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Home & ConstructionBuy too little paint and you're making a mid-project store run hoping the color matches. Buy too much and you've paid for gallons that harden in the garage. The fix is five minutes of measuring and one simple formula.
This guide shows you how to calculate paint for any room — walls, ceilings, doors and windows subtracted — with the standard coverage rates painters actually use.
Try it yourself: Enter your room dimensions and coats for an instant paint estimate. Paint Calculator →
Know your coverage rate
Most interior latex paint covers 350–400 square feet per gallon for one coat (roughly 9–11 m² per liter). That's the manufacturer's figure on smooth, primed walls. Textured walls, unprimed drywall, or dramatic color changes drop coverage by 20% or more — plan for the low end (350) unless your walls are ideal.
The formula, step by step
Wall area = 2 × (length + width) × height − doors − windows
Gallons = (wall area × coats) ÷ 350, then round up + 10%
Gallons = (wall area × coats) ÷ 350, then round up + 10%
Standard deductions: a door is about 21 sq ft (3×7 ft), an average window about 15 sq ft. Don't skip this step — in a room with a door and two windows, you're removing 50+ sq ft that would otherwise inflate your order.
Worked example: a 12 × 14 ft bedroom
Walls: 2 × (12 + 14) × 8 ft ceiling = 416 sq ft. Subtract one door (21) and two windows (30): 416 − 51 = 365 sq ft.
Two coats: 365 × 2 = 730 sq ft ÷ 350 = 2.09 gallons. Add 10% waste (0.2 gal) → ~2.3 gallons. Buy 3 gallons — or 2 gallons plus a quart — since you can't buy 2.3 gallons and leftover paint is your touch-up supply.
Ceilings and trim
Ceiling area is just length × width (168 sq ft in our example — about half a gallon per coat). Trim and doors need far less paint but a different type — semi-gloss or gloss enamel — so calculate them separately rather than folding them into the wall gallons.
One coat or two?
One coat works when repainting the same or a similar color over paint in good condition. Two coats are the professional standard for color changes, new drywall, or patched walls — the second coat is what gives even color and proper durability, so most projects should budget for two.
Painting the whole house? Scale up
For a whole interior, measure each room and add them — or estimate: a 1,500 sq ft home typically has 3,500–4,500 sq ft of paintable wall (walls run ~2.5–3× the floor area after subtracting openings). At 350 sq ft per gallon per coat, that's 10–13 gallons per coat, or 20–26 gallons for two coats. Exterior siding follows the same math using the wall's outer dimensions — just use exterior-rated paint and add extra for rough textures like stucco, which can drink 30% more.
Sheen matters too
| Sheen | Best for | Notes |
|---|---|---|
| Flat/matte | Ceilings, low-traffic walls | Hides flaws; hard to clean |
| Eggshell/satin | Living rooms, bedrooms | The all-rounder |
| Semi-gloss | Trim, doors, kitchens | Wipeable; shows flaws |
| Gloss | Accents, furniture | Most durable, least forgiving |
Sheen doesn't change how much paint you need, but it changes what you buy — calculate trim separately and get it in semi-gloss while walls get eggshell.
Storing leftovers
Seal the can tightly (tap the lid with a mallet, not a hammer), store indoors where it won't freeze, and label it with the room and date. Properly stored latex lasts 2–10 years unopened; once opened, use within a couple of years for reliable touch-ups.
Frequently asked questions
About 350–400 sq ft per gallon for one coat on smooth walls (9–11 m² per liter). Use the low end — 350 — for textured walls, new drywall, or big color changes.
Yes — a door is ~21 sq ft and a window ~15 sq ft. Skipping this step routinely overestimates by half a gallon or more in a typical room.
On new drywall, bare wood, patched spots, stains, and drastic color changes. Primer seals porous surfaces so topcoat coverage matches the can's rating — skip it and you'll use extra topcoat instead.
About 10% beyond the calculation, rounded up to the next purchasable size. Leftover paint isn't waste — it's your touch-up supply for scuffs and nail holes.
Disclaimer: Coverage varies by paint brand, surface texture, and application method. Check your specific product's label for its rated coverage before buying.