🏗️ Building & DIY

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.

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Wall Paint Calculator

Building & DIY

Unit
m
m
m

Standard door: 2.8 ft × 6.7 ft (21 sq ft) deducted

Standard window: 4 ft × 5 ft (20 sq ft) deducted

Paint ceiling too?
Number of Coats
8 sq ft/gal
5 m²/l12 m²/l

Check your paint can label. Standard latex paint: ~300–400 sq ft/gal

Paint Needed
10,0 Gallons
Total Area
45,0 m²
Wall Area (excl. doors/windows)
39,5 m²
Ceiling Area
–
Deducted Area
5,5 m²
Recommended Can Size
1 × 10 l

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.

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

RoomDimensionsSmooth wall, 1 coatSmooth wall, 2 coatsTextured, 2 coatsNote
Small bath8 × 6 ft, 8 ft ceil.0.6 gal1.2 gal1.5 gal1 window, no door
Small bedroom10 × 10 ft, 8 ft ceil.1.0 gal2.1 gal2.5 gal1 door, 1 window
Standard bedroom12 × 11 ft, 8 ft ceil.1.2 gal2.4 gal2.9 gal1 door, 1 window
Living room16 × 13 ft, 8.5 ft ceil.1.5 gal2.9 gal3.5 gal1 door, 2 windows
Large living room20 × 15 ft, 9 ft ceil.2.1 gal4.2 gal5.0 gal2 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

SurfaceFactorPaint for 430 sq ft net, 2 coats, 350 sq ft/galRecommended sizeNote
Smooth (previously painted)1.02.5 gal3 × 1 galStandard coverage per label
Drywall1.12.7 gal3 × 1 galSlightly absorbent; primer recommended
Fresh plaster1.152.8 gal3 × 1 galHighly absorbent; primer required
Textured wallpaper1.23.0 gal1 × 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

How much paint do I need for a 12×12 room?
A 12×12 ft room with 8 ft ceilings has roughly 352 sq ft of gross wall area. Subtract 1 door (21 sq ft) and 1 window (20 sq ft) to get 311 sq ft net. With 2 coats at 350 sq ft/gal: 311 × 2 / 350 = 1.8 gallons. Round up to 2 gallons — one standard gallon can per coat works here.
How much paint does a gallon cover?
Most standard latex paints cover 350–400 sq ft per gallon on smooth, previously painted surfaces. For porous surfaces like fresh drywall or new plaster, expect 15–20% less coverage. The coverage rate is always printed on the can label — use that number for the most accurate result.
Do I need primer before painting?
On fresh drywall and new plaster, primer is highly recommended. It seals the porous surface so your paint goes on evenly and achieves better coverage. Without primer, you may need an extra coat and still end up with uneven sheen.
Is 1 coat enough or do I need 2?
Light colors over white may cover in 1 coat. Dark colors, color changes (e.g. navy to white), or absorbent surfaces almost always need 2 coats. A second coat delivers uniform coverage, richer color, and better durability — skip it only on touch-ups.
How do I include the ceiling in my paint calculation?
Turn on the 'Paint ceiling too?' option in the calculator. The ceiling area (length × width) gets added automatically. If you're using a separate ceiling white, leave it off. Ceilings typically use a flat/matte finish that differs from wall paint — that's worth a separate calculation.

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.