Paint Calculator
(How Much Paint Do You Need?)
Enter your room's size, doors, windows and coats to find how many gallons or litres of paint to buy, with optional ceiling and primer.
Units
Each door is deducted as about 20 sq ft (1.9 m²)
Each window is deducted as about 15 sq ft (1.4 m²)
Paint the ceiling too?
Read it on the can. Blank uses 350 sq ft per gallon (about 8.6 m² per litre)
Include primer (one coat)?
Result
💡 Formula Guide
How to Work Out How Much Paint You Need
To find how much paint you need, calculate the paintable wall area, multiply by the number of coats and divide by the area one gallon covers. It is one of the most common home-improvement questions, and buying too little means a second trip and possible colour differences while buying too much wastes money. The steps are the same whether you paint a small bathroom or a whole house, and the calculator above does each one for you, rounding up to a quantity you can actually buy.
Example. A 14 × 12 ft room with 8 ft walls has a perimeter of 52 ft and a wall area of 416 sq ft. One door (20 sq ft) and two windows (30 sq ft) come off, leaving 366 sq ft. Two coats at 350 sq ft per gallon need 366 × 2 ÷ 350 = 2.09 gallons, so buy 3 gallons, or 9 quarts if you prefer smaller cans. Adding the 168 sq ft ceiling raises the total to 534 sq ft, or 3.05 gallons for two coats, which pushes you to 4 gallons.
Coverage Figures and Deductions Used
| Item | Typical planning figure | Notes |
|---|---|---|
| Paint coverage | 350–400 sq ft per gallon | Check the can; rough surfaces need more |
| Primer coverage | 200–300 sq ft per gallon | The calculator uses 250 sq ft per gallon |
| Standard door | About 20 sq ft | 3 ft × 7 ft opening |
| Standard window | About 15 sq ft | 3 ft × 5 ft opening |
| Coats | Usually 2 | One coat for same-colour touch-ups on sound paint |
The calculator defaults to 350 sq ft per gallon, the low end of the usual range, which builds in a little safety margin. If your can states a different figure, enter it in the coverage box and the result updates. Metric users can switch to metres and litres, where 350 sq ft per gallon is about 8.6 square metres per litre.
When You Will Need More Paint
- Colour changes. Painting light over dark or bold over neutral often needs an extra coat or a tinted primer.
- Porous or new surfaces. Bare drywall, plaster and unsealed wood absorb paint, so prime first and expect more usage.
- Texture. Textured walls have more surface area than flat ones and use more paint per square foot.
- Trim and doors. These usually take different paint or sheen, so calculate them separately.
💡 Tip: buy all the paint for one wall or room at the same time, and mix the cans together (called boxing) before you start. That evens out tiny colour differences between batches.
Planning the Job
Measure the room carefully or use the Room Area Calculator to confirm the dimensions, since the wall area depends on the perimeter and height. Decide whether the ceiling is being painted and in what finish, count the doors and windows and check the label of the paint you plan to use. Then run the calculator, round up and add a small reserve for touch-ups. For wall coverings instead of paint, the Wallpaper Coverage Calculator follows the same method with rolls instead of gallons, and the Tile & Flooring Calculator covers the floor.
These figures are planning estimates rather than guarantees. Application method, paint quality and the surface all change coverage, so treat the result as a starting point and follow the manufacturer's guidance on the can.
Frequently Asked Questions About How Much Paint to Buy
How much paint do I need for a room?
Multiply the wall area by the number of coats and divide by the coverage per gallon. For a 14 × 12 ft room with 8 ft walls, one door and two windows, the net wall area is 366 sq ft. Two coats at 350 sq ft per gallon need 2.09 gallons, so plan on 3 gallons or 9 quarts.
How much area does a gallon of paint cover?
Paint is usually quoted at roughly 350 to 400 square feet per gallon, while primer is often 200 to 300 square feet per gallon. Rough or porous surfaces soak up more. Check the can, because the label figure for your product is more reliable than a general rule.
How do I calculate wall area?
Add the length and width of the room, double it to get the perimeter and multiply by the ceiling height. A 14 × 12 ft room has a perimeter of 52 ft, so 52 × 8 = 416 sq ft of wall before deducting doors and windows.
How much do I subtract for doors and windows?
A standard door is about 20 square feet and a standard window about 15 square feet, so the calculator deducts those amounts. Adjust by entering fewer or more openings if yours are unusually large or small, and remember that painted door frames and trim are usually done separately.
Do I need two coats?
Two coats is the common plan for a uniform finish, particularly over a different colour. A single coat can work when repainting the same colour over sound paint or when the product is designed for one-coat coverage, but check the can. The calculator lets you set any number of coats.
Should I include the ceiling?
Only if you are painting it. Ceiling paint is often a separate flat finish, so you may want to run the calculation twice, once for walls and once for the ceiling, and buy accordingly. The calculator can add the ceiling area to the walls for a single-colour job.
Do I need primer?
Use primer on bare drywall, patched areas, stains, glossy surfaces or dramatic colour changes. The calculator adds primer for one coat at about 250 sq ft per gallon, the middle of the 200 to 300 range; check your primer's label.
Why buy extra paint?
Leftover paint is useful for touch-ups, and colour can vary slightly between batches, so running short later can leave a visible mismatch. Rounding up to the next can and keeping the remainder is usually cheaper than buying a partial can and matching it later.
Last Updated: September 2026 | Uses common planning figures: about 350 sq ft per gallon of paint, 200–300 for primer, 20 sq ft per door and 15 per window. Always check your paint's label for its actual coverage.