Paint Calculator
Work out how many gallons of paint you need for a room: walls minus doors and windows, number of coats, ceiling and primer, with gallons and quarts to buy.
Coverage, opening sizes and price
Check the can or the manufacturer’s data sheet for coverage — many interior paints list roughly 250–400 sq ft per gallon, and rough or porous surfaces need more paint.
| Wall area: 2 × (14 + 12) × 8 | 416 sq ft |
|---|---|
| Minus doors and windows | −50 sq ft |
| Paintable wall area | 366 sq ft |
| × 2 coats ÷ 350 sq ft per gallon | 2.09 gallons |
Buying a little extra from the same batch makes touch-ups match. Trim, doors and cabinets usually take a separate paint and sheen.
What Paint Calculator Does
This paint calculator tells you how much paint a room needs and what to buy. Enter the room’s length, width and wall height, the number of doors and windows, and how many coats you plan; it works out the paintable wall area, adds the ceiling and a primer coat if you want them, and converts the total into gallons, then into whole gallons plus quarts, the way paint is sold.
Coverage per gallon varies by product and surface, so it is an editable input rather than a fixed assumption; the default is 350 square feet per gallon. Door and window sizes are editable too, for rooms with patio doors or large picture windows.
How to Use Paint Calculator
- Enter the room length, width and wall height
- Enter the number of doors, windows and coats
- Tick ceiling or primer if needed
- Adjust coverage and opening sizes to match your paint and room
- Read gallons needed and the gallons and quarts to buy
Formula Used by Paint Calculator
Gallons of paint
Gallons = (2 × (length + width) × height − doors − windows) × coats ÷ coverage
Worked example
A 14 × 12 ft room, 8 ft walls, 1 door, 2 windows, 2 coats, 350 sq ft per gallon.
- 2 × (14 + 12) × 8 = 416 sq ft
- 416 − 20 − 2 × 15 = 366 sq ft
- 366 × 2 ÷ 350 = 2.09 gallons
Result: Buy 2 gallons and 1 quart.
Typical Paint Needed for Two Coats
Walls only, 8 ft ceilings, 1 door and 2 windows, 350 sq ft per gallon.
| Room size | Paintable wall area | Gallons for 2 coats |
|---|---|---|
| 10 × 10 ft | 270 sq ft | 1.5 |
| 12 × 12 ft | 334 sq ft | 1.9 |
| 14 × 12 ft | 366 sq ft | 2.1 |
| 16 × 14 ft | 430 sq ft | 2.5 |
| 20 × 16 ft | 526 sq ft | 3.0 |
How to Read Your Result
Surfaces that drink paint
New drywall, bare plaster, unpainted wood and heavily textured walls absorb more paint than smooth, previously painted walls. Prime porous surfaces first, and budget extra for textures such as knockdown or popcorn ceilings.
Buy from one batch
Tints can vary slightly between batches, so buy all the paint for a room at once, and keep a little left over for touch-ups. Mixing all the cans together in a bucket (boxing) before you start evens out any small differences.
Limitations & Accuracy Notes
- Trim, doors and cabinets are not included; they usually take a different sheen and much less paint.
- Rooms with vaulted ceilings or unusual shapes need their wall areas measured separately.
- Coverage on the can assumes a smooth, sealed surface and the recommended application method.
Frequently Asked Questions
How much paint do I need for a room?
Measure the wall area (the room’s perimeter times the wall height), subtract doors and windows, multiply by the number of coats and divide by the coverage on the can. A 14 × 12 ft room with 8 ft walls, one door and two windows needs about 2.1 gallons for two coats at 350 sq ft per gallon.
How much does a gallon of paint cover?
The can or product data sheet tells you; many interior paints list roughly 250–400 sq ft per gallon for one coat on a smooth, primed surface. Textured, porous or new drywall surfaces use more.
How many coats of paint do I need?
Two coats is the usual recommendation for even color and durability. One coat can work when repainting the same color; dark-to-light changes and new drywall may need a primer coat first.
How much do doors and windows take off?
The calculator subtracts 20 sq ft per door and 15 sq ft per window by default, which fits typical interior doors and mid-size windows. Change them under Coverage, opening sizes and price for large patio doors or picture windows.
Should I buy gallons or quarts?
Gallons are cheaper per square foot. The calculator suggests whole gallons plus quarts for a small remainder, and rounds up to another gallon when three or more quarts would be needed.
Do I need primer?
Use primer on new drywall, bare wood, patched areas, stains, and big color changes. Many paint-and-primer products reduce the need, but a separate primer is still recommended for bare or stained surfaces.