Free Paint Calculator — How Much Paint Do I Need?

Calculate gallons of paint needed for a room. Enter dimensions, count doors and windows, and pick the number of coats — get gallons needed plus estimated cost.

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Standard door = 21 sq ft subtracted from paintable area.

Standard window = 12 sq ft subtracted from paintable area.

Result
Enter your details on the left, then press Calculate.

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What is this calculator for?

You're standing in the paint aisle at Home Depot with a Behr swatch in your hand, looking at the 1-gallon ($46) versus 5-gallon ($168) cans, and you have no idea whether your bedroom needs 1 gallon, 2 gallons, or 4. Buy too little and you have to come back mid-project (and the new can won't quite match if it's a custom tint). Buy too much and you have $130 of paint sitting in your garage for the next decade. The paint calculator answers the only question that matters at this moment: how much paint to buy.

Standard paint coverage is 350 sq ft per gallon for one coat on smooth, primed surfaces. Two coats is the norm for color changes, repaints over different colors, or any wall going from light to dark or vice versa. New drywall or unprimed surfaces require a coat of primer plus two coats of color — effectively 3x coverage for the same area. Textured walls and rough surfaces drink 20-30% more paint than smooth.

This calculator takes your room dimensions (length, width, ceiling height), subtracts standard door and window openings (or the count you specify), and outputs the gallons needed for one coat, two coats, and total project. It handles single rooms, whole-house projects, ceiling-only repaints, and trim-only jobs. Use it before each Home Depot trip; saves you from buying twice or storing surplus.

How to use this calculator

Enter your room dimensions: length and width in feet, ceiling height (most standard rooms are 8 ft; older homes may be 9-10 ft; modern suburban builds often 9 ft). The calculator computes wall surface area as 2 × (length + width) × height.

Enter doors and windows to subtract. The default assumes one door (21 sq ft) and two windows (15 sq ft each) per room — typical of bedrooms. Add or remove based on your actual room. Don't subtract trim — trim painting is separate from wall painting (and uses different paint). For built-in cabinets, fireplace surrounds, or other features, subtract that surface area too.

Set number of coats. Two coats is standard for any color change. One coat works only for refreshing the same color on previously painted walls (touch-ups in the same shade). Going from any color to white or white to any color requires two coats; going from dark to light may need three. New drywall needs a primer coat plus two color coats — effectively three.

Indicate surface type: smooth (standard drywall, plaster) uses standard coverage. Textured (knockdown, orange peel, popcorn ceiling) consumes 15-25% more paint. Rough or porous (stucco, brick, unprimed wood) can consume 35-50% more. Pick the right category to avoid under-buying.

The calculator outputs gallons needed, recommended paint can size (multiple gallon cans vs single 5-gallon), and an estimated cost based on the price range you specify.

Understanding your results

The calculator returns total square footage to paint, gallons needed for your specified coat count, and a buy recommendation (e.g., "1 × 5-gallon bucket + 1 quart" or "2 × 1-gallon cans"). Plus a rough cost estimate.

How to read it. A 12 × 14 ft bedroom with 8 ft ceilings has 416 sq ft of wall area (minus ~50 sq ft for door + windows = 366 sq ft net). For two coats: 366 × 2 = 732 sq ft of paint to apply. At 350 sq ft/gallon: 2.09 gallons. Buy 3 gallons (or 1 gallon + 1 quart, or just round up to 3 single gallons). Cost at $40-50/gallon: $120-150.

The buy-too-much trap. People buy 5-gallon buckets thinking "economy size" without doing the math. A 5-gallon bucket covers 1,750 sq ft on one coat — that's TWO bedrooms two-coated. If you're painting one bedroom, the 5-gallon bucket leaves you with 2.5 unused gallons sitting in your garage. Paint has a shelf life (1-2 years if sealed well, less if opened), so this is real money wasted. The 5-gallon bucket is right for whole-floor projects (4-5 rooms); the 1-gallon can is right for single-room projects.

The buy-too-little trap. People underestimate by ignoring two-coat requirements or by not adding the texture factor. Running out mid-project means either: (a) driving back to Home Depot with a wet brush in your sink, or (b) accepting an obvious roller-line break where the new can starts. Custom-tinted paint cans rarely match exactly batch-to-batch — even 1% color variation is visible on adjacent walls. Better to buy slightly extra than slightly short.

For trim and accent walls. Trim is typically painted in a different finish (semi-gloss or satin) than walls (eggshell or matte). Trim consumes about 1 quart per 200 linear feet of trim. A standard bedroom has 50-70 linear feet of trim (baseboards plus door and window casings) — so 1 quart of trim paint per room. Accent walls follow the same wall math but for just one wall.

A worked example

Marcus is refreshing his 1,650 sq ft three-bedroom condo. He plans to repaint three bedrooms, the living/dining room, the kitchen (walls only — cabinets stay), one hallway, and one bathroom. All currently a builder-grade off-white; he wants two coats of a new warm gray.

Room-by-room calculation:

Bedroom 1 (master) 14 × 16 × 9 ft: 540 sq ft wall, minus 60 sq ft for door + 2 windows = 480 sq ft. Two coats = 960 sq ft. 2.75 gallons.

Bedroom 2 (12 × 14 × 9): 468 minus 50 = 418 sq ft. Two coats: 836. 2.4 gallons.

Bedroom 3 (10 × 12 × 9): 396 minus 35 = 361 sq ft. Two coats: 722. 2.1 gallons.

Living/dining (combined 16 × 22 × 9): 684 minus 100 (door, sliding glass, openings) = 584 sq ft. Two coats: 1,168. 3.34 gallons.

Kitchen walls (10 × 14 × 9): 432 minus 80 (door, window, cabinet area) = 352 sq ft. Two coats: 704. 2.0 gallons.

Hallway (3 × 18 × 9): 378 minus 30 = 348 sq ft. Two coats: 696. 2.0 gallons.

Bathroom (6 × 8 × 9): 252 minus 40 (door, tub surround, vanity area) = 212 sq ft. Two coats: 424. 1.2 gallons.

Total: 15.79 gallons. Realistic buy: three 5-gallon buckets ($168 each = $504) = 15 gallons. Plus one additional 1-gallon can ($45). Total paint: $549 plus tax. Or four 5-gallon buckets ($672) which gives plenty of cushion plus surplus for touch-ups over the next 2-3 years.

Plus trim and ceiling. Trim is 250 linear feet at 1 qt/200 lf = 1.25 quarts of semi-gloss trim paint = 2 quarts to be safe. Ceiling-only paint (white, flat or matte) for ceilings totaling 1,650 sq ft × 2 coats = 3,300 sq ft / 350 = 9.4 gallons. Two 5-gallon buckets of ceiling white = $80-180 depending on brand.

All-in for the project: $700-900 in paint plus brushes/rollers/tape/drop cloths ($75-120) plus possible primer for stained spots (1 gallon = $35). Total project cost: $850-1,100 for materials. Time investment: about 35-50 hours of solo painting, or 18-25 hours with one helper.

Related resources

For broader renovation cost estimation, see other home-improvement calculators: Flooring Calculator, Concrete Calculator, Wallpaper Calculator. For square footage calculations, the Square Footage Calculator. For overall home-improvement budgeting context, the Savings Goal Calculator. Benjamin Moore's paint coverage guide publishes professional contractor coverage data for different paint products and surface types.

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Frequently asked questions

How much area does one gallon of paint cover?

Standard interior latex paint covers about 350 sq ft per gallon on a smooth, primed surface. Heavy-bodied premium paints (Behr Marquee, Sherwin-Williams Emerald) advertise 400 sq ft per gallon. Textured walls and ceilings drop coverage to ~250 sq ft. Exterior paint over rough siding can be as low as 200 sq ft per gallon. Always read the back of the can — manufacturers print actual coverage.

Do I need primer?

Yes for: bare drywall, bare wood, dramatic color changes (light over dark or vice versa), stained surfaces, glossy old paint. No primer needed for: same-color refresh on existing latex paint, or when using paint-and-primer-in-one products on lightly tinted existing paint. When unsure, prime — primer is cheaper than a third topcoat.

How do I calculate ceiling paint separately?

Ceiling area equals the floor area: length × width of the room. A 12×10 room has 120 sq ft of ceiling, requiring 1 gallon for 2 coats (240 sq ft of coverage needed, well under one gallon's 700 sq ft for 2-coat coverage). Most rooms only need 1 gallon of ceiling paint. Use a flat-finish ceiling paint, not the same paint as the walls.

How many coats of paint do I need?

Two coats for any color change or for going over previously painted surfaces (the vast majority of repaints). One coat works for refreshing the same color on previously painted walls — a touch-up situation. Three coats may be needed for going from a very dark color to a light color (the dark base shows through two coats), for very rough surfaces, or when using premium one-coat-coverage paints that still don't deliver in practice on extreme contrast changes. New drywall requires a primer coat plus two color coats — primer fills the porous drywall surface so color coats apply uniformly.

Do I need to prime before painting?

Depends on the situation. (1) New drywall or unprimed wood: yes, prime first (the surface absorbs paint unevenly without it). (2) Painting over a stain (water damage, smoke, marker, blood): yes, prime with a stain-blocker like Kilz or Zinsser. (3) Painting over a glossy surface: yes, prime to give the new paint adhesion. (4) Painting over a similar color on previously painted walls in good condition: no, prime is optional. The 'paint and primer in one' marketing on many cans means the paint has slightly more solids — it does NOT replace a true primer for difficult surfaces. When in doubt, prime; the cost is $30-40 in primer to avoid hours of wasted color-coat work.

How much does professional painting cost compared to DIY?

Professional interior painting averages $3-6/sq ft of floor area as of 2025 (varies by region, condition, and paint quality). For a 1,650 sq ft condo: $4,950-9,900 for professional labor + materials. DIY materials only: $700-1,100. The labor savings: $4,000-8,000. Time cost: 40-60 hours of your weekend over 1-2 weeks. Effective hourly rate of DIY: $80-160. For most homeowners with the time, DIY is the better deal. For those who are working full-time and value evenings/weekends, hiring out can be worth it. The work itself is not hard but it is tedious, requires patience for taping and cutting in, and produces visible results that reflect care or lack thereof.

What's the difference between flat, eggshell, satin, and semi-gloss paint?

Sheen level — how reflective the dried paint is. Flat (matte): no sheen, hides imperfections best, but stains easily and is hard to clean. Used on ceilings and bedroom walls. Eggshell: slight sheen, easier to clean than flat. Standard for living rooms and bedrooms. Satin: more sheen, cleans well. Common for hallways, kitchens, kids' rooms. Semi-gloss: high sheen, very durable and easy to clean. Standard for trim, doors, kitchen cabinets, bathrooms. Gloss: maximum sheen, used for trim and doors when a polished look is wanted. Each step up in sheen shows wall imperfections more, requires better surface prep, and is more durable. Match sheen to use case, not to aesthetic preference alone.

How long does paint last in storage?

Sealed cans, properly stored (cool basement, not freezing, not garage that hits 100°F+ in summer): 1-2 years for color matching purposes, 5-7 years for usability. Opened cans: 6 months to a year before skinning or hardening. Tips for shelf life: (1) Tap the lid down with a hammer to ensure airtight seal. (2) Store can upside down so any air seals against the top, not the paint. (3) Mark date opened on lid with permanent marker. (4) Before reusing old paint, stir thoroughly and strain through cheesecloth or a paint strainer to remove any skin or clumps. Custom-tinted paint matches batch-to-batch reasonably well within 6 months; after a year, even the same SKU may not match exactly — buy a fresh quart for major touch-ups if more than a year has passed.

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