Paint Coverage Calculator
Calculate exactly how many gallons of paint to buy for any room — accounting for doors, windows, coats, and a 10% waste buffer.
Buying too little means a mid-project store run. Buying too much wastes money. One accurate measurement takes 30 seconds.
350 sq ft
Coverage per gallon of standard interior latex paint — the professional industry benchmark
2 coats
Standard application for full coverage — especially on colour changes or previously dark walls
10%
Buffer to add to your purchase — covers spills, touch-ups, and minor measurement errors
What most paint calculators get wrong
Simple area calculators miss doors, windows, and coating factors — this one doesn't.
Measure walls, not floors
A common mistake is confusing floor area with wall area. A 12 × 14 ft room has 168 sq ft of floor — but over 450 sq ft of wall if ceilings are 9 ft. Wall area is always larger than floor area in any standard room. Always measure from the walls up.
Doors and windows mean less paint needed
A standard interior door is roughly 21 sq ft (7 ft × 3 ft). A standard window is roughly 15 sq ft (3 ft × 5 ft). On a room with 2 doors and 3 windows, that's 87 sq ft you don't need to paint — potentially saving a full gallon on a large project.
Buy in the right can size
Paint typically comes in quart (0.25 gal), one-gallon, and five-gallon containers. Five-gallon buckets are cheapest per gallon but wasteful for small rooms. If your estimate lands between sizes, round up one size — leftover paint is useful for future touch-ups.
Paint estimate guide — 9 ft ceilings, standard doors/windows, 10% waste
A standard gallon of paint covers approximately 350–400 square feet with one coat on a smooth, primed surface. Rough or porous surfaces, dark colour changes, or unprepared walls can reduce coverage to 250–300 sq ft per gallon and require an additional coat. Buying the right amount matters: buying too little means a mid-project trip to the hardware store risking dye lot mismatches; buying too much wastes $35–$70 per unused gallon.
| Room size | Coats | Gallons needed | Notes |
|---|---|---|---|
| 10×10 room | 1 coat | 1.0182857142857145 gal | Accent wall or touch-up |
| 14×12 room | 2 coats | 2.6211428571428574 gal | Standard bedroom |
| 20×16 room | 2 coats | 3.5262857142857142 gal | Large living room |
| Any room | 3 coats | Approx 1.5× | Dark colour over light |
How do you calculate how much paint you need for a room?
1. Calculate wall area: (room perimeter × ceiling height) − (windows × 15 sq ft) − (doors × 20 sq ft). For a 12×14 room with 8ft ceilings, 2 windows, 1 door: perimeter = 52ft × 8ft = 416 sq ft − 30 − 20 = 366 sq ft. 2. Divide by coverage rate per coat (350 sq ft/gallon standard): 366 ÷ 350 = 1.05 gallons per coat. 3. Multiply by number of coats (typically 2 for full coverage, especially colour changes): 1.05 × 2 = 2.1 gallons. Round up to the nearest quart or gallon: buy 2.25 or 2.5 gallons.
How many coats of paint does a typical room need?
New drywall (after primer): 2 coats. Repainting same or similar colour: 1–2 coats depending on application quality. Dark to light colour change: 2–3 coats (the dark undercolour bleeds through lighter paints). Light to dark: 2 coats usually sufficient. Repairing patched areas: always 2 coats — patches absorb more paint and look uneven with single coats. High-quality paint in the $50–$70/gallon range (Benjamin Moore Aura, Sherwin-Williams Emerald) is formulated for better one-coat coverage — the premium price often pays off in labour savings for professional painters.
Should I use primer, and does it reduce how much paint I need?
Primer is essential for: new drywall (required — paint soaks directly into unprimed drywall); dark-to-light colour changes (tinted primer in a mid-tone dramatically reduces topcoat count); stain-blocking (water stains, smoke, marker); and when switching between paint types (oil to latex). Properly primed surfaces typically require one fewer topcoat, saving $35–$70 in paint and 1–2 hours in application. Primer costs $25–$40/gallon but its coverage rate is similar to paint. For standard repaints of previously painted walls in similar colours, skip primer and apply 2 quality topcoats.
How the Paint Coverage Calculator Works
Formula
Wall Area = 2 × (Length + Width) × Height
Net Area = Wall Area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
Gallons Needed = (Net Area × Coats) / 350
Recommended Purchase = Gallons Needed × 1.10 (10% buffer)Enter room dimensions
Length, width, and ceiling height in feet. Standard US ceilings are 8–9 ft.
Count doors and windows
Each door subtracts ~21 sq ft · Each window subtracts ~15 sq ft from the paintable area.
Choose number of coats
2 coats is standard for most repaints · 1 coat for same-colour refresh · 3 coats for dark-to-light colour changes.
See gallons + buffer
The result shows exact gallons needed and the recommended purchase with a 10% safety buffer.
The most common painting mistake is buying paint based on floor area rather than wall area. In a standard 12×14 ft room with 9 ft ceilings, the floor is 168 sq ft but the walls are over 450 sq ft — nearly three times more. Always calculate wall area, not floor space.
This calculator uses the 350 sq ft/gallon industry standard, which gives a slightly conservative estimate on smooth surfaces. Premium paints may cover up to 400 sq ft/gallon, so if you're using a high-quality product, you may have a little extra — useful for touch-ups.
Frequently Asked Questions
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