Quick Room Paint Calculator

Quick wall area math

wall area = 2 × (length + width) × height

This is the rectangle of all four walls multiplied out. A 15×12 room with 8 ft ceilings: 2 × (15+12) × 8 = 432 sq ft of wall. You can then subtract doors and windows for more precision.

Paint coverage benchmarks

This calculator uses 350 sq ft per gallon, not the 400 printed on most cans. That number comes from a lab roller on sealed, perfectly smooth drywall. Your walls are neither.

Why most rooms need two coats

Paint isn’t actually opaque on the first coat. The first coat is mostly absorbed by the surface (especially fresh drywall or porous primer) and provides a uniform base for the color. The second coat is when you actually see the finished color come up. Skipping the second coat is the #1 reason DIY paint jobs look streaky.

The exception: high-quality “paint and primer in one” formulas (Behr Marquee, Sherwin-Williams Emerald, Benjamin Moore Aura) sometimes genuinely cover in one coat, when the wall is already a similar color or neutral. Going from dark to light or vice versa still needs two regardless of marketing claims.

Door and window deductions

If you want precision, subtract:

• Standard interior door: 20 sq ft (3 × 6.67 ft)
• Standard window: 15 sq ft (3 × 5 ft)
• Sliding glass door: 40 sq ft
• Picture window: 20 to 35 sq ft

Most professional painters skip deductions on small rooms because the leftover paint covers touch-ups and edge work. For a single 15×12 room with 1 door and 2 windows, deductions total 50 sq ft, about 12% of the 432 sq ft wall area. Worth subtracting on a tight budget, optional otherwise.

This calculator does not subtract them. If you want the deduction, use the room paint quantity calculator, which takes door and window counts and handles the ceiling separately.

Color transition rules of thumb

Tinted primer is the secret. Asking your paint store to tint primer halfway between your old wall color and new wall color cuts a coat off the job. It turns a 3-coat marathon into a 2-coat afternoon.

Sheen choice affects coverage

Going from gloss to flat usually needs an extra coat because flat paints are thinner.

Pricing: what a gallon actually costs

Every figure below is a rough US retail range and will drift with the year, the state, and whichever sale is running. Treat them as a sanity check on a quote, not as a price list.

A 432 sq ft room in two coats needs about 3 gallons. At mid-tier pricing that is $105 to $165 for the paint alone. Add rollers ($15), brushes ($15), painter’s tape ($10), a drop cloth ($15), and patching compound for nail holes ($5), which comes to $60 of sundries you mostly keep for next time.

So one room lands at roughly $165 to $225 in materials, or $195 to $275 if the walls need primer at $30 to $50.

Time estimate (often under-budgeted)

Most DIY painters underestimate time by 50 to 100%. A single bedroom (12×12) realistically takes:

• Prep (move furniture, tape, fill holes, sand): 1.5 to 3 hours
• Cutting in (brushing edges, around trim): 1.5 to 2 hours
• Rolling first coat: 0.5 to 1 hour
• Dry time before second coat: 2 to 4 hours
• Second coat (cutting in + rolling): 1.5 to 2 hours
• Cleanup and re-hanging: 1 hour

Total: 6 to 12 hours of active work, plus the drying time. Most rooms take a full weekend if you also moved furniture and patched.

When to call a pro

• Ceilings (especially high, vaulted, or with crown molding) - a back and neck nightmare
• Older homes with lead paint - testing and remediation matters
• Exterior siding - the equipment investment isn’t worth it for one job
• Cabinets and trim - a pro spray finish is dramatically better
• Color changes that demand 3+ coats - pros are 3x faster than DIY at scale

A pro painter typically quotes $2 to $6 per sq ft of wall for two coats including labor and standard-grade paint. That 432 sq ft bedroom would run $850 to $2,600, versus about $165 in DIY materials plus a weekend of your own time.