon if you're painting the ceiling.
Should I buy gallons or a 5-gallon bucket?
For 15+ gallons of the same color, a 5-gallon bucket is cheaper per gallon and keeps color consistent across rooms. For 1–3 gallons, buy individual gallons — they're easier to carry, shake, and store for touch-ups.
Estimates use standard coverage assumptions and are for planning purposes. Actual needs vary by surface, paint quality, and application.