Calculate gallons needed, primer, and total cost for any room, exterior, or multi-room project.
Every paint calculator starts with one number: 350 square feet per gallon. That's the industry-standard coverage rate for standard interior latex paint applied by roller to a smooth surface. Premium paints often cover 400 square feet per gallon because they contain more pigment solids. Low-quality budget paint may only cover 300. The math cascades from there: multiply your paintable square footage by your number of coats, divide by the coverage rate, and round up to the nearest half-gallon (since paint is sold in quarts and gallons, not arbitrary decimal quantities).
Paintable area is where most DIYers get it wrong. The gross wall area of a room — calculated as the perimeter (2 × length + 2 × width) multiplied by the ceiling height — includes areas you're not actually painting. Standard interior doors cover 21 square feet (3 ft × 7 ft). A typical window covers 15 square feet (3 ft × 5 ft). Subtract these from your gross wall area to get the net paintable area. It sounds minor, but a living room with 4 windows and 2 doors can have over 120 square feet of non-paintable surface — enough to account for nearly half a gallon in a typical calculation.
The ceiling is often the largest single surface in a room and typically requires its own gallon. A 12×14 bedroom has 168 square feet of ceiling — nearly half a gallon at standard coverage, or a full gallon for two coats. Ceiling paint and wall paint are different products. Ceiling paint is ultra-flat to hide imperfections and prevent drips, and its formulation is optimized for horizontal application. Using wall paint on ceilings creates drip marks. Using ceiling paint on walls leaves a flat, chalky surface that marks easily and cannot be cleaned. Buy them separately and calculate them as separate surfaces.
Paint finish affects durability, cleanability, and how much it shows surface imperfections. The rule is simple: higher traffic and moisture = higher sheen. Flat and matte finishes hide imperfections beautifully but cannot be scrubbed. Semi-gloss and gloss finishes are bulletproof in terms of cleanability but will highlight every dent, crack, and texture irregularity on your walls. The sweet spot for most living spaces is eggshell or satin.
| Finish | Sheen Level | Best For | Cleanability | Hides Flaws |
|---|---|---|---|---|
| Flat / Matte | 0–5% | Ceilings, low-traffic rooms | Poor — no scrubbing | Excellent |
| Eggshell | 10–25% | Bedrooms, dining rooms | Good — wipe clean | Good |
| Satin | 25–35% | Kitchens, kids' rooms, hallways | Very good — scrub safe | Fair |
| Semi-Gloss | 35–70% | Trim, doors, bathrooms | Excellent | Poor |
| Gloss | 70–90% | Cabinets, furniture | Best in class | Very poor |
Primer is a sealer, stain blocker, and adhesion promoter. It is not just watered-down paint. You absolutely need primer when painting new, unprimed drywall — without it, the porous paper face absorbs paint unevenly and you'll spend twice as much on paint for the same result. You need primer when making a dramatic color change, particularly going from a dark color to a light one or covering a red or orange — these colors are notoriously difficult to cover and require primer plus multiple paint coats. Stains from water damage, smoke, or markers will bleed through multiple coats of regular paint; only a shellac-based or stain-blocking primer stops them.
You don't need primer when repainting with a similar color over existing paint that's in good condition — just clean the walls, lightly scuff with 220-grit sandpaper, and apply two coats. Paint-and-primer-in-one products (common from Sherwin-Williams and Benjamin Moore) are legitimate for standard repaints but should not be treated as a substitute for full primer on problem surfaces. The "primer" component in these products is a marketing claim more than a functional reality for challenging applications.
The 350 sq ft/gallon standard assumes smooth, properly primed drywall. Textured surfaces have more actual surface area than their measured square footage, so they absorb more paint. Light orange-peel or knockdown texture adds roughly 10% more paint consumption. Medium knockdown or skip-trowel texture requires about 20% more. Heavy stucco, brick, or popcorn texture can increase paint consumption by 30–40%. If your room has textured walls, increase your gallon estimate accordingly before purchasing. Our calculator includes a coverage rate field specifically for this adjustment — drop it from 350 to 280–300 for textured surfaces.
| Surface Type | Coverage Rate | Adjustment Factor |
|---|---|---|
| Smooth drywall (primed) | 350–400 sq ft/gal | Baseline |
| Light orange-peel texture | 310–340 sq ft/gal | +10% |
| Medium knockdown / skip trowel | 280–310 sq ft/gal | +20% |
| Heavy popcorn / stipple | 210–260 sq ft/gal | +30–40% |
| Smooth exterior siding (wood) | 300–350 sq ft/gal | Baseline exterior |
| Rough-sawn / weathered wood | 200–280 sq ft/gal | +25–35% |
| Stucco / masonry | 150–250 sq ft/gal | +40–60% |
Exterior painting is a more involved calculation than interior work because the surfaces are more varied and the stakes are higher — a bad exterior paint job peels within two or three years, while an interior mistake is much cheaper to fix. The calculation starts with gross siding area: the full perimeter of the house multiplied by the wall height. For a two-story home, the wall height is typically 18–20 feet (two 9-foot ceilings plus floor joists and framing). For a ranch, 9–10 feet per story is standard.
From that gross area, subtract openings — windows average 15 square feet each and doors average 21 square feet. Garage doors are much larger, averaging 63 square feet for a standard single (7×9 ft) and 126 square feet for a double. Many homeowners forget that garage doors are often painted separately or with a different paint type, so track them as a distinct line item. After subtracting all openings, you have your net siding area. This is what gets painted.
Exterior trim — window surrounds, corner boards, fascia, soffits, and door trim — accounts for approximately 15–20% of total siding area on a typical home. Trim is almost always painted with a semi-gloss or gloss exterior paint in a contrasting or complementary color to the main siding. Use a separate gallon count for trim and buy it in a separate color. For a 1,500 square foot ranch house with approximately 1,200 square feet of net siding area, expect 180–240 square feet of trim area — about half a gallon to a full gallon per coat.
Soffits (the underside of roof overhangs) are often neglected in exterior painting calculations. A ranch-style home with 2-foot overhangs around all four sides might have 200+ square feet of soffit area. Soffit paint should be exterior flat or low-sheen — you don't need durability as soffits don't get touched, but you do need UV resistance and moisture resistance. Calculate soffit area as overhang depth × perimeter.
Materials represent only 25–35% of the total cost of a professional painting job. Labor is the dominant expense, and understanding this split helps you make smarter decisions about DIY versus professional work. For interior painting, professional labor runs $1.50–$3.50 per square foot of paintable surface, depending on prep work required, ceiling height, and the region of the country. A 2,000 square foot interior in average condition costs $3,000–$7,000 professionally — but only $300–$600 in materials if you DIY.
| Project Type | DIY Materials | Professional Total | DIY Savings |
|---|---|---|---|
| Single bedroom (200 sq ft) | $50–$120 | $350–$600 | $250–$480 |
| Full interior (1,500 sq ft) | $350–$600 | $2,500–$5,000 | $2,000–$4,400 |
| Full interior (2,500 sq ft) | $550–$950 | $4,000–$8,000 | $3,500–$7,000 |
| Exterior — ranch (1,200 sq ft siding) | $300–$550 | $2,500–$4,500 | $2,000–$4,000 |
| Exterior — two-story (2,000 sq ft siding) | $500–$900 | $4,000–$7,000 | $3,500–$6,000 |
Budget paint at $20–$28 per gallon seems economical but frequently requires an extra coat, delivers mediocre color richness, and fades or scuffs within 3–5 years on walls. Premium paint at $55–$70 per gallon typically covers in one coat fewer, lasts 7–10 years before needing a refresh, and provides noticeably better color depth and washability. Over a 10-year horizon, the cost difference between budget and premium is often negligible or negative — you repaint less frequently with premium product and save on labor (professional or your own time).
Designer and specialty paints at $80–$100+ per gallon are formulated for specific applications — high-humidity commercial kitchens, heavy-traffic schools, hospitals — and are overkill for most residential applications. The meaningful decision for most homeowners is between standard-grade and premium paint. For high-traffic surfaces like hallways, stairways, kitchens, and kids' rooms, premium is clearly worth it. For a spare bedroom or infrequently used dining room, standard paint is a reasonable choice.