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A fresh coat of paint transforms any space, but buying the right amount is trickier than most people think. Too little paint means an extra trip to the store mid-project, and too much means wasted money sitting in your garage for years. Our free online painting calculator solves this problem by computing the exact wall and ceiling area that needs coverage, factoring in windows, doors, and the number of coats you plan to apply.

Whether you are repainting a living room, painting the exterior of a house, or touching up trim work across an entire floor, this tool gives you a precise gallon count and cost estimate in seconds. Paint coverage varies wildly depending on the surface texture, the paint brand, and whether you are applying primer first. Our calculator uses industry-standard coverage rates — roughly 350 square feet per gallon for smooth drywall and 250 square feet per gallon for textured or previously unpainted surfaces. Just enter your room dimensions, subtract the area of doors and windows, choose your number of coats, and the calculator tells you exactly how many gallons to buy.

For exterior jobs, it accounts for siding type and eave overhang. We have seen homeowners save hundreds of dollars by avoiding overbuying, and contractors use this tool to produce fast, accurate bids that win jobs. It supports both interior and exterior projects, and the cost estimator multiplies the gallon count by your chosen price per gallon for a complete material budget.

📖 この塗装計算機の使い方

Step 1: Measure the Room

Enter the length, width, and ceiling height of your room. For exterior projects, enter the total wall dimensions including gable ends.

Step 2: Subtract Openings

Enter the number and size of doors and windows to be excluded from the paint area. Standard door is 21 sq ft, standard window is 15 sq ft.

Step 3: Choose Paint Type

Select interior or exterior paint and the surface type. Smooth drywall covers about 350 sq ft per gallon; textured surfaces cover 250 to 300 sq ft per gallon.

Step 4: Select Number of Coats

Most paint jobs require two coats for even coverage. For dramatic color changes or bare drywall, select three coats to account for primer and two finish coats.

Step 5: Enter Price per Gallon

Input the cost per gallon for your chosen paint. Premium paints run 40 to 60 dollars per gallon, while standard paints are 25 to 35 dollars.

Step 6: Review Results

The calculator shows total paintable area in square feet, gallons needed, and total estimated cost. Round up to the nearest whole gallon when ordering.

📐 塗料カバレッジの計算式

The core formula for paint coverage is Gallons Needed = Wall Area / Coverage per Gallon. Wall area is calculated from the room perimeter multiplied by ceiling height: Wall Area = 2 × (Length + Width) × Height. You then subtract the area of doors and windows. A standard door occupies about 21 square feet (3 ft × 7 ft) and a standard window about 15 square feet (3 ft × 5 ft).

For example, a 12 ft by 14 ft room with 8 ft ceilings has a perimeter of 52 ft. The gross wall area is 52 × 8 = 416 square feet. Subtracting two doors (42 sq ft) and one window (15 sq ft) gives 359 net square feet. At a coverage rate of 350 square feet per gallon, you need 359 / 350 = 1.03 gallons per coat. For two coats, that is 2.06 gallons, which rounds up to 3 gallons when purchasing.

Coverage rates vary by surface: smooth drywall gets about 350 sq ft per gallon, lightly textured surfaces get 250 to 300 sq ft, and heavily textured or stucco surfaces may get as low as 150 to 200 sq ft per gallon. Primer typically covers 200 to 300 sq ft per gallon depending on the formula. The cost calculation multiplies the total gallons needed by the price per gallon, and you should always round up to the nearest whole gallon since paint is sold in full containers.

⚠️ 避けたいよくあるミス

Not subtracting doors and windows

✓ A standard door is 21 sq ft and a window is 15 sq ft. For a room with two doors and a window, that is 57 sq ft you do not need to paint — nearly two gallons worth.

Using the wrong coverage rate

✓ Smooth drywall gets 350 sq ft per gallon, but textured surfaces only get 250 to 300 sq ft. Using the wrong rate leaves you short on paint.

Forgetting to account for a second coat

✓ Most jobs need two coats for uniform coverage and true color. One coat looks patchy and will not hold up to cleaning or UV exposure.

Not buying enough primer

✓ New drywall and dramatic color changes need primer before topcoat. Primer covers 200 to 300 sq ft per gallon, which is less than finish paint.

Ignoring ceiling and trim

✓ Ceiling area is separate from wall area. For a 12x14 room, the ceiling adds 168 sq ft of coverage needed. Trim and baseboards add another 10 to 15 percent.

💡 プロのヒント

✓

Buy all the paint for a room from the same batch. Different batches can have slight color variations that show as visible lines on the wall.

✓

Cut in all edges with a brush before rolling. This gives you a clean line along ceilings, corners, and trim that a roller cannot achieve.

✓

Keep the wet edge while rolling to avoid lap marks. Work from one end of the wall to the other without stopping mid-section.

✓

Paint the ceiling first, then the walls, then the trim. Gravity causes drips, and you do not want to touch up freshly painted walls.

✓

In humid conditions, extend drying time between coats to 4 to 6 hours instead of the standard 2 to 4 hours listed on the can.

✓

Use a high-density foam roller for smooth surfaces and a 3/8-inch nap roller for lightly textured walls.

❓ よくある質問

A 12 ft by 12 ft room with 8 ft ceilings has about 384 square feet of wall area. Subtracting one door (21 sq ft) and one window (15 sq ft) gives 348 net square feet. At 350 sq ft per gallon, you need about 1 gallon per coat. For two coats, buy 2 gallons.

For a standard 12x12 room, you need 2 gallons of paint at 25 to 45 dollars per gallon, totaling 50 to 90 dollars in materials. Primer adds another 20 to 30 dollars. Professional labor for a standard room runs 200 to 400 dollars.

Yes, primer is essential for new drywall, patched areas, and dramatic color changes. Primer seals porous surfaces so the finish coat goes on evenly and uses less paint.

Latex paint is dry to the touch in 1 to 2 hours and ready for a second coat in 4 hours. Oil-based paint takes 6 to 8 hours to dry and 24 hours before recoating.

Eggshell and satin are the most popular for living rooms and bedrooms. Flat or matte is best for ceilings. Semi-gloss is ideal for trim, doors, and bathrooms.

🏗️ 実際の用途

Painting is one of the highest-ROI home improvements, and accurate material estimation separates a profitable paint job from a money-losing one. For interior projects, a typical three-bedroom house with 1,500 square feet of living space requires 15 to 20 gallons of wall paint, 5 to 7 gallons of ceiling paint, and 3 to 5 gallons of trim paint. At 35 dollars per gallon, material costs alone run 800 to 1,100 dollars.

Exterior painting is more complex because of height, weather exposure, and surface preparation. A typical two-story house with 2,000 square feet of exterior wall area requires 15 to 20 gallons of exterior paint. Siding type matters: smooth lap siding covers 350 sq ft per gallon, but stucco and rough-sawn wood may only cover 200 sq ft per gallon.

Contractors use this tool during the estimating phase to produce fast, accurate bids. Instead of walking the property with a clipboard, they enter the dimensions and get a material list in seconds. For multi-unit properties like apartment buildings or office complexes, the calculator scales easily to larger areas and helps with bulk purchasing decisions.

✅Method, Sources & Review

Maintained by theBuildingCalc methods desk· Last reviewedSeptember 23, 2026. This calculator implements the standards and references below; every figure can be rechecked against the formulas and worked example on this page.

PDCA standards Manufacturer coverage data

Results are estimates, not engineering advice. Verify against local codes and licensed professionals for structural, electrical, and gas work. See ourTerms. Spotted an error? Report it— corrections ship with the next review.

Brand	Latex	Oil	Epoxy
Asian Paints	350-400	300-350	250-300
Dulux	350-420	320-380	260-320
Sherwin-Williams	350-400	300-370	250-310
Benjamin Moore	400-450	350-400	280-340

🎨 Painting Calculator PRO

Living Room

Master Bedroom

Office Space

Kitchen Walls

Dimension Diagram — first wall, updates as you type

1 Rooms

Walls

2 Paint Selection

3 Application Method

4 Cost & Labour

Labour Cost

Equipment Rental

Results

Per-Room Area

Areas & Coverage

Material Breakdown

⚠️ Pre-Export Checklist

Professional Painting Guide

Core Formulas

Brand Coverage Rates

Finish Guide

Surface Prep Checklist

Application Tips

Common Mistakes

Painting — 3D Preview

関連する計算機

Client & Quote (prints on PDF & Excel)

Adjustments (applies to exports)

Project notes (prints on PDF & Excel)

Result compare (A/B)