Yes if you are painting over bare drywall, a much darker color, or oil-based paint. Otherwise high-quality paint-and-primer-in-one is fine.

What temperature can I paint at?

Latex paint: 50–85°F, humidity below 70%. Check our weather window for the next 7 days at your location.

How many square feet does a gallon of paint cover?

About 350–400 ft² on a smooth, previously painted surface. Bare drywall, fresh stucco, and rough-sawn wood absorb far more — as little as 200 ft² per gallon — which is what priming exists to prevent.

Should I add a waste factor to a paint estimate?

No. Manufacturer coverage figures already account for normal application loss, so adding a percentage on top over-orders. What actually changes the quantity is surface porosity and texture, not waste.

How many coats of paint do I need?

Two over a primed or previously painted surface in a similar colour. Three when going dramatically lighter, covering a strong colour, or using deep reds and yellows, which have weaker hiding pigments regardless of price.

What is the difference between paint sheens?

Flat hides surface defects but marks easily; eggshell and satin balance cleanability with a soft look and suit most walls; semi-gloss and gloss go on trim, doors, and wet areas. Higher sheen shows more of every imperfection underneath.

How much paint do I need for a ceiling?

Multiply room length by width, then divide by 350. A 12 × 12 ft ceiling is 144 ft², so a single gallon covers it in two coats with paint left over.

Can I paint over wallpaper?

It is possible with an oil-based or shellac primer, but seams and texture telegraph through and any loose edge lifts once wet. Stripping is more work up front and produces a far better result.

How much paint do I need?

Add up the wall area — perimeter times ceiling height — subtract large openings, then divide by the coverage on the can and multiply by the number of coats. Most interior latex covers 350 to 400 square feet per gallon on a smooth, previously painted surface. Unlike almost every other material on this site, paint needs no waste factor: manufacturer coverage figures already account for normal application loss, so adding a percentage on top simply over-orders. What genuinely changes the quantity is what you are painting onto. Bare drywall, fresh stucco, and rough-sawn wood are porous and can drop coverage to 200 square feet per gallon, which is precisely the problem primer solves — it is cheaper than finish paint and seals the surface so the topcoat spreads properly. Plan on two coats over a primed or similar-coloured surface, and three when going dramatically lighter or using deep reds and yellows, whose pigments hide poorly regardless of price.

Worked example

A 12 × 14 ft room with 8 ft ceilings, two coats

1Perimeter: (12 + 14) × 2 = 52 ft
2Wall area: 52 × 8 = 416 ft²
3Deduct a door and two windows: 416 − 60 = 356 ft²
4Two coats at 350 ft²/gal: 356 × 2 ÷ 350 = 2.03 gal

Buy 2 gallons, plus 1 gallon of primer if the walls are bare.

Run your own numbers below — the Paint Calculator handles the unit conversions and waste factor for you.

Paint Calculator

Calculate gallons of paint and primer for any project. Defaults to 350 ft²/gallon coverage and 2 coats — adjust for textured surfaces.

Formula and figures last reviewed August 6, 2026 — Cosyslabs · how we verify

Inputs

Surface area(ft²)

Number of coats

Coverage (ft²/gal)

Include primer?

Price per gallon (optional)($)

Result

Paint

2.3 gal

Primer

0.0 gal

Total (rounded)

3 gal

Coats: 2
Coverage: 350 ft²/gal

Diagram

Estimates only. Verify with a licensed contractor or supplier before purchasing. Rough Estimator! is not responsible for shortages, overages, or material variances. See our methodology for formulas and sources.

Formula

Gallons = area × coats ÷ coverage

References

Measure what the tool asks for
This calculator works from surface area in ft² and price per gallon (optional) in $. Measure the finished area you are covering, not the material — an estimate is only as good as the dimensions you feed it.
What it does with those numbers
Gallons = area × coats ÷ coverage Every figure on the page comes out of that one expression, so you can check it by hand.
Choosing a number of coats
Pro tip: Going from dark to light (e.g., charcoal → white) usually requires 3 coats. Same-family color changes are often fine with 2.
Where the numbers come from
The constants are taken from PCA (Painting Contractors Association) — Estimating and coverage standards, ASTM D5324 — Standard Guide for Testing Water-Borne Architectural Coatings and Sherwin-Williams — Product coverage data sheets, linked in full below. If a figure here disagrees with your supplier's spec sheet, use the spec sheet — it describes the product you are buying.
What this will not tell you
This is a material takeoff, not a bid. It does not price labour, permits, delivery, equipment hire or site preparation, and it assumes a straightforward layout. Complex cuts, difficult access and awkward shapes all cost more than the geometry suggests.

Pour-window forecast

Free 7-day forecast from Open-Meteo. Green = safe to paint. We never share your location.

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Frequently asked questions

A typical 12×12 room (~400 ft² wall area) needs about 2.3 gallons for two coats at 350 ft²/gal coverage.

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Cosyslabs. (2026). Paint Calculator. Rough Estimator!. https://roughestimator.com/paint-calculator

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