How much paint do I need?

Take net wall area (walls minus doors and windows), divide by coverage per gallon for your surface, then multiply by coats. On smooth walls plan about 350–400 sq ft per gallon per coat; textured walls closer to 250–300. This calculator does that math and rounds up with waste.

Why subtract doors and windows?

Openings are not painted. Leaving them in the area is the most common reason DIY calculators overbuy. We subtract about 21 sq ft per door and 15 sq ft per window by default.

How many coats of paint?

Two coats is the default for durable, even coverage. One coat rarely hides prior color evenly. Dramatic color changes may need a third coat or a tinted primer first.

Is primer included in the paint gallons?

No. Primer is a separate line at one coat so you can buy the right product. It is never folded into the finish-paint total.

Smooth vs textured — which coverage should I use?

Smooth drywall or skim coat: 350–400 sq ft per gallon per coat. Orange peel, knockdown, or popcorn: 250–300. When unsure, pick textured — buying a little extra is cheaper than a mid-job store run.

Does this cover exterior paint?

This page is for interior rooms. For siding, stucco, and masonry coverage by substrate, use the exterior paint calculator.

Quantity · Paint

Paint Calculator

Size interior paint from room length, width, and height. Subtract doors and windows — the step most online calculators skip. Two coats is the default; primer is a separate line, not folded into paint gallons. Coverage differs for smooth vs textured walls.

Room dimensions

LengthftWidthftHeightft

Include ceilingAdd length × width to paintable area.

Openings to subtract

Doors ~21 sq ft each (3'×7'); windows ~15 sq ft each. Most ranking calculators skip this — and overbuy paint.

DoorsqtyWindowsqty

Surface

350–400 sq ft per gallon per coat on smooth walls.

Paint coatsWaste%10% covers touch-ups and can variation; bump for dark-over-light or porous walls.

Two coats is the default for durable coverage — one coat rarely hides evenly.

Include primerPrimer is listed separately and is not folded into paint gallons.

Paint gallons to buy

2gal

1.66 gal raw · 2 coats · 312 sq ft net

Primer (separate line)

2gal

Not added into paint total — buy primer as its own product.

Walls: 384 sq ft
Ceiling: —
Openings: −72 sq ft
Net area: 312 sq ft
Coverage / gal: ~375 (350–400)
With waste: 1.83 gal

Gross = walls (+ ceiling) − doors − windows
Paint gal = net ÷ coverage × coats
Primer = net ÷ primer coverage (1 coat, separate)

Price data updated 2026-07-27. National US ranges from published supplier and contractor averages. Not a bid. Local quotes vary.

How paint quantity is calculated

Net area is wall (and optional ceiling) square footage minus openings. Gallons equal net area divided by coverage per gallon, times coats. Smooth walls cover farther per gallon than textured. Primer uses one coat at its own coverage and is never added into the paint total.

wall_sq_ft     = 2 × (length + width) × height
gross_sq_ft    = wall_sq_ft + (ceiling ? length × width : 0)
openings       = doors × 21 + windows × 15   // sq ft each
net_sq_ft      = max(0, gross − openings)
paint_gallons  = (net / coverage) × coats × (1 + waste %)
primer_gallons = (net / primer_coverage) × 1 × (1 + waste %)  // separate line

Worked example. A 12 × 12 × 8 ft room (walls only): gross 384 sq ft. Two doors (42) + two windows (30) = 72 sq ft openings → 312 sq ft net. Smooth coverage ~375 sq ft/gal, two coats → 1.66 gal raw → 2 gal with 10% waste. Primer (separate): ~1.04 gal raw → 2 gal — not included in the paint total.

Paint coverage by surface (sq ft per gallon per coat)

Unique reference for this interior page. Smooth vs textured coverage is the trust signal missing from thin calculators. Exterior paint uses a different weather/sheen/substrate table on its own URL.

Surface	Sq ft / gal / coat	Typical use
Smooth drywall / skim	350–400	New or well-prepped interior walls
Light texture	300–350	Light orange peel
Heavy texture / popcorn	250–300	Knockdown, popcorn, rough plaster
Primer (1 coat, separate)	250–350	Bare drywall, stains, color changes

Two coats is the default for finish paint; one coat rarely hides evenly.
Primer gallons are never folded into the paint total.

Opening sizes subtracted

Defaults used when you enter door and window counts. Adjust counts for oversized units.

Opening	Typical size	Sq ft each
Interior door	3' × 7'	21
Average window	~3' × 5'	15
Large picture window	varies	Count extra or raise window qty