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Paint Calculator

Calculate gallons of paint and primer for walls, ceilings and trim by coat.

What are you painting?

Interior wallsFour walls of a roomCeilingFloor area, flatExterior wallsPerimeter × heightTrimMeasured in linear feet

Measurements

Measure by

Room dimensionsLength, width, heightSquare footageIf you already measured

Length

Width

Wall height

Openings to deduct

Doors and windows come off at their actual area, unlike drywall estimating where small openings stay in.

Doors

Area per door

sq ft

A 3'0" × 7'0" door is 21 square feet.

Windows

Area per window

sq ft

A 3' × 5' window is 15 square feet.

Coverage and coats

Number of coats

Surface condition

Prime firstWorth it over bare drywall, over patches, over gloss, and for a large color change. Over sound similar-colored paint, a second finish coat does more good.

Pricing

Paint price per gallon

Paint to buy

3gallons

2.09 gallons is the exact requirement — 365 sq ft × 2 coats ÷ 350 sq ft per gallon.

Paintable area416 sq ft gross less 51 sq ft of openings: 365 sq ft
Total cost3 gal at $48: $144
Area covered per coat350 sq ft per gallon on this surface: 365 sq ft
Leftover after the jobKeep it sealed and labeled for touch-ups: 0.91 gal

Order one extra quart, not one extra gallon

2.09 gallons rounds up to 3, which leaves 0.91 gallons over. If you go back for more later, the new batch will not match exactly — have the store box your cans together now, or buy the extra quart from the same batch.

Surface by surface
Surface	Dimensions	Area
Wall — length side	14.0 × 8.0 ft	112.0 sq ft
Wall — length side (opposite)	14.0 × 8.0 ft	112.0 sq ft
Wall — width side	12.0 × 8.0 ft	96.0 sq ft
Wall — width side (opposite)	12.0 × 8.0 ft	96.0 sq ft
Doors deducted	1 × 21.0 sq ft	−21.0 sq ft
Windows deducted	2 × 15.0 sq ft	−30.0 sq ft
Paintable area	2 coats at 350 sq ft/gal	365.0 sq ft

Materials and cost
Item	Exact need	Buy	Cost
Finish paint, 2 coats	2.09 gal	3 gallons	$144
Coverage assumed	350 sq ft/gal	Smooth, previously painted	—
Total	730 sq ft of coating		$144

The coverage printed on a can assumes a smooth, sealed, similarly colored surface applied by spray. Rolling over texture, or covering a dark color with a light one, routinely lands closer to 250 square feet per gallon in practice. Deep bases and saturated colors cover worse still, because the tint displaces the solids that do the hiding — plan on three coats for a true red or a deep blue regardless of what this estimate says.

What this assumes

Standard coverage is 350 square feet per gallon on smooth, previously painted surfaces.
Textured, porous or bare surfaces reduce coverage to roughly 250 square feet per gallon.
Doors and windows are deducted at actual area.

Understanding the result

Coverage claims are optimistic

The 350–400 square feet per gallon printed on a can assumes a smooth, sealed, similarly colored surface applied by spray. Rolling over texture, or covering a dark color, routinely lands closer to 250 square feet per gallon.

When primer actually pays

Primer is worth the extra coat over bare drywall, over patches, over glossy surfaces, and when making a large color change. Over sound, similar-colored paint, a second coat of finish paint does more good than a coat of primer.

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The next decision usually follows directly from this one.

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