Calculating paint by square foot doesn't have to be complicated. This USA-focused guide breaks down exactly how much paint you need per square foot, with a real example and quick-reference table.
How Much Paint Per Square Foot? USA Paint Calculator Guide
You've got your room dimensions, you're ready to head to the hardware store — but how do you translate square footage into actual gallons of paint?
It's a question every homeowner and painter faces. Buy too little and you're making a second trip mid-job. Buy too much and you've got a garage full of half-used cans.
This guide gives you the exact answer: how much paint per square foot, how to calculate it yourself, and a quick-reference table so you can shop with confidence.
The Standard Rule: How Much Does a Gallon of Paint Cover?
In the USA, paint coverage is typically measured in square feet per gallon.
The industry standard:
One gallon of paint covers approximately 350–400 square feet per coat.
This applies to smooth, interior walls with standard latex or acrylic paint. The number shifts depending on surface texture, paint quality, and application method.
Here's a quick breakdown by paint type:
| Paint Type | Coverage per Gallon | Coverage per Quart |
|---|---|---|
| Standard interior latex | 350–400 sq ft | 87–100 sq ft |
| Premium interior | 400–450 sq ft | 100–112 sq ft |
| Exterior paint | 250–350 sq ft | 62–87 sq ft |
| Primer | 200–300 sq ft | 50–75 sq ft |
| Ceiling paint | 350–400 sq ft | 87–100 sq ft |
For reference: 1 gallon = 3.785 litres | 1 quart = 0.946 litres | 1 sq ft = 0.093 m²
The Paint Calculator Formula in Square Feet
Here’s the full step-by-step formula:
Gallons needed = Net wall area (sq ft) ÷ Coverage rate (sq ft/gal) × Number of coats
Step 1 — Calculate Total Wall Area in Sq Ft
Measure each wall: height (ft) × width (ft)
For a rectangular room:
- Perimeter = (length + width) × 2
- Wall area = Perimeter × ceiling height
Example: Room: 12 ft × 10 ft, ceiling height 9 ft Perimeter = (12 + 10) × 2 = 44 ft Wall area = 44 × 9 = 396 sq ft
Step 2 — Subtract Doors and Windows
| Opening | Typical Size |
|---|---|
| Interior door | ~20 sq ft (1.8 m²) |
| Standard window | ~15 sq ft (1.4 m²) |
| Large/picture window | ~25 sq ft (2.3 m²) |
| Sliding glass door | ~40 sq ft (3.7 m²) |
Step 3 — Divide by Coverage Rate
Use 350 sq ft/gallon as a conservative baseline for standard paint.
Step 4 — Multiply by Number of Coats
Most rooms need 2 coats for proper, even coverage.
Step 5 — Add 10% Buffer
Always add a small buffer for overspray, uneven surfaces, and touch-ups.
Real Example: Paint Calculator for a US Bedroom (Square Feet)
Let’s run through a complete real-world calculation.
Room details:
- Dimensions: 14 ft × 12 ft (4.3m × 3.7m)
- Ceiling height: 9 ft (2.7m)
- 1 door (20 sq ft), 2 windows (15 sq ft each)
- Standard interior latex paint (coverage: 375 sq ft/gallon)
- 2 coats
Step 1 — Total wall area: Perimeter = (14 + 12) × 2 = 52 ft Total area = 52 × 9 = 468 sq ft
Step 2 — Subtract openings: Door + 2 windows = 20 + 30 = 50 sq ft Net area = 468 − 50 = 418 sq ft
Step 3 — Paint for 2 coats: (418 × 2) ÷ 375 = 2.23 gallons
Step 4 — Add 10% buffer: 2.23 × 1.1 = 2.45 gallons
What to buy: 2.5 gallons (or one gallon + two quarts, rounded up)
In metric: 2.5 gallons ≈ 9.5 litres
Quick Reference: How Many Gallons for Common Room Sizes?
| Room | Dimensions (sq ft) | 2 Coats Needed |
|---|---|---|
| Small bathroom | 6 × 6 ft (36 sq ft floor) | ~1 quart |
| Small bedroom | 10 × 10 ft (100 sq ft floor) | ~1 gallon |
| Medium bedroom | 12 × 11 ft (132 sq ft floor) | ~1.5 gallons |
| Large bedroom | 14 × 12 ft (168 sq ft floor) | ~2–2.5 gallons |
| Living room | 16 × 13 ft (208 sq ft floor) | ~2.5–3 gallons |
| Open-plan living | 22 × 16 ft (352 sq ft floor) | ~4–5 gallons |
Assumes 9 ft ceilings, 2 coats, standard paint at 375 sq ft/gal, 1 door + 1–2 windows deducted.
What Affects Paint Coverage Per Square Foot?
The 350–400 sq ft per gallon figure is a guideline, not a guarantee. Several factors can push your usage higher:
Surface Texture
Smooth drywall: full coverage at standard rates Orange peel or knockdown texture: expect 10–15% more paint Skip trowel or heavy texture: up to 20–25% more Raw concrete or brick: can need 2× the standard amount
Paint Colour
Dark colours going over light walls often achieve full coverage in 2 coats. Light colours going over dark walls may need 3 coats — or a tinted primer first.
Paint Quality
Budget paint tends to have lower pigment density, meaning more coats for the same result. Premium paints from brands like Sherwin-Williams, Benjamin Moore, or Behr Premium often genuinely cover better and save money overall.
Application Method
- Roller — most efficient for walls; standard coverage rates apply
- Brush — slightly more paint used, especially on trim
- Sprayer — faster but uses 20–30% more paint due to overspray; always mask generously
Do I Need Primer? How Does It Affect Square Footage?
Primer is necessary when:
- Painting new drywall (it’s highly absorbent — skip primer and you’ll use twice the topcoat)
- Covering stains, smoke damage, or water marks
- Making a dramatic colour change
- Painting over glossy surfaces
Primer covers around 200–300 sq ft per gallon — less than topcoat. But using it usually means you can get away with just one topcoat instead of two, saving paint and money in the long run.
Rule of thumb: If you’re unsure, use a primer-sealer on new or heavily repaired walls. It’s almost always worth it.
Ceiling Paint: How Many Sq Ft Per Gallon?
If you’re painting the ceiling too, calculate it separately:
Ceiling area = Length × Width
For the 14 × 12 ft bedroom above: Ceiling = 14 × 12 = 168 sq ft At 375 sq ft/gallon × 1 coat = 168 ÷ 375 = 0.45 gallons → buy 1 quart
Ceiling paint is usually applied in just 1–2 coats. If the ceiling is already white, 1 coat is often enough.
How to Convert Square Feet to Litres (For International Buyers)
If you’re shopping internationally or using a metric-based paint:
- 1 gallon = 3.785 litres
- 1 quart = 0.946 litres
- 1 sq ft = 0.093 m²
- 350 sq ft/gallon ≈ 9.5 m²/litre
So a room needing 2.5 gallons = approximately 9.5 litres.
Use Paint Pro Calc to instantly calculate paint, cost, and profit — in both US gallons and litres, automatically.
5 Tips to Nail Your Paint Estimate in Square Feet
1. Measure walls, not floor area. The most common mistake homeowners make is using floor area to estimate wall paint. You need to measure the actual wall surface — height × width of each wall.
2. Use a conservative coverage rate. Better to estimate at 350 sq ft/gallon than 400. If your walls are textured or the paint is cheap, you’ll be glad you did.
3. Always buy 10% more than your calculation. This covers uneven application, touch-ups, and the inevitable drip. You can always return unopened cans.
4. Keep the colour code and batch number. Custom colours are mixed per batch. If you need more later and can’t match it, you’ll have to repaint the entire wall. Keep a photo of the label.
5. Don’t forget trim and baseboards. Door frames, window casings, baseboards, and crown moulding need their own separate calculation — usually in a semi-gloss or gloss finish.
Summary: Paint Per Square Foot at a Glance
| Paint type | Sq ft per gallon | Litres per m² |
|---|---|---|
| Standard interior | 350–400 | 0.08–0.10 L |
| Premium interior | 400–450 | 0.07–0.09 L |
| Exterior | 250–350 | 0.10–0.13 L |
| Primer | 200–300 | 0.10–0.14 L |
For most rooms, plan on 1 gallon per 350 sq ft of wall area, per coat.
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