How accurate is the default coverage rate?

350-400 sq ft/gallon is a widely cited manufacturer-published average for a smooth interior wall — real coverage varies by paint brand/line, surface texture, and application method. Check your specific product's label for its actual figure.

Why do textured walls need more paint?

Textured, porous, or heavily patched surfaces absorb more paint per square foot than a smooth, primed wall — dropping effective coverage to roughly 300-350 sq ft/gallon or lower.

Are the door and window size defaults accurate for my house?

They're standard-size rules of thumb (about 20 sq ft per door, 15 sq ft per window) — adjust the per-opening area if your doors or windows run noticeably larger or smaller than standard.

Do I need 2 coats for every paint job?

It depends on the paint, the color change, and the existing wall condition — 2 coats is a common default for full coverage and even color, but a single coat of a high-hiding paint over a similar existing color can sometimes suffice.

Does this work for ceilings too?

Yes, if you use the direct area mode and enter your ceiling's square footage instead of room dimensions — the coverage math is identical.

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Math & Everyday

Paint Calculator

Gallons of paint needed for a room, after door/window deductions and coats.

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About the Paint Calculator

Calculates how much paint to buy for a room, from wall dimensions (or a direct area entry), minus standard door/window deductions, scaled by the number of coats and your paint's coverage rate — always rounding up to a purchasable amount.

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How to use it

Choose units (imperial or metric) and whether to measure by room dimensions or direct wall area.
Enter room length/width/height, or total wall area directly.
Enter the number of doors and windows, and each one's typical area (defaults provided).
Enter the number of coats and your paint's coverage rate per gallon (or liter).
Read the paint amount to buy, rounded up to a purchasable unit.

Formula

Gross wall area = 2 × (length + width) × height (or entered directly). Net paintable area = gross area − (doors × area per door) − (windows × area per window). Paint needed = (net area × number of coats) ÷ coverage rate per gallon, then rounded up to the next whole gallon (or gallon + quart combination).

Worked example

A 12ft × 10ft room with 8ft walls (352 sq ft gross), 1 door (20 sq ft) and 2 windows (30 sq ft) deducted (302 sq ft net), 2 coats, at 350 sq ft/gallon coverage: 1.73 gallons needed, rounded up to 2 gallons to buy (or 1 gallon + 3 quarts).

Paint Coverage by Surface/Product Type (per gallon)

Paint Coverage by Surface/Product Type (per gallon)
Surface / Paint Type	Coverage Rate
Textured or rough surface	300-350 sq ft
Smooth interior wall (standard average)	350-400 sq ft
Premium formula, smooth surface	400-450 sq ft

Interpreting your result

Coverage varies by paint brand/line, surface porosity or texture, and application method (roller nap, sprayer, brush) — 350-400 sq ft/gallon (about 8.6 m²/L) is a manufacturer-published average for a smooth interior wall, not a guarantee for your specific product. Textured or rough surfaces can drop coverage to roughly 300-350 sq ft/gallon; premium formulas can reach 400-450 sq ft/gallon — adjust the coverage rate if you know your product's actual figure. Door/window deductions use standard-size rules of thumb, not your exact openings. Always round up: paint is sold by the gallon/quart (or liter can), and running short mid-coat risks a visible touch-up line from batch-to-batch color variation.

Recommendations

• Always round up rather than buying the exact calculated amount — running out mid-coat risks a visible touch-up line if the replacement can is from a different batch/mix.
• If you know your specific paint product's actual coverage rate (check the can label), use it instead of the default — premium and budget paints can differ by 100+ sq ft/gallon.
• Textured or heavily patched walls absorb more paint than a smooth wall — increase coats or lower your coverage-rate input if your walls aren't freshly smooth drywall.

Frequently asked questions

Paint is sold in fixed can sizes (gallons/quarts or liters) — you can't buy the exact fractional amount calculated, and running short mid-project risks a visible color mismatch from a different production batch.