Paint Calculator
Work out the paintable wall area of a room and the exact number of gallons or liters of paint to buy.
Paint Calculator — input your values
What does this construction calculator do?
The paint calculator estimates how much paint you need to cover the walls of any rectangular room, subtracting the standard areas of doors and windows from the total wall surface.
It accounts for the number of coats and the coverage rate of the paint you plan to buy, then reports the cans you need in gallons or liters — rounded up so you never run out mid-roll.
Formula and variables
Coverage rates vary with surface texture, primer and application method — treat the estimate as a starting point.
| Symbol | Meaning |
|---|---|
| Door | ≈ 20 sq ft each |
| Window | ≈ 15 sq ft each |
| Standard paint | ≈ 350 sq ft per gallon |
| 1 US gallon | ≈ 3.79 liters |
Worked example
A 12 ft × 10 ft room with 8 ft ceilings, 2 doors, 2 windows, painted with 2 coats of standard paint in gallons.
- Perimeter = 2 × (12 + 10) = 44 ft.
- Gross wall area = 44 × 8 = 352 sq ft.
- Subtract doors (40) and windows (30): 352 − 70 = 282 sq ft.
- Two coats: 282 × 2 = 564 sq ft.
- Gallons = 564 ÷ 350 = 1.6, rounded up = 2 gallons.
Answer: Buy ≈ 2 gallons of paint.
Tips and common mistakes
Tips
- Always prime bare drywall first — primer seals the surface and lets one coat of paint do the work of two.
- Dark-to-light color changes often need an extra coat; light-to-dark rarely does.
- Rollers use more paint than brushes but cover faster; keep a wet edge to avoid lap marks.
- Store leftover paint with the lid sealed tight and the can upside down briefly to create an airtight skin.
Common mistakes to avoid
- Painting the ceiling by accident — this calculator covers walls only; add ceiling area separately if needed.
- Ignoring texture: knockdown and orange-peel walls soak up noticeably more paint.
- Rounding down to save a few dollars — a second trip to the store costs more than one extra can.
Paint Calculator — frequently asked questions
How much paint do I need for a 12×12 room?
A 12×12 ft room with 8 ft ceilings has about 384 sq ft of gross wall area. After subtracting a door and a window, expect roughly 310 sq ft per coat — about one gallon of standard paint per coat.
How many square feet does a gallon of paint cover?
Standard interior paint covers about 350 sq ft per gallon on smooth, primed surfaces. Premium paints often cover closer to 400 sq ft.
Should I include the ceiling in my paint order?
Ceilings are usually painted separately with flat ceiling paint. A 12×10 ft room has a 120 sq ft ceiling, which adds roughly a third of a gallon per coat.
How much paint do I need for two coats?
Simply double the single-coat figure: 282 sq ft of paintable wall needs about 1.6 gallons for two coats, so buy 2 gallons.
Does paint color affect coverage?
Yes — reds and dark blues have less pigmented coverage per can, and covering a dark wall with white often requires an extra coat or a tinted primer.
Is a liter of paint enough for one wall?
A liter of standard paint covers about 92 sq ft. A typical 10 ft × 8 ft wall is 80 sq ft, so one liter is close to enough for a single coat.