Keep ceiling paint separate from wall paint
Assign a ceiling product on purpose, then read two can counts instead of one blended purchase.
A ceiling is not a taller wall. Ceiling area is length × width. Wall area is 2 × (length + width) × height, minus the openings you type. The paint calculator will add the ceiling into a product only when that product is selected in the ceiling field. Leaving the ceiling as “Not included” omits it. Giving the ceiling its own product entry rounds that entry on its own, even if both names say white.
What the two fields actually do
Each room has a wall product and a ceiling product. The wall product receives net wall area. The ceiling product receives length × width, and openings never reduce that figure. Choosing the same product entry for both surfaces adds the two areas before coats, allowance and whole-can rounding. Choosing a second product entry does not merge them because the names look alike. The grouping key is the product entry, not the words you typed.
Load “Try the test example” on the paint calculator. The synthetic bedroom is 4 m × 3 m × 2.5 m with one 2 m × 2 m opening. Gross walls are 35 m². Net walls are 31 m². The ceiling, if you assign it, is 12 m². The example leaves the ceiling unassigned. Confirm 31 m² on the room row before you change products. Measuring the shell is covered in measure a room for paint. Opening rows are covered in deduct doors and windows.
One product entry, both surfaces
Test product, not a recommendation: two coats, 10 m²/L, 10% allowance, 2.5 L cans, 0 L owned.
Walls only: 31 × 2 ÷ 10 = 6.2 L, then 6.82 L with allowance → 3 cans (7.5 L).
Walls and ceiling on that same entry: 31 + 12 = 43 m².
43 × 2 ÷ 10 = 8.6 L. With 10% allowance, 9.46 L → 4 cans (10 L).
The room row should show 43 m² once the ceiling uses the wall product. Four cans is the purchase for that single entry. It is not a promise that one sheen is right for both surfaces. If the label you will actually buy is the same specification on walls and ceiling, one entry is the matching model, because remainder litres share a can.
A second product changes the can count without changing the areas
Same coverage, coats, allowance and can size, but two product entries.
Walls stay at 6.82 L → 3 cans.
Ceiling alone: 12 × 2 ÷ 10 = 2.4 L, then 2.64 L → 2 cans.
Across both entries: 5 cans, against 4 cans when one entry receives both surfaces.
Create the second entry only when the specification differs: another sheen, another coverage, another can size, or a ceiling flat that is not the wall finish. Rename them so the assignment list is obvious, for example “Walls — eggshell” and “Ceiling — flat”. Two cards both called “White” stay separate.
A different ceiling specification does not inherit the wall coats. This synthetic ceiling flat is 12 m²/L, one coat, 10% allowance and 2.5 L cans, with nothing owned. The ceiling need is 12 × 1 ÷ 12 = 1.0 L, then 1.1 L, which is one can. The walls remain three cans. The shopping list is four cans across two lines. That is not the four cans of the shared two-coat entry above: the coats and the coverage are different, so the litres are different even though both totals say four.
Do not invent the ceiling by stretching the walls
Height is the wall height, not a place to hide the ceiling. Raising the test room from 2.5 m to 3.5 m adds 2 × (4 + 3) × 1 = 14 m² of gross wall, before openings. The ceiling is 12 m². Those are not the same number, and the extra wall area is still reduced by openings while a real ceiling is not. The tool will not warn you that the extra metre was standing in for a lid.
A second room is also the wrong substitute. There is no ceiling-shaped room type. If you need only the ceiling, clear the wall product, assign the ceiling product, and enter an explicit allowance. With the test room, allowance 0, two coats and 10 m²/L, the area is 12 m² and the need is 2.4 L, so one 2.5 L can. Wall height and openings are not required for that ceiling-only case. Assign the walls as well and height becomes required again.
Primer is another product, not an extra coat number
The coats field is the finish coats you intend to apply for that product entry. Primer is not inserted automatically. If the ceiling is primed and the walls are not, add a product entry that uses the primer’s own coverage, coats, allowance and can size, and assign it only where the primer goes. A shared colour name does not combine the primer with the finish. Read coverage and coats before you paste a two-coat label figure into the one-coat field, and how rooms combine before rounding before you split a finish only to keep the list tidy.
Check the lines, then the cans
- Confirm net walls and, separately, ceiling area. Openings touch walls only.
- Decide whether the ceiling specification is actually the same product entry.
- If it differs, create a second entry and select it in the ceiling field. Do not raise the wall height to “include” it.
- Read each product’s litres and cans. Add purchases only after each entry has rounded.
- Enter 0 for allowance and owned paint when you mean none. A blank is an error, not zero.
When those lines match the sketch, calculate the cans. The home page is where the other material calculators start. The count is the shortage you entered, packed into whole cans. It does not inspect texture, a colour change or whether stored paint is still usable.
Sources and scope
For definitions of wallpaper match, repeat and packaging, see Brewster's product properties and Graham & Brown's wallpaper FAQ. Product instructions take precedence over generic examples. Unit conversions use the exact international foot (0.3048 m) and US liquid gallon (3.785411784 L); see NIST conversion factors.