Paint Calculator
Work out gallons or litres of paint for a room, with doors, windows, coats, ceiling and primer — plus the tin sizes to buy.
✓ Method shownUpdated By DIYCalcLab Editorial TeamSources (3)How we calculate
Quick answer: A 12 × 12 ft room with 8 ft walls, one door and two windows has about 334 sq ft of wall. Two coats at 350 sq ft per gallon need 1.91 gal — buy 2 gal. A 4 × 3.5 m room with 2.4 m walls needs about 7 L for two coats at 9 m²/L (1 × 5 L + 1 × 2.5 L). Coverage varies by product; check the tin.
Result
Paint needed
1.91 gal
- Paintable area
- 334 sq ft
- Suggested purchase
- 2 gal
- In litres
- 7.2 L
Show the math
- Wall area = 2 × (L + W) × H − doors × 20 sq ft − windows × 15 sq ft = 334 sq ft
- Ceiling not included
- Paint = area × 2 coats ÷ coverage = 1.91 gal
- Rounded up to purchasable sizes: 2 gal
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Paint | 2 coats | 2 gal | — |
| Roller covers, tray, brush, tape | per room | 1 set | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Diagram
How to use this calculator
- Measure the room length, width and wall height in feet or metres.
- Count doors and windows; the calculator subtracts typical sizes (20 sq ft / 1.86 m² per door, 15 sq ft / 1.39 m² per window).
- Choose coats — two is standard; colour changes from dark to light may need three or a primer.
- Enter the coverage printed on your tin under Advanced; it ranges widely between products.
- Use the suggested purchase to buy the fewest tins with the least leftover paint.
Method & assumptions
Wall area = 2 × (Length + Width) × Height − openings Paint (gal or L) = Area × Coats ÷ Coverage per coat
- Coverage
- area one unit of paint covers in one coat (sq ft/gal or m²/L)
- Openings
- 20 sq ft per door, 15 sq ft per window (typical)
Assumptions & limits
- Walls are smooth and previously painted. Bare plaster, new drywall and textured surfaces absorb more.
- Trim, skirting and doors are painted separately.
- Ceiling uses the same coverage as the walls when included.
Rounding: purchase quantities are always rounded up to whole bags, boxes, sheets or bundles. See our methodology.
Worked examples
Imperial: 14 × 12 ft bedroom, 9 ft ceilings
- 2 × (14 + 12) × 9 = 468 sq ft
- Minus 1 door, 2 windows: 468 − 20 − 30 = 418 sq ft
- 418 × 2 ÷ 350 = 2.39 gal → buy 2 gal + 2 qt
Metric: 3.5 × 3 m bathroom, 2.4 m walls
- 2 × 6.5 × 2.4 = 31.2 m² − 1.86 (door) − 1.39 (window) = 27.95 m²
- 27.95 × 2 ÷ 10 m²/L = 5.6 L → 1 × 5 L + 1 × 1 L
In depth: paint planning notes
Coverage, coats and why primer changes the answer
Paint manufacturers state coverage per gallon or litre for one coat on a smooth, sealed surface — typically 350–400 sq ft per gallon in the US and 10–12 m² per litre for good interior emulsion in the UK and Australia. Real walls are rarely ideal. Fresh plaster, bare drywall and textured surfaces absorb more, and strong colour changes need extra coats. That is why our default is a conservative 350 sq ft per gallon (8.5 m² per litre) and two coats.
Primer is a separate product with its own coverage. Use it on new drywall, patched areas, stains and when going from dark to light. A tinted primer can save a top coat. The calculator adds primer as a separate line so you can buy the right product rather than doubling the top coat.
Buying the right mix of tins
Paint is sold in fixed sizes: gallons and quarts in North America, 5 L, 2.5 L and 1 L tins in the UK and 4 L, 2 L and 1 L in Australia, with 10 L tubs for trade. Our purchase list picks the combination that covers the volume with the least leftover, so a 1.2 gallon requirement becomes one gallon and one quart rather than two gallons. When the totals are close, buying one larger tin can still be cheaper than several small ones — compare prices at your store.
For a consistent finish, buy all the paint for a room from the same batch, or box (mix) several tins together in a bucket before you start. Keep a small amount of the leftover in a labelled jar for touch-ups. Ceilings are usually painted with a dedicated flat white, so the calculator shows ceiling paint separately when you include it.
Doors and windows are deducted using typical sizes — about 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window. If you have patio doors or large windows, enter a wider room measurement or adjust the count.
Reference table
| Surface | Coverage |
|---|---|
| Smooth, previously painted wall | 350–400 sq ft/gal (8.5–10 m²/L) |
| New drywall / bare plaster (with primer) | 250–300 sq ft/gal (6–7 m²/L) |
| Textured or rough surfaces | 200–300 sq ft/gal (5–7 m²/L) |
| Masonry / exterior render | 100–200 sq ft/gal (2.5–5 m²/L) |
Key facts
- 1 US gallon
- 3.785 L = 4 quarts
- Typical interior coverage
- 350 sq ft/gal (≈ 8.6 m²/L)
- Standard UK/AU tin sizes
- 1 L, 2.5 L, 5 L (10 L common in AU)
Tips & common mistakes
- Buy all of one colour from the same batch, or box (mix) tins together before you start.
- A cheap paint with poor coverage can need an extra coat — compare price per square foot covered, not per gallon.
- Keep a small amount for touch-ups and label the tin with the room and date.
Frequently asked questions
How much paint do I need for a 12×12 room?
With 8 ft walls, one door and two windows, about 1.9 gallons for two coats — buy 2 gallons. Add a gallon if you are painting the ceiling too.
How many square feet does a gallon of paint cover?
Most interior paints state 250–400 sq ft per gallon per coat; 350 is a common figure. Rough or porous surfaces cover less.
Do I really need two coats?
Usually, yes: two coats give even colour and sheen. A one-coat paint can work over a similar colour in good condition.
Should I prime first?
Prime new drywall, bare plaster, stains and big colour changes. Turn on "Add a primer coat" to include it.
How much paint for a ceiling?
Ceiling area = length × width. A 12 × 12 ft ceiling is 144 sq ft, about 0.8 gallon for two coats.
Sources
- Sherwin-Williams – paint coverage guidance — checked 2026-10; verify before launch
- Benjamin Moore – product data sheets (coverage per gallon) — checked 2026-10; verify before launch
- Dulux (UK) – coverage per litre on product pages — checked 2026-10; verify before launch
Default values (bag yields, coverage rates, densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Drywall CalculatorCount drywall or plasterboard sheets for walls and ceilings, plus joint compound, tape and screws. 4×8, 4×12 and 1200×2400 mm.
- Square Footage CalculatorFind the area of a room or yard in square feet, square metres and square yards — rectangles, triangles, circles and trapezoids.
- Flooring CalculatorCalculate boxes of laminate, vinyl plank or hardwood flooring for any room, with layout waste, underlay and baseboard length.
Learn more in our guides
Cite or embed this calculator
DIYCalcLab. “Paint Calculator.” Oct 18, 2026. https://diycalclab.com/paint/paint-calculator/
An embeddable version for blogs and contractor sites is planned.
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.