Paint Calculator - Gallons Needed, Doors Deducted

Buying too much paint wastes money.

Reviewed by: CalcMojo Editorial Team

Buying too little means a second trip to the store and the risk that the next batch does not match perfectly. This paint calculator tells you exactly how many gallons you need before you pick up a brush, based on your room dimensions, the number of coats, and the type of surface you are covering.

Enter the room’s length, width, and ceiling height, and the calculator computes the total wall area. It then subtracts the area of doors and windows you specify, applies the coverage rate for your paint type, and returns the number of gallons required. You can adjust for multiple coats, which most paint jobs require, and the tool accounts for different coverage rates between primer, flat, satin, and semi-gloss finishes.

Whether you are painting a single accent wall or an entire house interior, running the numbers first saves time, money, and the frustration of color-matching issues from buying paint in multiple batches. Pair this tool with our Square Footage Calculator if you need to calculate the floor area of a room as well.

How Paint Coverage Is Calculated

The core calculation behind paint estimation is straightforward: divide the total paintable surface area by the coverage rate of your paint, then multiply by the number of coats.

Gallons = (Total Wall Area – Unpainted Areas) x Number of Coats / Coverage Rate per Gallon

A standard gallon of interior paint covers approximately 350 to 400 square feet per coat on smooth, previously painted drywall. The actual coverage depends on the paint quality, surface texture, and application method.

For a rectangular room, the total wall area is the perimeter multiplied by the ceiling height:

Wall Area = (2 x Length + 2 x Width) x Ceiling Height

A 14 x 12 foot room with 8-foot ceilings has a perimeter of 52 feet and a total wall area of 416 square feet. Subtract approximately 20 square feet per standard door and 15 square feet per standard window. If this room has one door and two windows, the paintable area is 416 – 20 – 30 = 366 square feet. At 375 square feet per gallon, one coat requires just under 1 gallon. Two coats require approximately 2 gallons.

Coverage Rates by Paint Type

Not all paints cover the same area per gallon. The coverage rate varies based on the finish, quality, and brand.

Interior paint coverage rates (per gallon, per coat):

Flat/Matte: 350-400 sq ft. Flat paint has the highest coverage because it contains more pigment relative to binder. It hides imperfections well but is less durable and harder to clean.
Eggshell: 350-400 sq ft. Similar coverage to flat with slightly more sheen and better washability. A popular choice for living rooms and bedrooms.
Satin: 350-400 sq ft. Moderate sheen with good durability. Common for hallways, family rooms, and children’s rooms where walls need occasional cleaning.
Semi-Gloss: 350-400 sq ft. Higher sheen, very durable and easy to clean. Standard for kitchens, bathrooms, trim, and doors.
High-Gloss: 300-350 sq ft. The most reflective finish. Uses more paint per square foot because the higher binder content reduces coverage. Used primarily for trim, doors, and cabinets.

Primer coverage rates:

Standard primer: 300-350 sq ft per gallon. Primer is thinner than paint and absorbs into the surface more, which is why coverage is slightly lower.
High-build primer: 200-300 sq ft per gallon. Thicker formulas designed to fill surface imperfections cover less area.

Exterior paint coverage rates:

Smooth siding: 350-400 sq ft per gallon.
Rough or textured siding (stucco, brick): 150-250 sq ft per gallon. Rough surfaces absorb significantly more paint due to their greater actual surface area.

When You Need Primer

Primer is a separate coat that goes on before the finish paint. It improves adhesion, provides a uniform base, and can block stains from bleeding through. You need primer in these situations:

New drywall. Unpainted drywall is porous and absorbs paint unevenly. Without primer, you will see differences between the paper face and the joint compound, resulting in a blotchy appearance even after two coats of paint.

Dark to light color changes. If you are covering a dark color with a light one, primer prevents the old color from showing through. A high-quality tinted primer in a shade close to your finish color can reduce the number of top coats from three to two.

Stain blocking. Water stains, smoke damage, crayon marks, and tannin bleed from wood all require a stain-blocking primer. Standard paint alone will not prevent these from reappearing.

Significant surface changes. Going from a glossy finish to a flat one, or painting over wallpaper adhesive residue, benefits from primer to ensure proper adhesion of the new paint.

Bare wood. Wood is porous and has natural oils and tannins that can discolor paint. A wood primer seals the surface and creates a stable base.

If you are repainting a wall in a similar color over a surface that is in good condition, you can often skip primer and apply two coats of a paint-and-primer combination product. These are not true primers but contain higher solids that provide adequate coverage in most standard repainting situations.

How Many Coats Do You Need

One coat is rarely sufficient except when touching up small areas with the exact same paint. Even high-quality one-coat paints typically look best with two coats.

Two coats is the industry standard for most interior painting. Two coats provide full, even coverage, better color depth, and longer-lasting results. The first coat seals and primes the surface; the second coat provides the final color and finish.

Three coats may be necessary when making dramatic color changes (dark to light or light to very saturated), painting over patched areas that absorb paint differently, or working with certain deep-base colors (deep reds, yellows, and oranges) that have lower hiding power due to their pigment chemistry.

This calculator defaults to two coats but lets you adjust the number based on your specific situation.

Calculating Paint for Ceilings and Trim

Ceilings. Ceiling area equals the room’s length multiplied by its width. A 14 x 12 room has a 168 square foot ceiling. At 400 square feet per gallon with flat ceiling paint, one gallon covers the ceiling in one coat. Most ceilings need two coats, so plan for approximately one gallon of ceiling paint for rooms up to 200 square feet.

Trim and baseboards. Measure the linear footage of all trim, then multiply by the trim width to get square footage. Standard baseboards are 3 to 5 inches wide. In a room with 52 linear feet of baseboard at 4 inches wide, the trim area is approximately 17 square feet. Door and window casings add more. A quart of trim paint (covering approximately 90-100 sq ft) is typically sufficient for trim in a single room.

Doors. A standard interior door has approximately 20 square feet of paintable surface (both sides). A quart of semi-gloss covers 2 to 3 doors.

Paint Cost Estimation

Paint prices vary significantly by brand and quality:

Budget paint: $20-$30 per gallon. Lower coverage, may require three coats.
Mid-range paint: $30-$50 per gallon. Good coverage, typically two coats.
Premium paint: $50-$80+ per gallon. Excellent coverage, often marketed as one-coat, better durability and color retention.

For a standard 14 x 12 room with 8-foot ceilings, you need approximately 2 gallons of wall paint and 1 gallon of ceiling paint. At mid-range pricing, materials cost $90 to $150 before primer, tape, rollers, and brushes. Use the Square Footage Calculator to get precise room dimensions before estimating costs.

Tips for Accurate Paint Estimation

Measure every wall. Do not assume opposite walls are the same length. Older homes especially may have walls that differ by several inches.

Count all openings. Subtract doors, windows, fireplaces, and any area that will not be painted. A large picture window can reduce paint needs significantly.

Textured walls and ceilings. Knockdown, orange peel, and popcorn textures increase the actual surface area by 15% to 50%. If your walls are textured, increase your paint estimate accordingly.

Buy the same batch. Paint colors can vary slightly between production batches. Buy all the paint you need at once from the same store, and ask for cans from the same batch. If you must buy extra later, keep the original color formula label.

Round up, not down. Always round up to the nearest full gallon or quart. Leftover paint is useful for touch-ups, and most stores accept returns of unopened cans within 30 days.

These estimates are approximate and based on standard material calculations. Actual quantities and costs vary by region, supplier, material grade, and site conditions.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

A 12 x 12 room with 8-foot ceilings has approximately 384 square feet of wall area. After subtracting one door (20 sq ft) and one window (15 sq ft), the paintable area is approximately 349 square feet. At 375 square feet per gallon coverage, you need approximately 1 gallon per coat. For two coats, buy 2 gallons. Add 1 gallon for the ceiling if painting it as well.

How many square feet does a gallon of paint cover?

A gallon of standard interior paint covers 350 to 400 square feet per coat on smooth, previously painted drywall. Rough or textured surfaces reduce coverage to 200 to 300 square feet. High-gloss finishes cover 300 to 350 square feet. These are per-coat figures, and most jobs require two coats.

Do I need primer before painting?

Primer is necessary for new drywall, dramatic color changes (especially dark to light), stain blocking, bare wood, and any surface with adhesion concerns. For repainting walls in a similar color that are in good condition, a quality paint-and-primer combination product may be sufficient without a separate primer coat.

Should I buy one-coat paint?

One-coat paints contain more pigment and solids to provide better coverage per coat, but two thin coats of standard paint almost always produce a more even, more durable finish than one thick coat of premium paint. One-coat products work best for touch-ups and when the new color closely matches the existing one.

How do I calculate paint for a vaulted ceiling?

Measure the length and width of the room for the base area, then account for the additional height of the vaulted section. For a simple peaked vault, the wall triangle above the standard ceiling line can be calculated as (base x peak height) / 2. Add this area to the standard ceiling area. For cathedral ceilings, measure the slope length rather than the horizontal span.

How much does it cost to paint a room?

For DIY painting, materials for a standard bedroom (12 x 12) cost approximately $60 to $120 for mid-range paint, primer, tape, rollers, and drop cloths. Professional painters charge $200 to $500 per room for labor, depending on room size, ceiling height, and preparation work needed.

What is the best paint finish for a bathroom?

Semi-gloss or satin finishes are best for bathrooms because they resist moisture and are easy to wipe clean. Flat and eggshell finishes absorb moisture and are more prone to mildew in humid environments. Use semi-gloss on trim and satin on walls for a good balance of durability and appearance.

Sources & Methodology

Coverage rates (350-400 sq ft per gallon) based on manufacturer specifications from major paint brands (Benjamin Moore, Sherwin-Williams, Behr) for smooth drywall surfaces.
Textured surface coverage reductions based on Painting and Decorating Contractors of America (PDCA) guidelines.
Cost ranges based on national average retail pricing from major home improvement retailers.
Primer recommendations follow manufacturer application guidelines and PDCA industry standards.

Default values shown are illustrative. Always verify measurements on site. Data accurate as of: March 2026