AJ Designer

Paint Coverage Calculator

21 ft² each

15 ft² each

2 = default

Optional buffer

Solution

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Gallons equals the ceiling of two times length plus width times ceiling height, minus door area and window area, all times the number of coats divided by coverage per gallon

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Single Room (Rectangle)

Enter the room's length, width, and ceiling height, plus the number of doors and windows. The calculator computes wall perimeter, multiplies by ceiling height, and subtracts standard 21 ft² doors and 15 ft² windows to get the paintable wall area.

A = 2·(L + W)·H − doors·21 − windows·15

Wall Only (Direct Area)

Already know the wall area in square feet? Enter it directly. Useful for accent walls, partial rooms, or when you've measured the area in a different way (e.g., from a contractor's takeoff).

G = ⌈ A_walls · coats / coverage ⌉

Multiple Walls

Enter each wall's width and height separately — best for irregular rooms, vaulted ceilings, or whole-house projects where every wall is a different size. Doors and windows are subtracted from the total.

A = Σ (wᵢ · hᵢ) − doors·21 − windows·15

How It Works

This paint coverage calculator computes how many gallons of paint you need to cover a room's interior walls. It takes the wall area (either computed from room length × width × ceiling height, entered directly, or summed from individual walls), subtracts standard door and window areas, multiplies by the number of coats, divides by the manufacturer-published coverage rate for the chosen paint type, and rounds up to the nearest gallon. Coverage varies by paint type — flat covers 400 ft²/gal, eggshell covers 350 ft²/gal, and semi-gloss covers 300 ft²/gal, because each finish builds film at a different thickness.

Example Problem

A homeowner wants to repaint a 12 ft × 15 ft bedroom with 8 ft ceilings. The room has 1 standard door (21 ft²) and 2 standard windows (15 ft² each = 30 ft² total). They plan 2 coats of eggshell paint (350 ft²/gallon coverage). How many gallons do they need?

  1. Compute perimeter: 2 × (12 + 15) = 54 ft.
  2. Gross wall area: 54 × 8 ft ceiling = 432 ft².
  3. Subtract openings: 1 × 21 + 2 × 15 = 21 + 30 = 51 ft².
  4. Net paintable area: 432 − 51 = 381 ft².
  5. Gallons per coat: 381 ÷ 350 ≈ 1.089 gallons.
  6. Total for 2 coats: 1.089 × 2 ≈ 2.18 gallons → round up → 3 gallons.

Buy 3 full gallons rather than 2 gallons + 1 quart — having a small amount left over is essential for touch-ups, and most paint stores will color-match a quart later but can't guarantee a perfect match across batches.

When to Use Each Variable

  • Painting One Room — for a rectangular bedroom, living room, dining room, or office. Enter length, width, ceiling height, and the number of doors and windows — the most common residential interior painting scenario.
  • Accent Wall or Known Area — for a single accent wall, an irregularly shaped surface you've already measured, or when you have wall area from a contractor's takeoff sheet.
  • Vaulted Ceilings or Whole Houses — for rooms with vaulted/cathedral ceilings, irregular floor plans, or whole-house projects where each wall has a different height. Add one row per wall.

Key Concepts

Paint coverage is the area one gallon will cover at the manufacturer's recommended film thickness — typically 350 ft² per gallon for standard interior eggshell on smooth drywall. Coverage drops on rough surfaces (textured walls, popcorn ceilings: 250 ft²/gal), heavier finishes (semi-gloss: 300 ft²/gal), and rises on smooth surfaces with flat paint (400 ft²/gal). Two coats is the residential default — one coat rarely achieves full hide over a different existing color, and primer plus one finish coat is the typical workflow for fresh drywall or color changes. Always round gallons up: paint stores don't sell partial gallons, and a short-buy means a second store trip and a possible batch mismatch.

Applications

  • Repainting a bedroom or living room (single-room mode)
  • Accent walls in any room (wall-only mode)
  • Whole-house interior repaints (multiple-walls mode, room by room)
  • Estimating primer needed for fresh drywall (use primer paint type)
  • Calculating deck or fence stain (use stain paint type, 200 ft²/gal)
  • Trim and door paint jobs (use semi-gloss, 300 ft²/gal)

Common Mistakes

  • Forgetting to subtract doors and windows — a standard 3 ft × 7 ft door is 21 ft², and a 3 ft × 5 ft window is 15 ft². Two windows + one door in a typical bedroom is ~50 ft² of paint you don't need to buy.
  • Using one-coat math when planning two coats — most color changes and most flat-to-eggshell transitions need two coats for full hide. Always multiply the gallons-per-coat number by the number of coats.
  • Using flat-paint coverage (400 ft²/gal) when the actual paint is semi-gloss (300 ft²/gal) — the wrong rate underbuys by 25%, leaving you short on the last wall.
  • Rounding gallons down — a calculation of 2.1 gallons means you need 3 full gallons, not 2 — paint is sold by the gallon and partial cans can't be returned.
  • Forgetting that textured walls (orange peel, knockdown, popcorn) absorb noticeably more paint than smooth drywall — drop coverage to 250 ft²/gal for textured surfaces.

Frequently Asked Questions

How many gallons of paint do I need?

Compute the wall area (length + width, doubled, times ceiling height), subtract doors and windows, multiply by the number of coats, divide by the coverage rate (350 ft²/gal for eggshell), then round up. A 12 × 15 ft room with 8 ft ceilings, 1 door, and 2 windows needs 3 gallons for 2 coats of eggshell.

What is the coverage of a gallon of paint?

Flat / matte latex covers 400 ft² per gallon on smooth walls. Eggshell and satin cover 350 ft²/gal (the most common interior finish). Semi-gloss and gloss cover 300 ft²/gal because the finish builds a thicker film. Textured walls and popcorn ceilings drop to 250 ft²/gal. Deck and wood stain covers 200 ft²/gal. These are manufacturer-published averages — always check the label for the specific product.

Do I need 2 coats of paint?

Two coats is the residential default for most repaints — one coat rarely achieves full hide, especially when changing colors or going from a darker to lighter shade. Use one coat only when refreshing the same color over a sound existing finish in good condition. Use primer plus one finish coat for fresh drywall, stain blockers, or major color changes (white over a dark accent wall).

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

Flat (or matte) paints have the lowest film build per coat — about 400 ft²/gal — because they contain less binder and more pigment, hiding minor wall imperfections. Eggshell and satin sit in the middle at 350 ft²/gal with a slight sheen and better washability. Semi-gloss and gloss build a thicker, more durable film at 300 ft²/gal, used for trim, doors, and kitchen/bathroom walls where you need to wipe surfaces clean.

How much paint primer do I need?

Primer covers about 350 ft² per gallon on fresh drywall — similar to eggshell. For most residential primer jobs, one coat is sufficient; the primer's job is to seal the drywall and provide a uniform base for the topcoat, not to build hide. Use the same area math as a regular paint coat: net wall area divided by 350. For a 12 × 15 ft bedroom (381 ft² net), one gallon of primer is plenty.

How much paint do I need for the ceiling vs the walls?

Ceiling area is just the room's floor area (length × width). A 12 × 15 ft room has a 180 ft² ceiling, or about 0.6 gallons of ceiling paint per coat at 300 ft²/gal (ceiling paint is usually flat and slightly thicker, covering closer to 300 than 400). Walls in the same room run ~381 ft² net — about double the area. Buy 1 gallon of ceiling paint and ~3 gallons of wall paint for a standard bedroom with 2 coats.

Can I paint a textured wall with the same coverage rate?

No — textured surfaces (orange peel, knockdown drywall, popcorn ceilings, stucco) absorb more paint than smooth walls. Drop coverage to 250 ft²/gal instead of 350 to avoid buying short. The extra surface area inside the texture pattern is the reason — a 'flat' textured wall is really a wrinkled surface with 20–40% more actual paint-contact area.

Should I add a waste factor to my paint estimate?

A waste factor isn't always needed because gallons are already rounded up — the ceiling already includes a buffer. However, for complex jobs (multiple colors, cutting in tight trim, spray application) a 10% waste factor is a reasonable buffer; for simple roll-and-cut interior jobs, the ceil-to-whole-gallon rounding usually covers spillage and tray waste. Painters often buy one extra quart of the main color for future touch-ups.

Reference: Coverage rates published by major US paint manufacturers (Sherwin-Williams, Benjamin Moore, Behr) and the Paint Quality Institute. Standard door (3 ft × 7 ft = 21 ft²) and window (3 ft × 5 ft = 15 ft²) dimensions from residential construction averages.

Worked Examples

Bedroom Repaint

How much paint for a 12 × 15 ft bedroom with 8 ft ceilings, 1 door, 2 windows?

Standard bedroom repaint with 2 coats of eggshell. The poster-child example used throughout this page.

  • Knowns: L = 12 ft, W = 15 ft, H = 8 ft, 1 door, 2 windows, 2 coats, eggshell (350 ft²/gal).
  • Perimeter: 2 × (12 + 15) = 54 ft.
  • Gross wall area: 54 × 8 = 432 ft².
  • Subtract: 1 × 21 + 2 × 15 = 51 ft². Net = 432 − 51 = 381 ft².
  • Raw gallons: 381 × 2 / 350 ≈ 2.18 → round up → 3 gallons.

3 gallons of eggshell (2 coats, 381 ft² paintable area)

Buy 3 full gallons. The leftover ~0.8 gallon is ideal for touch-ups over the life of the paint job.

Accent Wall

How much paint for a 10 × 8 ft accent wall in semi-gloss?

A single accent wall, no doors or windows, 2 coats of semi-gloss (300 ft²/gal). Small job — should fit in quarts, not gallons.

  • Wall area: 10 × 8 = 80 ft².
  • No openings: net area = 80 ft².
  • Raw gallons: 80 × 2 / 300 ≈ 0.533 gallons.
  • Round up to whole gallons → 1 gallon, OR 3 quarts (0.533 × 4 = 2.13 → 3 quarts).
  • Buy 3 quarts for the exact accent color; cheaper than a full gallon.

1 gallon, or 3 quarts (160 ft² paintable, 2 coats)

Quarts are perfect for accent walls — full gallons leave most of the can unused, while quarts let you buy just enough for the small job.

Whole House Primer

How much primer for 2,000 ft² of fresh drywall (1 coat)?

Whole-house primer job before the topcoat. Fresh drywall, primer covers 350 ft²/gal, one coat is sufficient.

  • Wall area: 2,000 ft² (entered directly).
  • No openings counted at this stage (already netted by the contractor).
  • Raw gallons: 2,000 × 1 / 350 ≈ 5.71 gallons.
  • Round up to whole gallons → 6 gallons of primer.
  • Buy a 5-gallon bucket (cheaper per gallon) plus 1 extra gallon for cut-in.

6 gallons of primer (2,000 ft², 1 coat, 350 ft²/gal)

A 5-gallon bucket is almost always cheaper per gallon than five 1-gallon cans. For large jobs, buy buckets and decant into a tray-friendly container.

Paint Coverage Formulas

Paint coverage is just "area divided by gallons," rounded up. The challenge is computing the right area — gross wall area minus openings, summed across all walls if the room isn't a simple rectangle.

Single room: A = 2·(L + W)·H − doors·21 − windows·15
Wall only: A = A_walls − doors·21 − windows·15
Multiple walls: A = Σ (wᵢ · hᵢ) − doors·21 − windows·15
Gallons: G = ⌈ A · coats / coverage ⌉

Where:

  • A — net paintable wall area in ft² (after subtracting doors and windows)
  • L, W, H — room length, width, and ceiling height (any consistent unit; calculator converts to feet)
  • doors, windows — counts; standard nominal sizes are 21 ft² (3 × 7 ft door) and 15 ft² (3 × 5 ft window)
  • coats — number of paint coats (2 is the residential default)
  • coverage — ft²/gallon for the chosen finish (eggshell 350, flat 400, semi-gloss 300, textured 250, primer 350, stain 200)
  • ⌈ ⌉ — ceiling function (round up to the nearest whole gallon — paint is sold by the gallon)

Paint Coverage Reference

Paint TypeCoverage (ft²/gal)Typical Use
Flat / Matte Latex400Ceilings, low-traffic walls
Eggshell / Satin350Bedrooms, living rooms (default)
Semi-Gloss / Gloss300Trim, doors, kitchens, bathrooms
Textured / Popcorn250Orange-peel walls, popcorn ceilings
Primer350Fresh drywall, color change base
Stain (Deck / Wood)200Decks, fences, exterior wood

Coverage rates are manufacturer-published averages for smooth interior surfaces. Rough or porous substrates may absorb 20–40% more paint per gallon; check the can label for the exact figure.

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