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Paint calculator

Measure the room, get the gallons, buy the right cans.

  • Last checked: October 10, 2026
  • By the Paint Gallons editorial team

Estimate your paint

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Measure by

Enter the room dimensions to calculate the combined wall area.

Units

Use feet, square feet and US gallons.

Room

Measured along the floor, wall to wall; assumes a rectangular room.

The other side of the room; assumes a rectangular room.

Measured from floor to ceiling; assumes all walls have the same height.

Ceiling area = length × width, with the same coats and coverage; ceiling paint is often sold separately.

Your result will appear hereEnter your room size to see how much paint you need.

Doors and windows

Each counted as 20 sq ft.

Each counted as 12 sq ft.

Paint

The number of coats you plan to apply; two for most repaints. See the coats guide.

Printed on the paint can, usually 350-400 sq ft per gallon.

Choose the wall finish; the calculator multiplies the can’s coverage by 1 for a painted wall, 0.8 for new plaster or drywall, or 0.75 for a textured or rough surface.

Allows for paint lost in the tray and roller, uneven walls and later touch-ups.

Enter your room’s size, the number of doors and windows and how many coats you plan. You get the gallons you need, the cans to buy and the full working.

  • Shows the formula and every assumption
  • Gallons and quarts, or litres
  • Accounts for new drywall and textured walls

How this is calculated

Wall area = 2 × (length + width) × height, minus the area of doors and windows. Gallons of paint = paintable wall area × coats ÷ coverage per gallon. Coverage is adjusted for the selected surface before dividing, and the optional 10% extra is added last.

Written and checked by the Paint Gallons editorial team, against the manufacturer coverage data below.

Door −20 sq ftWindow −12 sq ftLengthHeightWidth

gallons = paintable area × coats ÷ (coverage × surface factor)

Assumptions

  • Each door is counted as 20 sq ft and each window as 12 sq ft (1.6 m² and 1.2 m² in metric).
  • Default coverage is 400 sq ft per gallon per coat; use the figure printed on your paint can. New drywall or plaster uses a surface factor of 0.8 and a textured or rough wall 0.75.
  • The estimate is rounded to a tenth of a gallon before cans are planned: 5-gallon buckets first, then gallons, then one or two quarts for a remainder up to half a gallon.
  • By default the estimate covers walls only. Include the ceiling adds the floor area as ceiling area. Trim, doors and primer are not included.

Sources

How much paint for common room sizes

Rooms with 8 ft walls, one door and two windows, painted with two coats at 400 sq ft per gallon. Use the calculator above for your exact room.

Gallons of wall paint for common room sizes (two coats)
RoomPaintable wallPaint neededCans to buy
10 × 10 ft276 sq ft1.4 gal1 × 1 gal and 2 × 1 qt
12 × 10 ft308 sq ft1.5 gal1 × 1 gal and 2 × 1 qt
12 × 12 ft340 sq ft1.7 gal2 × 1 gal
14 × 12 ft372 sq ft1.9 gal2 × 1 gal
16 × 14 ft436 sq ft2.2 gal2 × 1 gal and 1 × 1 qt
20 × 16 ft532 sq ft2.7 gal3 × 1 gal

Frequently asked questions

How much wall does a gallon of paint cover?

Most interior wall paints list about 350 to 400 sq ft per gallon for one coat on a smooth, previously painted wall. Check the figure on your can and enter it in the calculator, which lowers it for new drywall and textured walls.

How many gallons of paint do I need for a 12 × 12 room?

A 12 × 12 ft room with 8 ft walls, one door and two windows has 340 sq ft of paintable wall. Two coats at 400 sq ft per gallon come to 1.7 gallons, so buy two 1-gallon cans. A dark-to-light color change or new drywall can need more.

Should I take doors and windows out of the wall area?

Yes, for a closer estimate. Enter their count and the calculator subtracts 20 sq ft per door and 12 sq ft per window. Leave a count at zero if you would rather keep that paint as a margin.

Do most walls need two coats?

Two coats are the usual plan for a repaint, and especially for a color change. One coat can be enough when refreshing the same color on a sound wall; three may be needed going from dark to light.

Should I buy extra paint?

Tick Add 10% extra to allow for paint left in the tray and roller, uneven walls and touch-ups later. Keep the leftover sealed and labelled with the color name so a later repair matches.

How accurate is this? Real use varies with the roller, wall porosity and color, so the amount you use can differ from the estimate. The can plan already rounds the estimate up; add 10% extra if you want a margin.

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