Paint Calculator

How much paint do I need? Estimate gallons from room length, width, and height in feet: calculate wall area, subtract door and window openings, add the ceiling if you want, multiply by coats, then divide by about 350 square feet of coverage per gallon per coat. This free calculator rounds up to whole gallons you can buy at the store.

Reviewed / updated August 2026

Full walkthrough: How to Estimate Materials for Any Home Project

Your inputs

ft
ft
ft

Most US homes use 8 ft ceilings; enter vaulted heights if needed

coats
count

Each door ≈ 21 sq ft deducted

count

Each window ≈ 15 sq ft deducted

Adds length × width to the paintable area (same coverage rate).

Result

Paint to buy

2 gal

333 sq ft paintable · 1.90 gal exact

Paintable area
333 sq ft
Exact gallons
1.90 gal
Buy (rounded up)
2 gal
Coverage assumption
350 sq ft/gal/coat

For 333 sq ft of paintable walls at 2 coats (~350 sq ft/gal/coat), buy 2 gallons. We round up from 1.90 gal so you buy whole containers.

1 coat vs 2 coats

Same room and openings — only the coat count changes.

CoatsExactBuy
1 coat0.95 gal1 gal
2 coats1.90 gal2 gal

Suggested next steps

  • Buy a quart of the same color for touch-ups after furniture scuffs and nail holes — a full extra gallon is usually more than you need for repairs.
  • Dark-to-light color changes may need an extra coat beyond this estimate — test a board or corner first.
  • Stir thoroughly between coats and finish one full wall at a time for even sheen.

How much paint do you need?

For a typical rectangular room, buy gallons = ceil( (paintable sq ft × coats) ÷ 350 ). Paintable area starts with the four walls, subtracts openings, and optionally adds the ceiling:

  1. Wall area = 2 × (length + width) × height
  2. Openings = (doors × 21) + (windows × 15) square feet
  3. Paintable = max(0, wall area − openings) + (ceiling? length × width : 0)
  4. Exact gallons = (paintable × coats) ÷ 350
  5. Buy = round up to the next whole gallon

Coverage assumption: ~350 sq ft of coverage per gallon per coat for interior latex on smooth, primed surfaces — a widely used US DIY planning rate consistent with consumer paint-label guidance from major brands (often listed near 250–400 sq ft depending on product and surface). Always confirm the rate printed on your can.

Source note: Coverage figures vary by brand, sheen, and substrate. Treat 350 sq ft/gal/coat as a planning estimate, not a warranty. Updated August 2026.

Worked example

Room 12 × 12 ft, ceiling height 8 ft, 2 coats, 1 door, 2 windows, ceiling not included:

  1. Walls = 2 × (12 + 12) × 8 = 384 sq ft
  2. Openings = 21 + (2 × 15) = 51 sq ft
  3. Paintable = 384 − 51 = 333 sq ft
  4. Exact = (333 × 2) ÷ 350 ≈ 1.90 gallons
  5. Buy = 2 gallons (rounded up)

That matches the default inputs on this page. Switch the coat comparison table to see the same room at one coat (~1 gallon to buy) versus two coats (2 gallons).

Tips for a better estimate

All math runs in your browser from the dimensions you enter — no account and no backend.

Frequently asked questions

How much paint do I need for a 12×12 room?

For a 12×12 ft room with 8 ft ceilings, one standard door, two windows, and two coats (walls only), paintable area is about 333 sq ft. At ~350 sq ft per gallon per coat that is about 1.90 gallons exact, so you buy 2 gallons. Add the ceiling or more coats and the total rises.

How many square feet does a gallon of paint cover?

This tool uses 350 square feet per gallon per coat — a common planning figure for interior latex on smooth, primed walls. Manufacturer labels often list a range (roughly 250–400 sq ft) depending on surface texture, color change, and product. Rough plaster, unprimed drywall, or dark-to-light changes usually need more paint.

Should I include the ceiling in my paint estimate?

Only if you plan to paint it. Toggle “Include ceiling” to add length × width to the paintable area at the same coverage rate. Ceilings are often a flatter sheen and sometimes a different product, so many DIYers estimate walls and ceiling as separate purchases.

Why do you subtract doors and windows?

Openings are not painted. We deduct about 21 sq ft per door and 15 sq ft per window — standard DIY planning sizes. Oversized patio doors or picture windows may need a larger deduction; tiny closet doors may need less.

Do I need primer in addition to these gallons?

Primer is separate. Bare drywall, stains, or drastic color changes often need primer first; coverage rates differ from finish paint. Use this calculator for finish coats, then check the primer can for its own sq ft per gallon guidance.

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