Paint Calculator

The formula

Coverage and yield assumptions

The default spread rate is 350 sq ft per gallon, the low end of the 350–400 sq ft range most major paint makers print on their labels for smooth, primed drywall. Rough or porous surfaces such as new plaster, bare wood, textured ceilings, block or stucco can drop that to 200–300 sq ft. Deep colors often need a tinted primer or a third coat, so raise the coat count rather than the spread rate.

One US gallon is 3.785 liters; a quart is a quarter gallon. Metric users can enter coverage in m² per liter (350 sq ft/gal ≈ 8.6 m²/L).

How to measure the walls

Walk the room with a tape and add the length of every wall you plan to paint. That sum is the perimeter. Measure height from the floor to the ceiling, not to the top of the baseboard; the small strip behind trim is useful slack. For a room with a sloped or vaulted ceiling, split each tall wall into a rectangle plus a triangle (half of base × rise) and enter the combined area by using an equivalent perimeter.

Count every door and window that sits in those walls. Large openings such as sliding patio doors or picture windows are worth measuring individually: subtract their real area from the perimeter × height figure by lowering the perimeter you enter. Closet interiors count if you paint them, and they often add 60–100 sq ft that people forget.

Primer, coats and sheen

Two coats is the norm for a color change and the default here. A single coat can work when you repaint the same color with a good-quality paint. Primer is a separate product with its own spread rate, usually a bit lower than finish paint, so run a second estimate with one coat and the primer label rate. Self-priming paints are thicker and may cover fewer square feet per gallon.

Sheen does not change quantity much, but flat and matte finishes hide surface flaws better, while satin and semi-gloss are easier to wash. Kitchens and bathrooms usually get satin or semi-gloss; ceilings almost always get flat.

Buying the right container sizes

Paint is sold in quarts, gallons and 5-gallon buckets. Five gallons usually costs less per gallon than five singles, and boxing several cans together in one bucket keeps the color consistent across walls. If the estimate lands just over a whole gallon, a quart may cover the extra; the result shows both. Keep a labeled quart of leftover paint for touch-ups and record the color formula in your phone.

Tips for a cleaner job

• Patch holes, sand and wipe the walls first; paint shows every dent.
• Cut in corners and edges with a brush, then roll while the edge is still wet to avoid lap marks.
• Use a 3/8 in nap roller on smooth walls and 1/2 in on light texture.
• If the house was built before 1978, test for lead before sanding and follow EPA lead-safe practices.

Common mistakes to avoid

• Measuring only the floor area: paint covers walls, so use perimeter × height.
• Skipping primer over fresh patches, which then flash through the finish coat.
• Buying the cheapest paint and then needing a third coat, which costs more overall.
• Mixing two cans of the same color from different batches on one wall; box them together first.
• Painting in high humidity or below the label minimum temperature, which slows drying and causes poor adhesion.

Waste factor

The default 10% covers paint left in the tray and roller, a little extra on the first coat over patched areas, and back-rolling. Drop it to 5% for small, smooth rooms painted with a sprayer you clean carefully; raise it to 15% for heavily textured walls or when you are switching from a dark color to a light one.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m, sq ft/gal ↔ m²/L. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Bedroom, 12 × 12 ft

Inputs: total wall length (perimeter) 48 ft; wall height 8 ft; doors 1; windows 2; coats 2; coverage per gallon 350 sq ft/gal; waste factor 10%.

Result: You need about 2.1 gallons (7.9 L).

• Units to buy (rounded up): 3 gallons
• Gross wall area: 384 sq ft (35.7 m²)
• Paintable area after doors/windows: 334 sq ft (31 m²)
• Buy (whole gallons): 3 gal
• Or in quarts: 9 qt

Living room with tall walls

Inputs: total wall length (perimeter) 70 ft; wall height 9 ft; doors 2; windows 4; coats 2; coverage per gallon 350 sq ft/gal; waste factor 10%; cost per unit $42.

Result: You need about 3.3 gallons (12.6 L).

• Units to buy (rounded up): 4 gallons
• Gross wall area: 630 sq ft (58.5 m²)
• Paintable area after doors/windows: 530 sq ft (49.2 m²)
• Buy (whole gallons): 4 gal
• Or in quarts: 14 qt
• Estimated material cost: $168.00

Same-color refresh, one coat

Inputs: total wall length (perimeter) 40 ft; wall height 8 ft; doors 1; windows 1; coats 1; coverage per gallon 400 sq ft/gal; waste factor 5%.

Result: You need about 0.7 gallons (2.8 L).

• Units to buy (rounded up): 1 gallons
• Gross wall area: 320 sq ft (29.7 m²)
• Paintable area after doors/windows: 285 sq ft (26.5 m²)
• Buy (whole gallons): 1 gal
• Or in quarts: 3 qt

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the paint calculator result scales.

Limitations

Results assume rectangular walls and average opening sizes. Actual coverage depends on the product, the applicator, the surface texture and how thickly you apply. Read the technical data sheet for the exact paint you buy and treat this as a shopping estimate, not a guarantee.