Paint Calculator
How much paint do I need?
A paint calculator converts your room dimensions into the gallons to order. Paint is sold by the gallon, so the result is given in gallons first, with the exact figure alongside it. Start from the wall area, which is the perimeter of the room times the wall height, then subtract the doors and windows, then divide by the coverage per gallon and multiply by the number of coats. Two coats is the standard. Add a waste allowance before you order: running short halfway through a wall is hard to fix, and a second trip for one more gallon costs more than the paint. If you know your price per gallon, enter it and the calculator gives a material cost. Leave it blank and no cost is shown.
Calculate paint
Enter a length greater than 0.
Enter a width greater than 0.
Enter a height greater than 0.
Enter a number of doors from 0 to 20, as a whole number.
Enter a door area from 1 to 100 sq ft.
Enter a number of windows from 0 to 20, as a whole number.
Enter a window area from 1 to 100 sq ft.
Enter coats from 1 to 3, as a whole number.
Enter a coverage from 250 to 400 sq ft per gallon.
Enter a waste allowance from 0 to 20 percent.
Enter a price of 0 or more, or leave it blank.
Enter your dimensions and click Calculate.
No result yet. Enter your dimensions and click Calculate.
Prices vary by supplier and location. Confirm with your local supplier before ordering.
The calculator takes the room length, width and wall height, each with its own unit selector (ft, in, m). Add the ceiling with the toggle if you are painting it. Enter the number of doors and windows and the area of each: a standard door is 21 sq ft and a standard window is 15 sq ft, and you can change both figures. Coats defaults to 2, and you can change it from 1 to 3. Coverage per gallon is an editable input, set to 350 sq ft per gallon by default, and you can change it from 250 to 400 sq ft per gallon. The waste allowance is a visible input, set to 10 percent by default as a starting point you can change, and you can change it from 0 to 20 percent. There is no single standard allowance. Rough or porous surfaces can need up to 20 percent more paint, and painting a surface for the first time can need up to 30 percent more. The primer toggle adds a primer line at its own coverage, set to 250 sq ft per gallon by default, and you can change it from 200 to 300 sq ft per gallon. Price per gallon is optional: leave it blank and no cost is shown. The formula used is shown directly under the result, so you can check the number.
What is a paint calculator?
A paint calculator works out how much paint a room needs from the room dimensions, the number of coats and the coverage per gallon. It gives the paintable wall area, the exact gallons and the gallons to buy, so you can order before you start and not run short halfway through a wall.
The method is perimeter times height, never floor area. Wall area is 2 x (length + width) x height. Paintable area is the wall area minus the doors and windows. Gallons are paintable area x coats / coverage per gallon. The ceiling, if you paint it, is length x width and is added without subtracting openings. Primer is a separate line at its own, lower coverage.
Coverage is a range, not a constant, so check the label on the can. The table below gives the coverage for each common surface. Interior wall paint on smooth, previously painted drywall covers 350 to 400 sq ft per gallon for one coat, and 350 is the usual planning figure. Primer covers 200 to 300 sq ft per gallon. Exterior paint covers 250 to 300 sq ft per gallon, less than interior paint, because the surface is rougher and the film is thicker. The 350 to 400 and 250 to 300 figures match the figures on the technical data sheets for common paints, including the Sherwin-Williams Emerald, SuperPaint and Duration lines.
| Surface | Coverage (sq ft per gallon, one coat) |
|---|---|
| Smooth drywall or plaster | 350 to 400 |
| Lightly textured drywall | 300 to 350 |
| Heavily textured or stucco | 200 to 250 |
| New bare drywall, unprimed | 200 to 300 |
| Primed wood or trim | 350 to 400 |
| Bare wood siding | 250 to 300 |
| Brick or block, unpainted | 100 to 200 |
| Metal, primed | 350 to 400 |
One common mistake is to start from the floor area. Paint covers the walls, so the estimate always starts from perimeter times height. The Sherwin-Williams calculator page itself shows a floor-area shortcut, a 10 ft by 10 ft room is 100 sq ft, which is the floor, not the wall. A 10 ft by 10 ft room with 8 ft walls has 320 sq ft of wall, more than three times the floor area.
One quart covers about 100 sq ft, a rounded trade figure, not the arithmetic quarter of a gallon, since 350 / 4 = 87.5 sq ft. Do not treat 100 sq ft per quart as exact.
No competitor page names an author or reviewer, no page works an example all the way through to an order with waste and primer, and no page cites a product technical data sheet by name for its coverage figures. This page shows every step of the arithmetic, so you can check the number.
How to calculate paint
- Measure the room. Measure the length, width and wall height in feet. Measure to the finished floor and the finished ceiling.
- Work out the wall area. Add the length and width, double it, and multiply by the height. A 12 x 10 ft room with 8 ft walls has 2 x (12 + 10) x 8 = 352 sq ft of wall.
- Subtract the doors and windows. Subtract 21 sq ft for each standard door (3 x 7 ft) and 15 sq ft for each standard window (3 x 5 ft). A room with 1 door and 1 window loses 36 sq ft. Some tools use 20 sq ft for a door.
- Add the ceiling if you paint it. The ceiling is length x width. A 12 x 12 ft ceiling adds 144 sq ft. Do not subtract openings from the ceiling.
- Divide by the coverage and multiply by the coats. Divide the paintable area by the coverage per gallon, then multiply by the number of coats. Two coats is the standard. One coat is enough only for a repaint in the same colour over a sound surface. Three coats are needed for a dark to light colour change, for the reverse, or over bare drywall.
- Add waste and round up. Multiply by 1.10 for 10 percent waste, then round up to whole gallons. Under a gallon, round up to the nearest quart. A gallon is 4 quarts.
Coverage: gallons per area at 350 sq ft per gallon, one coat and two coats.
| Paintable area | 1 coat | 2 coats |
|---|---|---|
| 100 sq ft | 0.29 gal | 0.57 gal |
| 200 sq ft | 0.57 gal | 1.14 gal |
| 300 sq ft | 0.86 gal | 1.71 gal |
| 316 sq ft (the worked example) | 0.90 gal | 1.81 gal |
| 400 sq ft | 1.14 gal | 2.29 gal |
| 500 sq ft | 1.43 gal | 2.86 gal |
These are typical values, not a specification. The paint your job needs depends on the surface, the colour change and the label on the can, so confirm it before you order.
Paint formula and worked example
Wall area (sq ft): Wall area = 2 x (Length + Width) x Height.
Paintable area (sq ft): Paintable area = Wall area – (Doors x Door area) – (Windows x Window area).
Ceiling area (sq ft): Ceiling area = Length x Width, added only if the ceiling toggle is on.
Exact gallons: Exact gallons = Paintable area x Coats / Coverage per gallon.
Order gallons: Order gallons = Exact gallons x (1 + waste percent / 100).
Gallons to buy: Gallons to buy = Order gallons rounded up to whole gallons, or to the nearest quart under a gallon.
Primer gallons: Primer gallons = Paintable area / Primer coverage, one coat, no waste.
Cost: Cost = Gallons to buy x Price per gallon (only if a price was entered).
Worked Example
Worked example, 12 x 10 ft room, 8 ft high, 1 door, 1 window, 2 coats, coverage 350 sq ft per gallon, waste 10 percent:
Gross wall area 2 x (12 + 10) x 8 = 352 sq ft
Openings 21 + 15 = 36 sq ft
Paintable area 352 – 36 = 316 sq ft
Paint per coat 316 / 350 = 0.90 gallons
Two coats 316 x 2 / 350 = 1.81 gallons
Order with 10 percent waste 1.81 x 1.10 = 1.99 gallons
Buy 2 gallons.
Check by the coverage table: 316 sq ft needs 0.90 gallons for one coat and 1.81 gallons for two, the same answer.
Primer for this room: at 250 sq ft per gallon, 316 / 250 = 1.26 gallons, buy 2 gallons. At 200 to 300 sq ft per gallon the range is 1.05 to 1.58 gallons, so buy 2 gallons of primer if priming.
Cross-check, 12 x 12 ft room, 8 ft walls, 1 door, 2 windows: wall area 2 x (12 + 12) x 8 = 384 sq ft, openings 21 + 30 = 51 sq ft, paintable area 333 sq ft. One coat 333 / 350 = 0.95 gallons, buy 1 gallon. Two coats 333 x 2 / 350 = 1.90 gallons, buy 2 gallons. Two coats plus the 144 sq ft ceiling: paintable area 477 sq ft, 477 x 2 / 350 = 2.73 gallons, buy 3 gallons.
Paint calculators
Drywall Calculator
Drywall sheets and screws for walls and ceilings.
Tile Calculator
Tiles, adhesive and grout for floors and walls.
Brick Calculator
Bricks and mortar for a wall.
Flooring Calculator
Flooring planks or tiles for a room.
Siding Calculator
Siding panels for exterior walls.
Linear Feet to Square Feet Calculator
Convert linear feet to square feet.
Wallpaper Calculator
Rolls of wallpaper for a room.
Insulation Calculator
Insulation batts or rolls for walls and ceilings.
Frequently asked questions
How much paint do I need?
Work out the wall area first: multiply the perimeter of the room by the wall height. A 12 x 10 ft room with 8 ft walls has 2 x (12 + 10) x 8 = 352 sq ft of wall. Subtract the openings: 1 door is 21 sq ft and 1 window is 15 sq ft, so the paintable area is 316 sq ft. Divide by the coverage and multiply by the coats: 316 x 2 / 350 = 1.81 gallons for two coats at 350 sq ft per gallon. Add waste before you order: 10 percent waste gives 1.99 gallons, so buy 2 gallons. The calculator on this page does the same sum.
How much does a gallon of paint cover?
One gallon of interior wall paint covers about 350 to 400 sq ft for one coat on smooth, previously painted drywall. Use 350 sq ft per gallon as the planning figure, and check the label on the can, because coverage is a range, not a constant. Primer covers less, about 200 to 300 sq ft per gallon. Exterior paint covers about 250 to 300 sq ft per gallon, because the surface is rougher and the film is thicker. Textured drywall, bare drywall and bare wood cover about 250 to 300 sq ft per gallon, heavily textured surfaces and stucco about 200 to 250, and unpainted brick or block about 100 to 200.
How many coats of paint do I need?
Two coats is the standard for most rooms. One coat is enough only for a repaint in the same colour over a sound surface. Three coats are needed for a dark to light colour change, for the reverse, or over bare drywall. New drywall drinks the first coat, so prime it first: a cheaper coat of primer usually avoids an extra finish coat. The calculator on this page defaults to 2 coats, and you can change it from 1 to 3.
Do I subtract doors and windows?
Yes. Paint covers the walls, so every opening is area you do not paint. Subtract 21 sq ft for a standard door (3 x 7 ft) and 15 sq ft for a standard window (3 x 5 ft). Some tools use 20 sq ft for a door. The calculator on this page uses 21 sq ft for a door and 15 sq ft for a window by default, and you can change both figures if your doors or windows are a different size.
How much paint for a 12 x 12 room?
A 12 x 12 ft room with 8 ft walls has 2 x (12 + 12) x 8 = 384 sq ft of wall. Minus 1 door (21 sq ft) and 2 windows (30 sq ft), the paintable area is 333 sq ft. Two coats at 350 sq ft per gallon need 333 x 2 / 350 = 1.90 gallons, so buy 2 gallons. With a 10 percent waste allowance the exact figure is 2.09 gallons, so buy 3 gallons. Painting the 144 sq ft ceiling as well takes the paintable area to 477 sq ft and the exact figure to 2.73 gallons, so buy 3 gallons.
How much paint for the ceiling?
A 12 x 12 ft ceiling is 144 sq ft. Two coats at 350 sq ft per gallon need 144 x 2 / 350 = 0.82 gallons, so buy 1 gallon. The ceiling is length x width, added to the paintable area without subtracting openings. Some calculators leave the ceiling out entirely, so check whether yours includes it. The calculator on this page has a ceiling toggle, off by default.