Paint Calculator

How to use the paint calculator

For a whole room, choose Room (four walls) and enter its length, width and wall height, plus how many doors and windows it has. For a feature wall, a stairwell or anything that is not four full walls, choose Single wall and enter its width and height. Add as many rooms and walls as you like. Then set the number of coats and the coverage from your paint can, and tick Include ceilings or Add one coat of primer if you need them.

The result shows the gallons (and liters) of paint needed and what to buy — whole gallons, or gallons plus quarts. Add prices to estimate the cost. The working shows every step with your numbers, and the page address keeps your rooms and settings so you can share them or check them in the store. Nothing is sent anywhere.

The formula

wall area (room) = 2 × (length + width) × wall height wall area (single wall) = width × height paint area = wall area − doors − windows (+ ceiling = length × width) gallons = paint area × coats ÷ coverage per gallon quarts = gallons × 4 (round up)

Door and window areas are width × height, converted from square inches by dividing by 144: a 36 × 80 in door is 2,880 ÷ 144 = 20 sq ft. Coverage is the area one gallon covers in one coat, so two coats need twice the paint.

Worked example

The calculator opens with a bedroom 14 ft × 12 ft with 8 ft walls, one door and two windows, painted with two coats:

• Walls: 2 × (14 + 12) × 8 = 416 sq ft.
• Openings: one 36 × 80 in door (20 sq ft) and two 36 × 48 in windows (12 sq ft each): 416 − 20 − 24 = 372 sq ft.
• Paint: 372 × 2 coats ÷ 350 sq ft per gallon = 2.13 gallons (8.05 liters).
• What to buy: 2.13 × 4 = 8.5 quarts, rounded up to 9: 2 gallons + 1 quart, or 3 gallons.
• With the ceiling: adding 14 × 12 = 168 sq ft gives 540 sq ft, and 540 × 2 ÷ 350 = 3.09 gallons — 3 gallons + 1 quart, or 4 gallons.

If the paint costs $40 a gallon (an illustration, not a typical price), 3 gallons for the walls cost 3 × $40 = $120.00. The calculator prices whole gallons; if you plan to buy quarts, compare the shelf prices before deciding.

Coverage, coats and primer

Textured walls, bare plaster, new drywall and raw wood absorb more paint than smooth, previously painted walls, so the paint may not go as far as the label says, and a big color change can need an extra coat. When you are unsure, use a slightly lower coverage figure rather than a higher one.

Primer is usually applied as one coat, which is how the calculator counts it. It is often used on new drywall, bare wood, patched areas, stains, or before a strong color change; the paint label will say whether the product needs a primer. Enter the primer’s own coverage, as it can differ from the paint’s.

Tips for buying paint

• Measure the height too. Wall area grows with height: the same 14 × 12 ft room with 9 ft walls has 468 sq ft of wall, not 416.
• Use your own door and window sizes. The defaults are only a starting point; a patio door or a bay window is much larger.
• Buy tinted paint in one go. Colors can vary slightly between batches, so buying enough at once helps the walls match.
• Keep a little for touch-ups. Rounding up to the next gallon leaves some for scuffs later. Label the can with the room and color.
• Trim and doors are separate. This calculator covers walls and ceilings; trim, doors and cabinets are often painted in a different finish.

To measure an irregular floor or ceiling first, use the square footage calculator. For conversions, see gallons to liters, quarts to liters and square meters to square feet. Planning a bigger project? The concrete calculator covers slabs, footings and steps.