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Paint Calculator

Estimate how much paint your project needs — in gallons and litres — from the wall area or room dimensions, number of coats, coverage rate, and any doors or windows to skip.

FreeNo sign-upPrivate — runs in your browser, nothing is uploaded

Paint needed
2.29 gal

Buy 3 gallons (8.7 L) to cover 400 ft² with 2 coats.

Computed in your browser. Assumes 1 gallon covers ~350 ft² per coat; each door subtracts 21 ft² and each window 15 ft².

Quick answer

To estimate paint, multiply the wall area by the number of coats and divide by the paint's coverage rate (about 350 ft² per gallon). For 400 ft² with two coats: (400 × 2) ÷ 350 = 2.29 gallons, so you'd buy 3 gallons. This calculator works from either a known wall area or room dimensions, subtracts doors and windows, and reports gallons, litres, and how many cans to buy.

Formula & method

Wall area is either entered directly or computed from the room as 2 × (length + width) × height. Each door subtracts 21 ft² and each window 15 ft². Paint needed = (net area × coats) ÷ coverage, where coverage defaults to 350 ft² per gallon. The result is rounded up to whole cans, and gallons are converted to litres (1 US gallon = 3.785 L).

Examples

Example 1: Known wall area
Input
400 ft², 2 coats, 350 ft²/gal
Result
2.29 gal (buy 3)
Why
(400 × 2) ÷ 350 = 2.29 gallons, rounded up to 3 cans.
Example 2: From room size
Input
12 × 12 ft room, 9 ft walls, 2 coats
Result
2.47 gal (buy 3)
Why
Walls = 2 × (12+12) × 9 = 432 ft²; (432 × 2) ÷ 350 = 2.47 gallons.
Example 3: One coat, with openings
Input
500 ft², 1 coat, 2 doors, 3 windows
Result
1.18 gal (buy 2)
Why
Net = 500 − 2×21 − 3×15 = 413 ft²; 413 ÷ 350 = 1.18 gallons.

When to use this tool

  • Buying the right number of paint cans for a room or wall before a trip to the store.
  • Budgeting a repaint from a quoted coverage rate.
  • Adjusting for doors, windows, and multiple coats.

Common mistakes

  • Painting only one coat in the estimate when most colors need two for even coverage.
  • Forgetting that the first coat on bare or porous walls covers less than the rated 350 ft²/gallon.
  • Counting the floor or ceiling area instead of the wall area (perimeter × height).

Frequently asked questions

How much paint do I need for a room?

Calculate the wall area (perimeter × height), multiply by the number of coats, and divide by the coverage rate (~350 ft²/gal). The tool does this from either the area or the room dimensions.

How many square feet does a gallon cover?

About 350 ft² per coat for typical interior wall paint on a primed, smooth surface. Rough, porous, or dark-to-light walls cover less, so the default is adjustable.

Do I subtract doors and windows?

Yes, for a tighter estimate. The calculator subtracts 21 ft² per door and 15 ft² per window from the wall area.

How many coats should I plan for?

Two is standard for an even finish, especially over a different color. One coat may do for a same-color refresh over primer.

Does it give litres too?

Yes — it shows both US gallons and litres (1 gallon = 3.785 L) and rounds up to whole cans to buy.

Is anything uploaded?

No. The estimate is calculated in your browser.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

  • ✓ Free to use
  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
Last updated: How we calculateReport a correctionDisclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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