Paint coverage calculator for walls, ceilings and can counts

Use this paint coverage calculator for walls, ceilings and can counts when you want a clear shopping list before you head to the store. It turns a room’s length, width and wall height into paintable area, subtracts doors and windows, multiplies by coats and adds a small waste allowance so you don’t run short. The output shows total area and paint volume in the unit you picked at the top, plus a sensible mix of can sizes so purchase is simple.

Coverage on the tin is your best input. If you don’t have it, start with 10 m² per liter or 350 ft² per gallon. Real coverage varies: fresh drywall, textured plaster, stucco, very deep colors, or big color changes often need more paint. Smooth, primed surfaces in a similar shade usually hit the default assumptions. If your first room feels tight or generous, nudge the coverage or waste and recalculate.

Measure quickly and consistently. Run a tape along the floor for length and width, and measure wall height from finished floor to ceiling. Round to two decimals; perfection isn’t required for a reliable estimate. If large built-ins permanently cover walls, subtract their footprint from length × width when you include the ceiling, or leave the ceiling unchecked to estimate walls only.

Primer and sheen matter for planning. New drywall, patched surfaces and unsealed wood almost always benefit from primer, which improves adhesion and reduces the number of finish coats. Flat and matte finishes hide minor flaws but scuff more easily; satin and eggshell offer a good balance for living spaces; semi-gloss is common on trim and doors. If you switch sheen or jump from dark to light, plan for an extra coat and set waste a touch higher.

Technique affects coverage. Load the roller evenly, maintain a wet edge, and finish with light passes in one direction to level the texture. Cut in two or three walls at a time so the roller blends with the brush work. When using multiple cans, box them by pouring into a larger bucket and mixing. The calculator’s can list helps you decide how many containers to buy so boxing is easy from the start.

Finally, remember that paint is cheaper than a second trip. Buying one small extra can in the same batch reduces inconsistency between coats and leaves a little for touch-ups later. If you end up with unopened cans, most stores accept returns; if not, seal tightly and store in a temperate place for future maintenance.

Wall area = 2 × (length + width) × height. From this we subtract door count × door width × door height and window count × window width × window height. If you tick the ceiling, we add length × width. The result becomes paintable area, which we multiply by coats and increase by the waste percentage to allow for tray and roller loss and touch-ups. Dividing by your coverage gives liters or gallons. Finally, a greedy packer proposes a near-minimal set of can sizes to cover the volume with a small buffer.

Here is a quick example in metric. A 4.5 m by 3.8 m room with 2.4 m walls yields 2 × (4.5 + 3.8) × 2.4 = 39.84 m² of wall area. Subtract one 0.9 × 2.0 m door and two 1.2 × 1.0 m windows to get roughly 36.84 m². With the ceiling included, add 17.10 m² for a subtotal of 53.94 m². Two coats at 10 percent waste produces 118.67 percent of that area, or about 64.06 m². At 10 m² per liter, you need about 6.41 L of paint. The can packer would likely suggest one 5 L can plus one 2.5 L can so you have enough with a cushion.

The same calculation in imperial uses feet, feet squared and gallons. If you switch the unit at the top, all labels adjust and the output shows only the chosen system so you can shop confidently without converting in your head.