Paint Calculator

How to use this paint calculator

1. Choose project type - Room/Interior calculates wall area from room length, width, and ceiling height (2 coats for standard rooms). Exterior uses house perimeter and wall height. Custom lets you enter a known area directly.
2. Enter dimensions - All measurements in feet. For exterior, measure the outside perimeter at ground level and use average wall height to eaves.
3. Enter doors and windows - Standard door ≈ 21 sq ft; standard window ≈ 15 sq ft. These are deducted from the total area. Skip if you want a conservative (over) estimate.
4. Set coats and coverage - 2 coats is the standard for most jobs. Coverage defaults to 350 sq ft/gal. Check your specific paint can label for the exact figure.
5. Review results - The calculator shows gallons per coat, total gallons, and gallons to order (rounded up with the waste buffer applied).

Paint coverage formula explained

Figuring paint coverage comes down to dividing your paintable area by the coverage rate of the product you are using.

Paint coverage by surface type

Coverage rate varies widely by surface texture and porosity. Always use the lower end of the range for rough, porous, or bare surfaces when figuring paint coverage.

These figures apply to a single coat. Multiply by your number of coats for total project paint. Actual coverage may vary by brand - always check the can label.

Understanding paint quantity and coverage

Why does surface texture matter so much?

Rough or porous surfaces have far more actual physical area than a flat surface of the same square footage. Every ridge in textured drywall, every pit in concrete, and every grain in rough wood adds to the real surface the paint must cover. A textured ceiling can use 40-50% more paint than a smooth one of the same size. Always use the conservative (lower) end of the coverage range for textured or porous materials.

Primer vs. paint coats

A coat of primer is not the same as a coat of paint in terms of coverage. Primer seals the surface and improves adhesion, allowing finish paint to go further. When painting over a bright or dark existing color, priming first and applying 1-2 coats of finish paint is usually more effective (and sometimes cheaper) than applying 3 coats of finish paint with no primer.

Deck stain and exterior estimating tips

The deck stain estimator logic is the same: measure total deck area in square feet, subtract areas you won't stain (house siding, planters), and divide by the stain's coverage rate. Most solid deck stains cover 200-300 sq ft/gal; semi-transparent stains cover 250-400 sq ft/gal on bare wood. Always apply deck stain to clean, dry wood for maximum coverage and durability.

Common mistakes when calculating paint

• Not deducting doors and windows - On a typical room, these represent 10-20% of the gross wall area. Skipping this step leads to significant over-ordering.
• Ignoring texture - Using the 400 sq ft/gal coverage number for a rough textured wall will lead to short delivery. Check your paint can.
• Buying only one can - Even if calculated at just over a gallon, buy two cans from the same batch code. Touch-ups years later require the same color.
• Forgetting the ceiling - This calculator focuses on walls. If you're painting the ceiling too, re-run the calculator with the floor area as your custom area (ceiling and floor area are the same).

Worked examples

Example 1 - Bedroom

A 12 ft x 10 ft bedroom with 9 ft ceilings: gross wall area = 2 x (12 + 10) x 9 = 396 sq ft. Subtract 1 door (21 sq ft) + 2 windows (30 sq ft) = 51 sq ft. Net = 345 sq ft. At 350 sq ft/gal x 2 coats with 10% waste: 345 / 350 x 2 x 1.1 = 2.17 gal. Order 3 gallons (or 2 gallons + 1 quart).

Example 2 - House exterior

A house with a 140 ft perimeter and 10 ft wall height: 140 x 10 = 1,400 sq ft gross. Subtract 4 doors (84 sq ft) + 8 windows (120 sq ft) = 204 sq ft. Net = 1,196 sq ft. At 375 sq ft/gal x 2 coats with 10% waste: 1,196 / 375 x 2 x 1.1 = 7.03 gal. Order 8 gallons.

Quick tips for accurate paint estimates

• Use the lower end of coverage for rough surfaces. Textured drywall, rough wood, and masonry absorb more paint. Using 350 sq ft/gal on a heavily textured wall instead of 250 sq ft/gal sends you back to the store mid-job.
• Buy all cans from the same batch code. Even the same paint name varies between production runs. Buy everything at once from the same store batch - touch-ups added later from a different batch may not match once dry.
• Prime first when changing colors dramatically. One coat of primer plus one coat of quality paint usually outperforms three coats of finish paint on a bare or strongly contrasting surface - and often costs less in product used.
• Calculate the ceiling as a separate input. Ceiling area equals floor area (length x width). Enter it as a custom area, use a flat-finish ceiling paint, and add the result to your wall total for a complete project estimate.

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