Quick Room Paint Calculator
Calculate paint needed from room dimensions with door and window offsets.
Returns gallons and liters based on standard 400 sq ft per gallon wall coverage rate.
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
| Unit | Coverage per container (one coat, smooth wall) |
|---|---|
| 1 US gallon | 350 sq ft (lab number is 400, real-world is closer to 320-350) |
| 1 liter | ~10 sq m |
| 1 imperial quart | ~88 sq ft |
| 1 spray can (12 oz) | 15 to 25 sq ft |
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 — 12% of the total. Worth subtracting on a tight budget, optional otherwise.
Color transition rules of thumb
| Going from | To | Coats |
|---|---|---|
| White | Light beige / cream | 2 |
| White | Mid-tone gray / blue | 2 |
| White | Dark navy / forest green | 3 plus tinted primer |
| White | Black / charcoal | 3 plus gray primer |
| Dark | Light (white, cream) | 3 (often) plus tinted primer |
| Mid-tone | Same-family color | 2 |
| Bold red | Anything | Always 3, with primer |
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 — turns a 3-coat marathon into a 2-coat afternoon.
Sheen choice affects coverage
| Sheen | Coverage | Where to use |
|---|---|---|
| Flat / matte | Best coverage; hides imperfections | Ceilings, low-traffic walls |
| Eggshell | Good coverage, slight wipe-ability | Living rooms, bedrooms |
| Satin | Lower coverage; more durable | Kids’ rooms, hallways |
| Semi-gloss | Lower coverage; very wipeable | Bathrooms, kitchens, trim |
| Gloss | Lowest coverage; very durable | Doors, trim, cabinetry |
Going from gloss to flat usually needs an extra coat because flat paints are thinner.
Pricing — what a gallon actually costs
| Tier | $/gallon | Examples |
|---|---|---|
| Builder grade | $20 to $30 | Glidden Premium, Behr Premium Plus |
| Standard | $35 to $55 | Behr Marquee, Sherwin-Williams ProClassic |
| Premium | $55 to $90 | Benjamin Moore Aura, Sherwin Emerald |
| Designer / specialty | $100+ | Farrow & Ball, Portola Paints |
A 432 sq ft room in two coats needs about 3 gallons. At mid-tier pricing: $105 to $165 for paint alone. Plus rollers ($15), brushes ($15), painter’s tape ($10), drop cloth ($15), primer if needed ($30 to $50), and patching compound for nail holes ($5). Total budget for one room: roughly $180 to $260 in materials.
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) — back/neck nightmare
- Older homes with lead paint — testing and remediation matters
- Exterior siding — equipment investment isn’t worth it for one job
- Cabinets and trim — 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 be $850 to $2,600 — versus $180 in DIY materials plus a weekend.