The ceiling uses the floor area
A flat ceiling covers the same area as the floor beneath it, so the area to paint is the floor area, whatever the height of the walls. Nothing is taken off for doors and windows, which are in the walls. This is also why the walls and the ceiling of one room are worked out from different numbers: the walls from the perimeter and the height, the ceiling from the length and the width.
For a room that is not a plain rectangle, the floor shape in step 1 of the planner gives the ceiling too:
- Rectangle: length × width.
- L-shape: the Overall length (A) × Overall width (B), minus the corner that is cut out, Cut-out length (a) × Cut-out width (b).
- Several rectangles: each rectangle on its own, added up. The rectangles must not overlap.
Step by step
- Measure the floor of each room: its length and width, or the overall size and the cut-out of an L-shape, or the rectangles of an open-plan room. How to measure a room shows the order.
- Work out the ceiling area as above. It needs no wall height.
- Choose the coats for the ceiling and multiply: area × coats.
- Divide by the coverage per coat from the label to get the paint before any allowance. Why the label figure is per coat, and why it is a best case, is explained in paint coverage and coats.
- Add every ceiling that gets the same paint, then the extra for touch-ups, then round up to whole cans.
Worked example in feet: an L-shaped living room
An invented living room, 20 × 16 ft overall, with an 8 × 6 ft corner cut out. Two coats of a paint with an example coverage of 350 sq ft per gallon per coat, 10% extra and one-gallon cans:
- Ceiling area: 20 × 16 − 8 × 6 = 320 − 48 = 272 sq ft
- Paint for two coats: 272 × 2 ÷ 350 = 1.554 gal
- With 10% extra: 1.71 gal
- Cans: 2 one-gallon cans, with about 0.29 gal left over
Worked out as the full 20 × 16 ft rectangle, the same ceiling would be 320 sq ft: 1.829 gal, or 2.01 gal with the extra, which is 3 cans. Leaving out the cut-out costs a whole can here.
Worked example in metric: two rectangles
An invented open-plan kitchen-diner, measured as two rectangles of 4.0 × 3.2 m and 2.5 × 1.8 m. Two coats, an example coverage of 10 m² per liter per coat, 10% extra and 2.5 L cans:
- Ceiling area: 12.8 + 4.5 = 17.3 m²
- Paint for two coats: 17.3 × 2 ÷ 10 = 3.46 L
- With 10% extra: 3.806 L
- Cans: 3.806 ÷ 2.5 = 1.52, so 2 cans (5 L), with about 1.19 L left over
No measured perimeter is needed for this. The walls of the same room would need the perimeter all around it and the wall height.
One paint entry for every ceiling of the same color
Every room and surface you assign to the same paint entry is added together before rounding. Painting several ceilings in one white, enter the white once and choose it for the ceiling of every room.
An invented flat with three ceilings, all with two coats, 10 m² per liter, 10% extra and 2.5 L cans: a bedroom of 3.6 × 3.0 m (10.8 m²), the kitchen-diner above (17.3 m²) and a hallway of 4.0 × 1.1 m (4.4 m²).
- One white entry: 32.5 m², 6.5 L, 7.15 L with the extra: 3 cans, about 0.35 L left over.
- The same white entered once per room: 2.376 L, 3.806 L and 0.968 L, each rounded on its own: 1 + 2 + 1 = 4 cans. The planner flags entries that look identical, so you can merge them.
The order of the steps, and why a leftover from one entry is never counted for another, is in how pack rounding works.
A ceiling in its own color
A ceiling in a different color from the walls, or a different finish, is a separate paint entry, worked out and rounded on its own: the leftover of the wall color cannot finish a white ceiling. In the flat above, painting the kitchen-diner ceiling in a cream and the other two in white gives 2 cans of cream (3.806 L) and 2 cans of white for 15.2 m² (3.344 L): 4 cans in all, one more than with a single white for all three ceilings.
The same applies to a primer: a primer on the ceiling is its own material, with its own coverage and cans, on its own line, Primer on ceiling.
If the walls and the ceiling are the same paint, choose the same entry for both: the two areas are added, each with its own coats, before rounding.
Common mistakes
- Using the wall area for the ceiling. Perimeter × height is the walls. The ceiling is length × width, and it stays the same if the walls are taller.
- Treating an L-shaped room as its overall rectangle. In the feet example that is 48 sq ft too much and a third can.
- One entry per room for the same white. Each entry is rounded on its own, so three ceilings can need a can more than one entry would.
- Two colors on one entry. If the ceiling and the walls are different colors, they need two entries, or the shopping list adds up paint that cannot be mixed.
- Counting a sloped ceiling as flat. A slope is longer than the floor under it. Choose None for the ceiling and measure the slopes, and any flat part, as painted areas instead.
Where to plan it
In step 2 of the paint calculator or the room refresh planner, use Add paint color for each color, with its Coverage per coat, Can size and Extra for touch-ups. Then choose a paint for the Ceiling of each room, or None, and set the Coats for that line. To compare can sizes, open Compare other can sizes of this paint on the paint card.