PAINT GALLONS

Enter your room length, width, and ceiling height. Set the number of coats — 2 is standard for new color or covering primer; 1 may be enough for refresh of the same color. Pick the surface type: smooth drywall and previously painted walls get the full 350 ft² per gallon, while textured walls (knockdown, orange peel, popcorn) drop to 275 ft² per gallon because the texture has more surface area.

The result rounds up to whole gallons since paint is sold by the gallon. Always buy a slight surplus — touchups years later are nearly impossible without a leftover can in the same lot. The calculator covers walls only; add a separate gallon for the ceiling and a quart of trim paint per ~100 linear feet of baseboard, casing, and crown.

For a 12 × 10 ft room with 8-foot ceilings, 2 coats, smooth walls:

Wall area = 2 × (12 + 10) × 8 = 352 ft². Total to cover = 352 × 2 = 704 ft². At 350 ft² per gallon: 2.01 gallons → round up to 3 gallons (the third gallon covers touchups, the closet you forgot, and stays sealed for future repairs).

If those same walls were textured: 704 ÷ 275 = 2.56 gallons → also 3 gallons. Same purchase, but you'll use more of the third can during the actual job.

Add 1 quart of trim paint per ~80–100 linear feet of baseboard, casing, and crown — and a separate gallon if you're painting the ceiling, since ceiling paint is a flatter formula than wall paint.

Skipping primer on bare drywall, water stains, or color changes from dark to light. Primer is cheaper than paint and seals the surface so your topcoat covers in one coat instead of three.

Confusing wall area with floor area. A 12 × 10 room has 120 ft² of floor but ~352 ft² of walls (more than 3× the floor area). Calculating from floor area alone leaves you 60% short.

Buying paint from different lots for the same job. Two cans labeled the same color can vary slightly between manufacturing batches. Have the store mix all your gallons in one batch, or "box mix" by intermixing cans yourself before applying.

Ignoring sheen for the room's use. Flat hides imperfections but won't scrub clean — bad for kitchens, bathrooms, hallways, and kids' rooms. Eggshell and satin clean better; semi-gloss for trim and doors.

Smooth wall: 350 ft² per gallon, one coat. Textured: 275 ft² per gallon. Two coats is the residential standard.

Ceilings: 1 gallon covers about 350 ft² of ceiling at one coat. A typical 12 × 10 ceiling is 120 ft² — 1 gallon does it twice with paint left over.

Trim and doors: 1 quart covers about 100 linear feet of baseboard or one full interior door (both sides + frame).

Wall area ≈ 3× floor area for a standard 8-foot ceiling — useful sanity check.

Allow 1 gallon of leftover per 4-5 gallons used for future touchups. Same lot will never come back.