Paint Calculator
Calculate how much paint you need for a room based on dimensions
Room Dimensions (feet)
How It Works
Overview
A paint calculator estimates how many gallons of paint you need to cover the walls of a room based on its dimensions and the openings (doors and windows) you won't paint over. The calculation starts from the surface area in square feet, subtracts openings, multiplies by the number of coats, and divides by the paint's coverage rate to give you a buying figure.
Most interior latex paints cover roughly 350 to 400 square feet per gallon on a smooth, primed surface. Rough textures, raw drywall, and dark-to-light color changes drink more paint, so always plan for at least two coats and add 10–15% to your final number. It's much cheaper to buy a spare quart for touch-ups than to interrupt a project halfway through to color-match a fresh batch.
The Formula
Where:
- L, W = room length and width in feet
- H = ceiling height in feet
- openings = total square feet of doors and windows (use 20 sq ft per door, 15 sq ft per window as defaults)
- coats = number of finish coats, typically 2
- coverage = manufacturer rating in sq ft per gallon, typically 350
The first part — 2 × (L + W) × H — is the perimeter times the height, which gives the total wall surface area for a rectangular room. Always round the final gallon count up to the nearest whole gallon when shopping.
Worked Example
Take a 14 ft × 12 ft bedroom with 9 ft ceilings, one door, and two windows, painting two coats with standard 350 sq ft/gallon paint:
- Wall perimeter: 2 × (14 + 12) = 52 ft
- Total wall area: 52 × 9 = 468 sq ft
- Openings: 1 door (20) + 2 windows (30) = 50 sq ft
- Paintable area: 468 − 50 = 418 sq ft
- Two coats: 418 × 2 = 836 sq ft of coverage needed
- Gallons: 836 ÷ 350 = 2.39 gallons → buy 3 gallons
That gives you a comfortable buffer for touch-ups. If the room has a textured wall finish or you're painting white over a navy accent wall, plan for an extra half-gallon to cover the absorption.
When to Use This
- Buying paint for a single-room repaint — bedroom, office, dining room.
- Estimating a whole-house exterior or interior repaint — sum each room individually.
- Comparing paint brands — better paints cover at 400+ sq ft/gallon and need only one coat in some cases.
- Budget planning for renovations — translate gallons × $40 per gallon into a real materials cost line.
- Picking a feature wall vs full room — see how much paint and money you save by accenting one wall.
- Estimating primer separately — primer uses the same coverage math as paint.
Common Mistakes to Avoid
- Forgetting to multiply by coats. Two coats is the norm; don't buy enough for one and run out.
- Subtracting too much for openings. A small kitchen window is more like 9 sq ft; an oversized sliding door is 40+ sq ft. Adjust if your openings deviate from the 20/15 sq ft defaults.
- Buying separate gallons that aren't the same batch. Mix all gallons together in a single 5-gallon bucket (called "boxing") before painting to ensure consistent color across walls.
- Skipping primer on patched or stained drywall. Spackled patches show through finish paint as dull halos. Spot-prime them first.
- Underestimating ceilings, trim, and closets. The room number doesn't include the ceiling or interior of closets — calculate those as separate paintable areas.
Frequently Asked Questions
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