How Much Paint Do I Need? Room-by-Room Calculator Guide

The Core Paint Formula

Paint is sold in quarts (1 qt) and gallons (1 gal). Coverage rates assume one coat on a smooth, primed surface:

Gallons needed = Total paintable area (sq ft) ÷ Coverage rate (sq ft/gal) x Number of coats

For walls: paintable area = wall area — door area — window area

Use our paint calculator to run all room dimensions at once.

Standard Coverage Rates by Paint Type

Paint Type	Coverage per Gallon	Notes
Standard latex (flat)	350-400 sq ft	Most common, good for ceilings
Eggshell / satin finish	350-400 sq ft	Walls, washable
Semi-gloss	300-350 sq ft	Trim, doors, bathrooms
High-gloss	250-300 sq ft	Doors, cabinetry
Premium (Benjamin Moore Aura, SW Emerald)	400 sq ft	Better hide, often 1-coat capable
Primer (water-based)	300-350 sq ft	Always prime before topcoat on new surfaces

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Calculating Wall Area

The standard wall calculation method:

1. Measure the perimeter of the room (add all wall lengths)
2. Multiply perimeter by ceiling height to get gross wall area
3. Subtract 21 sq ft per standard door (3 ft x 7 ft)
4. Subtract 15 sq ft per average window (3 ft x 5 ft)

Formula: Wall area = (Perimeter x Height) — (Doors x 21) — (Windows x 15)

Example — 12x12 room, 8 ft ceiling, 1 door, 2 windows: Perimeter = (12+12+12+12) = 48 ft Wall area = 48 x 8 = 384 sq ft Deductions = 21 + (2 x 15) = 51 sq ft Net paintable wall area = 333 sq ft

At 350 sq ft/gallon for 2 coats: 333 x 2 ÷ 350 = 1.9 gallons — buy 2 gallons.

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Pre-Calculated Room Sizes

These estimates assume 8 ft ceilings, 1 door, 2 windows, 2 coats of wall paint, and standard coverage (350 sq ft/gal). Ceiling is calculated separately.

Walls Only (2 Coats)

Room Size	Net Wall Area	Gallons Needed	Buy This Much
8 x 8 ft (small bedroom)	213 sq ft	1.22 gal	2 quarts
10 x 10 ft (bedroom)	261 sq ft	1.49 gal	2 gallons
12 x 12 ft (bedroom)	333 sq ft	1.9 gal	2 gallons
12 x 16 ft (master bedroom)	397 sq ft	2.27 gal	3 gallons
14 x 14 ft (living area)	381 sq ft	2.18 gal	3 gallons
16 x 20 ft (living room)	477 sq ft	2.72 gal	3 gallons
20 x 24 ft (great room)	597 sq ft	3.41 gal	4 gallons

When in doubt, round up. Most retailers accept returns on unopened paint gallons, and leftover paint is useful for touch-ups.

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Ceiling Paint

Ceilings are calculated separately from walls. Use flat ceiling paint (lower sheen hides imperfections). One coat is often sufficient for white-on-white repaints; two coats for new construction or color changes.

Ceiling area = Length x Width (no deductions)

Room Size	Ceiling Area	Gallons (1 coat)	Gallons (2 coats)
10 x 10 ft	100 sq ft	0.28 gal	0.57 gal
12 x 12 ft	144 sq ft	0.41 gal	0.82 gal
12 x 16 ft	192 sq ft	0.55 gal	1.1 gal
16 x 20 ft	320 sq ft	0.91 gal	1.82 gal
20 x 24 ft	480 sq ft	1.37 gal	2.74 gal

Ceiling paint is typically cheaper than wall paint — $25-$40/gallon vs $35-$70/gallon for premium wall paint. Use ceiling white (slightly different sheen than regular flat) for best results.

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Primer: When You Need It and How Much

Primer is a separate product from paint. “Paint-and-primer-in-one” products exist but perform like thick paint — they’re not true primers.

Use dedicated primer when:

• Painting bare drywall or new construction
• Covering stains (water stains, smoke, crayon) — use shellac-based primer
• Dramatic color change (especially dark to light)
• Painting over glossy surfaces
• Covering repairs (spackle, joint compound)

Primer coverage: 300-350 sq ft per gallon (one coat is usually sufficient)

Primer cost: $15-$30/gallon (water-based), $20-$35/gallon (oil-based), $25-$40 for shellac-based stain-blocking.

For a 12x12 room with 333 sq ft of net wall area: 1 gallon of primer, then 2 gallons of wall paint.

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Number of Coats

Two coats is the industry standard for interior painting. Here’s when you need more or fewer:

One coat is fine:

• Same color or near-match repaints
• Using a premium high-hide paint ($55+/gal) over a similar color
• Light touch-up work

Two coats are standard:

• All new painting projects
• Color change (same tone family)
• Repainting after repairs

Three coats may be needed:

• White or very light colors over dark (navy, forest green, charcoal)
• Bold reds, oranges, and yellows (low pigment hide — a known industry issue)
• Going from any color to a bright white without priming first

Reds and oranges are notorious. Consumer Reports testing found that even premium paints needed 3-4 coats to fully cover a true red. Budget accordingly: a red accent wall that’s 200 sq ft could require 1.7 gallons instead of the expected 1.1 gallons.

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Paint Cost Comparison by Brand (2026)

Brand / Line	Price per Gallon	Coverage	Quality
Behr Premium Plus (HD)	$30-$38	350 sq ft	Good
Sherwin-Williams SuperPaint	$60-$68	400 sq ft	Very good
Sherwin-Williams Emerald	$84-$92	400 sq ft	Excellent
Benjamin Moore Regal Select	$65-$74	400 sq ft	Very good
Benjamin Moore Aura	$90-$102	400 sq ft	Excellent
PPG Diamond	$35-$45	350 sq ft	Good
Valspar Signature (Lowes)	$33-$40	350 sq ft	Good

The price-per-square-foot difference between Behr ($30) and Benjamin Moore Aura ($96) is about $0.17/sq ft on a 12x12 room — roughly $22 extra. Premium paints offer better coverage, better color retention, and a finish that holds up to cleaning. For high-traffic areas and kids’ rooms, the upgrade is worth it.

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Trim, Doors, and Accent Walls

Trim and Baseboards

Trim paint (semi-gloss or gloss) is calculated differently. Baseboards are roughly 0.5 ft wide — multiply the room perimeter by 0.5 ft for baseboard area. Window and door casings add roughly 20 sq ft per opening.

A 12x12 room: perimeter 48 ft x 0.5 ft = 24 sq ft baseboards + 2 windows + 1 door = 24 + 40 + 20 = 84 sq ft of trim area. One quart of trim paint handles this with coverage to spare.

Accent Walls

An accent wall in a 12x12 room is one 12 x 8 ft wall = 96 sq ft. Subtract window if present: 96 — 15 = 81 sq ft.

For 2 coats at 350 sq ft/gallon: 81 x 2 ÷ 350 = 0.46 gallons. Buy 1 quart (covers ~100 sq ft/coat) and expect it to be tight, or buy 2 quarts for comfortable coverage.

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Full Room Summary: 12x12 Master Bedroom

Item	Area	Coats	Gallons Needed	Buy
Walls	333 sq ft	2	1.9 gal	2 gal
Ceiling	144 sq ft	1	0.41 gal	1 qt
Trim/baseboards	84 sq ft	2	0.48 gal	1 qt
Primer (walls)	333 sq ft	1	0.95 gal	1 gal

Total materials: 2 gal wall paint + 1 gal primer + 1 qt ceiling + 1 qt trim Estimated DIY cost: $85-$180 depending on brand

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Frequently Asked Questions

How many square feet does a gallon of paint cover?

350-400 sq ft on smooth, primed surfaces. Expect 250-300 sq ft on rough or unprimed surfaces like bare drywall. Premium paints with higher pigment loads maintain 400 sq ft/gallon even over dark colors.

Do I need primer before painting?

Yes for bare drywall, significant color changes, stain coverage, and any surface that’s never been painted. No for same-color repaints over sound existing paint. Priming before a dramatic color change saves money on paint overall.

How many coats of paint do I need?

Two coats is standard. One coat works for same-color repaints. Three coats may be needed for dark-to-light changes or when painting bold reds and oranges, which have poor hide even in premium formulations.

Should I buy by the gallon or quart?

Gallons at $35-$70 are almost always better value than quarts at $15-$25. Buy quarts only for small accent areas under 100 sq ft or trim work on a single room.

How much does it cost to paint a room?

DIY for a 12x12 bedroom: $85-$180 in materials. Professional painting of the same room runs $380-$750 including labor, prep, and paint.

TL;DR

• Coverage rate: One gallon of interior latex covers 350-400 sq ft on a smooth, primed surface — plan on 2 gallons of wall paint for a standard 12x12 room with two coats.
• Deduct openings: Subtract 21 sq ft per door and 15 sq ft per window from gross wall area before calculating gallons needed.
• Two coats is the standard: Budget for three coats only when covering bold reds, oranges, or dark colors going to white — these colors have poor hide even in premium formulations.
• Buy gallons, not quarts: A gallon at $35-$70 covers 400 sq ft; a quart at $15-$25 covers only 100 sq ft — quarts make sense only for trim or accent areas under 100 sq ft.
• Pro vs. DIY cost gap: A 12x12 bedroom costs $85-$180 in DIY materials versus $380-$750 for professional painting — the labor premium is 3-5x the materials cost.