Paint Calculator
Estimate how much paint you need for your room. Accounts for walls, ceiling, openings, and multiple coats.
Paint Calculator
How Much Paint Do I Need?
Real talk: paint calculation comes down to room size, coats you're putting on, paint quality, and wall condition. Get it right and you're not making a second trip or throwing money away.
Simple math: calculate paintable area (walls minus doors and windows, plus ceiling if you're painting it), divide by coverage per gallon, multiply by number of coats, round up. That's it.
Coverage Rates by Paint Type
Different finishes cover differently. It's mostly about how thick they go on and how they level out:
| Finish Type | Coverage | Best Use | Drying Time | Sheen Level |
|---|---|---|---|---|
| Flat | 350 sq ft/gal | Living rooms, bedrooms | 2-4 hours | Matte (non-reflective) |
| Eggshell | 350 sq ft/gal | Most rooms (family choice) | 2-4 hours | Low sheen |
| Satin | 350 sq ft/gal | Kitchens, bathrooms | 2-4 hours | Medium sheen, wipeable |
| Semi-gloss | 400 sq ft/gal | Trim, doors, cabinets | 4-8 hours | High sheen, very durable |
| High-gloss | 400 sq ft/gal | Specialty applications | 4-8 hours | Glossy, reflective |
How to Measure a Room for Paint
Measure every wall length and the ceiling height. For weird-shaped rooms, break them into sections and measure separately.
Wall area: add all wall lengths and multiply by height. A 12×12 room: (12+12+12+12) × 8 = 384 sq ft of wall.
Ceiling: length × width. That 12×12 ceiling is 144 sq ft.
Subtract openings. Standard door is roughly 20 sq ft. Standard window is about 12 sq ft. These don't need paint, so you subtract them.
Do Ceilings Need Different Paint?
Flat paint on ceilings. That's the rule. Hides imperfections and doesn't show brush marks or dust like glossy would. Walls get eggshell or satin, ceiling gets flat white or off-white. Some people swear by ceiling-specific flat paint — it's got additives that prevent sagging. Worth the extra cost if your ceiling is tricky.
Primer Coverage Rates
Primer covers about 350–400 sq ft per gallon, same as paint. Use it when you:
- Paint new drywall (always)
- Cover dark colors with light colors
- Paint over glossy surfaces
- Make dramatic color jumps
- Cover stains and water damage
Primer's cheaper than finish paint. Don't skip it — it saves you paint and coats later.
One Coat vs. Two Coats
Two coats is the rule. One might work for touch-ups or premium paint, but two coats gives you:
- Real color uniformity
- Better durability and scuff protection
- Stains don't bleed through
- Lasts longer before repainting
Dark colors? Three coats or primer plus two. Plan for it in your budget.
Paint for Trim and Doors Separately
Trim and doors go semi-gloss or high-gloss — different sheen than walls. Calculate separately:
- Measure all linear feet of baseboard, crown, door frames, window casings
- 12×12 room has about 48 linear feet of base
- Convert to sq ft: 48 LF × 1.5-inch brush width ÷ 12 = 6 sq ft
- Plus 2 doors at roughly 25 sq ft each = 50 sq ft for trim
Semi-gloss and gloss may cover more (400 sq ft per gallon) but work slower — thicker, shinier finishes take more time.
Textured Walls and Ceilings
Textured surfaces eat paint. Coverage drops to 200–250 sq ft per gallon depending on how thick the texture is. Heavy texture? Plan for 30–50% more paint than smooth walls. Benjamin Moore Regal or Sherwin-Williams Duration will cover 350-400 sqft per gallon on smooth walls. The $25/gallon stuff from big box stores? More like 250-300 sqft, and you'll probably need a third coat.
Worked Example: 12×12 Room Calculation
Room: 12'L × 12'W × 8'H, 1 door, painting walls and ceiling, 2 coats eggshell:
- Wall area: (12+12+12+12) × 8 = 384 sq ft
- Ceiling area: 12 × 12 = 144 sq ft
- Door opening: 20 sq ft
- Total: 384 + 144 – 20 = 508 sq ft
- Coverage: 350 sq ft per gallon
- For 2 coats: (508 ÷ 350) × 2 = 2.9 gallons
- Buy: 3 gallons
- Cost at $30/gal: $90
