Paint Calculator

About This Tool

Buying the right amount of paint prevents two expensive mistakes: running out mid-project and creating visible lap marks from dried edges, or buying too much and wasting money on gallons that sit unused in your garage. This calculator determines exactly how many gallons you need by accounting for wall area, openings like doors and windows, number of coats, and the specific coverage rate of your paint. Professional painters use this same formula to bid jobs accurately, and now you can calculate your needs in seconds before you head to the store. The formula behind every paint estimate starts with total wall area. Multiply the room perimeter by the wall height to get gross wall area in square feet, then subtract openings. Standard doors occupy about 20 square feet each, and standard windows roughly 15 square feet. The remaining paintable area gets multiplied by the number of coats to determine total coverage needed. Dividing that total by the paint coverage rate per gallon (usually 300 to 400 square feet depending on finish type and surface texture) gives you the gallons required, always rounded up because paint stores do not sell partial gallons. Getting this number right before you visit the store saves both money and the frustration of a second trip mid-project.

How to Measure Your Room for Paint

Accurate measurements lead to accurate estimates. Follow these steps:

Measure length and width: Use a tape measure to get the dimensions of the room at floor level. For irregularly shaped rooms, break them into rectangular sections.
Measure wall height: Measure from floor to ceiling. Standard residential ceilings are 8 feet, but measure to confirm.
Count openings: Count all doors and windows. Standard doors are approximately 20 sq ft (3ft × 6.5ft), and average windows are 15 sq ft (3ft × 5ft).
Skip trim and baseboards: This calculator assumes you're painting walls only. Trim is typically painted separately and requires minimal paint.

For precise calculations, you can measure each door and window individually, but using the standard sizes (20 sq ft for doors, 15 sq ft for windows) provides a close estimate that accounts for the area you won't paint.

One Coat vs Two Coats: When to Use Each

Most professional painters recommend two coats for a quality finish, but there are exceptions:

One coat works when: You're painting the same color over a surface in good condition, using premium paint with excellent coverage, or doing touch-up work in small areas.
Two coats required when: Covering a darker color with lighter paint, painting over stains or patches, using budget paint with lower pigment density, or painting new drywall (which is very porous).
Three coats needed when: Making dramatic color changes (red to white), covering severe stains, or painting over glossy surfaces that weren't properly primed.

The second coat doesn't just provide better coverage. It also improves durability. Paint that's too thin wears through faster and shows scuff marks more easily. For high-traffic areas like hallways and kids' rooms, two coats are insurance against early wear.

Paint Finish Types and Coverage Differences

Paint finish affects both appearance and coverage rate:

Flat (Matte): Highest coverage (350-400 sq ft/gal). Hides surface imperfections well. Best for low-traffic areas like ceilings and adult bedrooms. Shows marks easily and hard to clean.
Eggshell: Moderate coverage (300-350 sq ft/gal). Slight sheen makes it washable. Ideal for living rooms and dining rooms. Hides minor wall flaws.
Satin: Moderate coverage (300-350 sq ft/gal). Smooth, velvety sheen. Great for high-traffic areas like hallways and kids' rooms. Resists moisture and easy to clean.
Semi-Gloss: Lower coverage (300-325 sq ft/gal). Shiny finish reflects light. Perfect for trim, doors, and kitchens/bathrooms. Very durable and scrubbable.
Gloss (High-Gloss): Lowest coverage (250-300 sq ft/gal). Mirror-like finish. Used for cabinets and furniture. Extremely durable but shows every surface flaw.

The higher the gloss, the more binder (resin) and less pigment the paint contains, which reduces coverage per gallon. Always check the manufacturer's coverage rate on the paint can, as premium paints often cover better than budget options.

How Surface Texture Impacts Paint Consumption

Smooth drywall requires the least paint, but many surfaces drink up more:

Textured walls (knockdown, orange peel): Reduce coverage by 20-30%. The texture has more surface area than flat drywall, so a gallon that normally covers 350 sq ft might only cover 250 sq ft.
Bare or unprimed drywall: First coat gets absorbed heavily. Always prime new drywall before painting because primer costs less than paint and seals the porous surface.
Concrete or cinder block: Extremely porous. Use concrete-specific paint and expect 50% reduction in coverage. A gallon rated for 350 sq ft on drywall might only cover 175 sq ft on block.
Wood (stained or raw): Tannins in wood can bleed through paint. Use stain-blocking primer first. Raw wood absorbs paint like a sponge, so primer is essential.
Previously glossy surfaces: Must be sanded or de-glossed. Without proper prep, new paint won't adhere, regardless of how much you use.

When in doubt, reduce the coverage rate in the calculator by 25% for textured or porous surfaces. It's better to have an extra quart for touch-ups than to run out mid-wall.

Cost Estimation and Budgeting for Paint Projects

Accurate material estimates save money on every painting project. Here's how to budget effectively:

Paint cost ranges: Budget latex paint costs $15-25 per gallon and covers 350-400 sq ft. Mid-range brands like Behr or Valspar run $30-45 per gallon with better coverage and durability. Premium paints from Benjamin Moore or Sherwin-Williams cost $50-80 per gallon but offer superior coverage, often requiring fewer coats to achieve full opacity.
Primer costs: Primer runs $15-30 per gallon and covers 300-400 sq ft. Paint-and-primer combos save a step but are not true replacements for dedicated primer on bare surfaces, stains, or dramatic color changes.
Supplies budget: Rollers, brushes, tape, drop cloths, and trays add $30-75 to a typical room project. Cheap rollers shed fibers into wet paint, so invest in quality roller covers ($8-15 each) for a smooth finish.
Labor costs (if hiring): Professional painters charge $1.50-4.00 per square foot of wall area, or $200-500 per room depending on complexity. Ceilings, trim, and prep work (patching holes, sanding, caulking) cost extra.

For a standard 12x10 room needing 2 gallons of mid-range paint plus supplies, expect to spend $100-150 for a DIY project or $400-700 with professional labor. Getting the gallon count right with this calculator prevents the $35-50 waste of buying an unnecessary extra gallon.

Frequently Asked Questions

How much paint do I need for a 12x12 room?

A 12x12 room with 8-foot ceilings has approximately 384 sq ft of wall space. Minus 1 door (20 sq ft) and 2 windows (30 sq ft), that's 334 sq ft of paintable area. For 2 coats, you need 668 sq ft of coverage, which requires 2 gallons of paint (at 350 sq ft per gallon). Always buy an extra quart for touch-ups.

Does primer count as a coat?

No, primer is separate from your finish coats. Primer seals the surface and provides a uniform base for paint to adhere to. After priming, you still need 1-2 coats of your final paint color. New drywall, bare wood, and stained surfaces should always be primed before painting.

Can I use ceiling paint on walls?

Technically yes, but it's not recommended. Ceiling paint is formulated to minimize splatter and drips when painting overhead, but it's less durable and harder to clean than wall paint. Wall paint is designed to withstand scrubbing and daily wear. Use the right paint for each surface.

Why do paint cans show different coverage rates?

Coverage varies by paint quality, finish, and color. Premium paints with higher pigment density cover better (sometimes 400+ sq ft per gallon). Dark colors often cover less area than light colors because they require more pigment. Glossy finishes cover less than flat finishes. Always check the specific product's label.

How long does paint last in the can after opening?

Properly sealed paint lasts 2-10 years depending on storage. Store leftover paint in a cool, dry place (not a freezing garage). Press plastic wrap over the paint surface before sealing to prevent skin formation. Label the can with the room name and date. If paint is lumpy, moldy, or smells rotten, discard it. Don't use it on walls.