Wall Paint Calculator: How Much Paint Do You Need?
Enter your room dimensions, surface type, and paint coverage – and get the exact amount of paint you need. With recommended can size.
Wall Paint Calculator
Building & DIY
🚪 Number of Doors, Number of Windows, Paint ceiling too?
Standard door: 2.8 ft × 6.7 ft (21 sq ft) deducted
Standard window: 4 ft × 5 ft (20 sq ft) deducted
Check your paint can label. Standard latex paint: ~300–400 sq ft/gal
Note: Always ensure good ventilation while painting. Latex paints are water-based and low-odor, but open your windows anyway. Wear safety goggles for overhead work. Paint sprayers require a respirator mask.
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How Much Paint Do I Need? The Number That Makes It Through Two Coats
Two trips to the paint store on the same Sunday is how most people learn this: the coverage figure on the can is a lab number, measured on a sealed, smooth surface with a sprayer. Your wall is textured, half-absorbent, and rolled by hand — so it drinks 15–20% more than the label promises. The number that actually makes it through both coats starts from the net wall area, not the room size, then adds back what the surface and the extra coat really cost.
That's the math behind the calculator. Gross wall area is (length + width) × 2 × ceiling height; standard deductions come off for openings — 21 sq ft per door (2.8 × 7.5 ft), 20 sq ft per window (4 × 5 ft) — leaving the net wall area. A surface factor adds for porosity (textured wallpaper +20%, fresh plaster +15%), then net area × surface factor × coats ÷ coverage = gallons. Worked through for a 16 × 13 ft room, 8.2 ft ceiling, 1 door, 2 windows, 2 coats, textured wallpaper (factor 1.2) at 350 sq ft/gal: gross (16+13) × 2 × 8.2 = 476 sq ft, minus 61 sq ft of openings = 415 net, × 1.2 × 2 ÷ 350 = 2.8 gallons → buy 3.
Paint Quantities for Common Room Sizes
| Room | Dimensions | Smooth wall, 1 coat | Smooth wall, 2 coats | Textured, 2 coats | Note |
|---|---|---|---|---|---|
| Small bath | 8 × 6 ft, 8 ft ceil. | 0.6 gal | 1.2 gal | 1.5 gal | 1 window, no door |
| Small bedroom | 10 × 10 ft, 8 ft ceil. | 1.0 gal | 2.1 gal | 2.5 gal | 1 door, 1 window |
| Standard bedroom | 12 × 11 ft, 8 ft ceil. | 1.2 gal | 2.4 gal | 2.9 gal | 1 door, 1 window |
| Living room | 16 × 13 ft, 8.5 ft ceil. | 1.5 gal | 2.9 gal | 3.5 gal | 1 door, 2 windows |
| Large living room | 20 × 15 ft, 9 ft ceil. | 2.1 gal | 4.2 gal | 5.0 gal | 2 doors, 2 windows |
All figures at 350 sq ft/gal (standard latex). Ceiling not included. Includes ~5% buffer for edges and corners.
Surface Type Impact on Paint Quantity
| Surface | Factor | Paint for 430 sq ft net, 2 coats, 350 sq ft/gal | Recommended size | Note |
|---|---|---|---|---|
| Smooth (previously painted) | 1.0 | 2.5 gal | 3 × 1 gal | Standard coverage per label |
| Drywall | 1.1 | 2.7 gal | 3 × 1 gal | Slightly absorbent; primer recommended |
| Fresh plaster | 1.15 | 2.8 gal | 3 × 1 gal | Highly absorbent; primer required |
| Textured wallpaper | 1.2 | 3.0 gal | 1 × 5 gal (partial) | Texture adds ~20% effective surface area |
Common Paint Buying Mistakes
❌ Ignoring the coverage rate on the can
Problem: Many people assume 400 sq ft/gal, but budget paints often deliver only 300 sq ft/gal. On a 400 sq ft room with 2 coats, that's 2.7 gallons instead of 2.0 — a whole extra quart you didn't budget for.
✅ Fix: Read the "theoretical coverage" on the paint can label and enter it into the calculator. It's always listed under the technical data.
❌ Skipping the surface adjustment
Problem: Painting textured wallpaper with the same quantity calculated for smooth walls leaves you short. The texture increases the real surface area by 20%.
✅ Fix: Select the correct surface type. With textured wallpaper (factor 1.2), always buy more than the smooth-wall estimate.
❌ Planning 1 coat for a dark-to-light color change
Problem: Going from deep navy to off-white can take 3 coats with budget paint. One coat leaves strong bleed-through and an uneven finish.
✅ Fix: Plan for 2 coats minimum when changing colors. For extreme changes, apply a gray or tinted primer first — it reduces the number of finish coats needed.
❌ Painting fresh drywall without primer
Problem: Unprimed drywall absorbs paint unevenly. The paper face drinks in the first coat, leaving blotchy sheen and poor adhesion.
✅ Fix: Apply a drywall primer first — standard practice for the pros at the Painting Contractors Association. It seals the surface, evens out absorption, and makes your finish coats go further — often saving you one full coat.
❌ Buying an extra small can just in case
Problem: Running short with 200 sq ft left to go and buying a quart at retail is expensive per gallon, and dye lots may not match what you already applied.
✅ Fix: Always buy the next size up. The calculator recommends the exact can combination — a 5-gallon bucket beats two separate gallons for large rooms, and it costs less per gallon.
Frequently Asked Questions
Also planning to wallpaper? Our Wallpaper Calculator gives you the exact number of rolls needed — with pattern repeat, waste, and door and window deductions.
Find more free calculators for home and garden projects on our tool overview page.