Paint Calculator

A paint calculator returns the gallons of paint a room needs, plus primer if you select it. It starts with the wall area, subtracts estimated door and window openings, and adds the ceiling when included. That area is multiplied by the coats and divided by the coverage rate, then rounded up to a whole gallon.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Paint Needed

2 gal

Shopping list

Paint2 gallons

Breakdown
Paint Needed	2 gal
Wall Area (openings subtracted)	301 sq ft
Ceiling Area	0 sq ft
Total Paintable Area	301 sq ft
Primer Needed	0 gal
Estimated Cost (Paint)
Estimated Cost (Primer)

How to Calculate Paint

Wall area for a rectangular room is `2 × (Length + Width) × Height`, the perimeter times the wall height. Door and window area is estimated by count (21 sq ft per standard door and 15 sq ft per standard window, both editable planning assumptions, not measurements). Those areas are subtracted, since paint doesn't go on an opening. If the ceiling is included, its area (Length × Width) is added to the total. Sherwin-Williams' own ProClassic product data sheet lists a coverage rate of 350-400 sq ft per gallon. It also specifies 1 coat of primer plus 2 coats of topcoat for drywall. This calculator uses 350 sq ft/gal as a conservative default within that stated range, because texture and porosity reduce real-world coverage toward the low end. Both the coverage rate and the coat count can be adjusted. Paint needed is `(total area × coats) ÷ coverage per gallon`, rounded up to a whole gallon. Primer, when included, is always one coat regardless of the topcoat count.

Example: 12 ft × 10 ft room, 8 ft walls, 1 door, 2 windows, no ceiling

A 12 ft × 10 ft room, 8 ft walls, 1 door, 2 windows, no ceiling, 2 coats, no primer, at 350 sq ft/gal: 1. Gross wall area: 2 × (12 + 10) × 8 = 352 sq ft 2. Openings: (1 × 21) + (2 × 15) = 21 + 30 = 51 sq ft 3. Net wall area: 352 − 51 = 301 sq ft 4. Total paintable area (no ceiling): 301 sq ft 5. Paint needed: (301 × 2) ÷ 350 = 1.72, rounded up to 2 gallons Adding the ceiling (12 × 10 = 120 sq ft) would bring the total to 421 sq ft, pushing paint needed to (421 × 2) ÷ 350 = 2.41, rounded up to 3 gallons. A coat of primer on top of that would need ceil(421 ÷ 350) = 2 more gallons.

Paint: Compare Scenarios

Scenario	Paint Needed	Wall Area (openings subtracted)	Ceiling Area	Total Paintable Area
Small (8×8 ft powder room)	2 gal	220 sq ft	0 sq ft	220 sq ft
Medium (12×10 ft bedroom)	2 gal	301 sq ft	0 sq ft	301 sq ft
Large (20×16 ft great room, ceiling included)	5 gal	474 sq ft	320 sq ft	794 sq ft

What Is Paint?

A paint calculator converts a room's dimensions into the gallons of paint (and primer) needed to cover it. It accounts for what reduces or adds to that number, including doors and windows that don't get painted, an optional ceiling, and multiple coats. It's built around a whole room rather than one flat wall, since that's how paint is actually bought and applied on a real job.

Paint: Pros and Cons

Pros

Handles a whole room (all four walls, plus an optional ceiling) rather than a single flat wall, matching how a real paint job is actually planned
Subtracts door and window area automatically instead of leaving that adjustment to the user
Coverage rate and coat count are both grounded in a real manufacturer's product data sheet (Sherwin-Williams ProClassic) but stay adjustable, since coverage varies by product, color, and surface porosity

Cons

Assumes a rectangular room, so an alcove, a vaulted ceiling, or an irregular floor plan needs manual adjustment or a room broken into rectangular sections calculated separately
Door and window counts use standard size assumptions (21 and 15 sq ft) rather than measured openings. For an unusually large or small opening, adjust the count or treat it as a partial opening
Doesn't total multiple rooms into one shopping list yet. For a whole-house job, run this once per room and add up the gallon figures

Paint: What a Pro Checks

Use the conservative end of your paint's coverage range for a textured or unprimed wall. Manufacturer data sheets like Sherwin-Williams' ProClassic list 350-400 sq ft/gal, and porosity pulls real-world coverage toward the low end.
Prime first on new drywall, a drastic color change, bare wood, or a stained or patched surface. The ProClassic drywall specification calls for 1 coat of primer plus 2 coats of topcoat.
If your raw total lands just over a gallon mark, check your brand's quart price against a full extra gallon before buying. A quart on top can be the better value.

Frequently Asked Questions

How many gallons of paint do I need for a room?

Take the total wall area (minus doors and windows), multiply by the number of coats, and divide by the paint's coverage rate. That rate is commonly 350-400 sq ft per gallon, according to Sherwin-Williams' ProClassic product data sheet. For a typical 12x10 room with 8 ft walls, one door, and two windows, that works out to about 2 gallons for two coats.

How much wall area does a gallon of paint cover?

Most quality interior paints cover 350-400 sq ft per gallon per coat on a smooth, previously painted or primed surface, per manufacturer data sheets like Sherwin-Williams' ProClassic. Textured walls, unprimed drywall, or a deep color over a light one typically cover less. This calculator defaults to 350 (the conservative end of that range) but lets you adjust it.

Do I need to prime before painting?

Priming is typically recommended for new drywall, a drastic color change, bare wood, or a stained/patched surface. Sherwin-Williams' own drywall specification calls for 1 coat of primer followed by 2 coats of topcoat. Repainting a wall in a similar color over sound existing paint often doesn't need it. This calculator includes one coat of primer, covering the full paintable area, whenever it's turned on.

How many coats of paint do I need?

Two coats is standard for full color hide and even sheen, which is what manufacturer specifications like Sherwin-Williams' ProClassic data sheet call for on drywall. A single coat can work for a very similar-color repaint with a high-hiding paint, and deep or dramatic color changes sometimes need a third coat. Adjust the coats input for your situation.

Should I round up to a whole gallon or buy quarts?

This calculator rounds up to whole gallons, which is usually the better value per square foot covered. If the raw calculation comes out just over a gallon mark (say, 2.1 gallons), a quart on top of 2 gallons can be more economical than buying a full 3rd gallon. Check your paint brand's quart pricing against its gallon price before deciding.

Does this work for an entire house, not just one room?

Not directly in one pass. This calculator is built around a single rectangular room. For a whole house, run it once per room and add the gallon totals together. Keep in mind that buying all the paint from one larger total often gets a better per-gallon price than several smaller purchases.

Sources

Sherwin-Williams ProClassic Waterborne Interior Acrylic Satin Product Data Sheet 101.16a, rev. 3/2013 (350-400 sq ft/gal at 4 mils wet; 1 coat primer + 2 coats topcoat for drywall) (opens in a new tab)

Wall & Ceiling