Paint Calculator — How Much Paint Do I Need?

Quick answer: a 12 × 12 ft room with 8 ft walls, 1 door and 2 windows needs about 2 gallons for two coats of interior latex.

Paint math comes down to one chain: paintable area × coats ÷ coverage per gallon. This calculator runs it for a full room (four walls) or a known wall area, subtracts standard door and window sizes, and turns the result into a real buy list — whole gallons plus quarts where that's the cheaper way to buy.

Coverage rates are per paint type, and you can override with the number from your can label — textured or bare surfaces drink noticeably more paint.

How to use it

Enter the room size (or the total wall area if you've already measured), plus wall height.
Count the doors and windows you won't paint — each door deducts ~21 ft², each window ~15 ft².
Set your coats (2 is standard) and pick the paint type so the coverage rate matches your can.
Read the buy list: exact gallons, the gallon + quart combination to buy, and the cost at your price.

How it works

Paintable area = wall area − (doors × 21 ft²) − (windows × 15 ft²). Gallons = paintable area × coats ÷ coverage (sq ft per gallon), rounded up to purchasable cans.

Standard deductions: 21 ft² per door, 15 ft² per window.
Coverage per gallon by type: interior latex 350, exterior 325, primer 300, flat 400, eggshell/satin 350, semi-gloss 300, high-gloss 250.
Two coats assumed by default — most paints and warranties assume two.
Trim, doors and ceilings are separate jobs — use the wall-area mode for ceilings.

Worked example

A 12 × 12 ft room, 8 ft walls, 1 door, 2 windows: perimeter 48 ft × 8 = 384 ft². Deduct 21 + 30 = 51 ft² → 333 ft² paintable. Two coats at 350 ft²/gal: 333 × 2 ÷ 350 = 1.9 gal → buy 2 gallons (or 1 gal + 3 quarts, usually pricier).

Quick reference

Coverage by paint type

Paint	Sq ft per gallon
Flat / matte	~400
Eggshell / satin	~350
Semi-gloss	~300
High-gloss	~250
Primer	~300
Textured / bare surfaces	250–300 (or less)

Quick room reference (2 coats, 8 ft walls)

Room	Walls only	With ceiling
10 × 10 ft	2 gal	2–3 gal
12 × 12 ft	2 gal	3 gal
12 × 16 ft	2–3 gal	3–4 gal
15 × 20 ft	3–4 gal	5 gal

FAQ

How much does a gallon of paint cover?

350–400 ft² per coat on smooth, sealed drywall — 350 is the safe planning number. Textured walls, bare drywall and masonry drop to 250–300 or less. Check the "spread rate" on your can.

How many coats of paint do I need?

Two coats is the standard for an even, durable finish and what most warranties assume. One coat only works for touch-ups in the same color; going light-over-dark or covering bold colors may take three coats or a primer coat.

Do I deduct windows and doors?

Yes — the calculator removes 21 ft² per door and 15 ft² per window. Large openings (patio doors, picture windows) are bigger; use the wall-area mode with your own net number for precision.

Should I paint the ceiling with the same paint?

Ceilings usually get their own flat ceiling paint — add the ceiling area (length × width) in wall-area mode and calculate it as a separate job, since coverage and price differ.

How much paint for a whole house interior?

A typical 1,500 ft² home takes roughly 8–12 gallons for two coats on all walls, more with ceilings and trim. Run each room through the calculator and add the results.

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Sources & verification

Data verified: October 11, 2026.

Coverage rates: industry ranges and can-label spread rates (350–400 ft²/gal typical).
Openings: standard door ≈ 21 ft², window ≈ 15 ft² (painter conventions).

Estimates are for planning only — always confirm quantities with your supplier and check local building codes before you buy or build.