How this paint calculator works
Paint quantity is a volume: the area you are coating, times the number of coats, divided by how much area one gallon or litre covers at one coat. A loss allowance covers what stays in the roller and tray. The result is rounded up to whole containers of the size you choose. The calculator never mixes container sizes for you — pick the size you intend to buy and read the leftover.
The formula
net area = Σ(selected walls and ceiling) − Σ(openings)
volume = net area × coats ÷ coverage per unit volume × (1 + loss %)
containers = ⌈ volume ÷ container size ⌉
Worked example
- Three 12 × 9 ft walls = 324 sq ft; less a 3 × 7 door and a 4 × 4 window = 287 sq ft
- 2 coats at 350 sq ft/gal: 287 × 2 ÷ 350 = 1.64 gal; +10% loss = 1.804 gal
- 1-gallon cans: ⌈1.804⌉ = 2 cans; leftover about 0.2 gal
- Primer, 1 coat at 300 sq ft/gal, no loss: 287 ÷ 300 = 0.957 gal → 1 can
Coverage reference
| Product type | Typical coverage, one coat | Notes |
|---|---|---|
| Interior wall paint | 350–400 sq ft/gal · 10–12 m²/L | Smooth, primed surface |
| Ceiling paint (flat) | 350–400 sq ft/gal | Textured ceilings cover less |
| Drywall primer / sealer | 250–350 sq ft/gal · 8–10 m²/L | New drywall absorbs more |
These are planning presets. Always confirm the coverage printed on your can; the planner asks you to tick that you have done so before a list is marked ready.
Measuring tips
- Wall area is perimeter × height, not floor area. A 12 × 12 room with 9 ft walls has 144 sq ft of floor and 432 sq ft of wall — see floor area vs paintable area.
- Deduct the opening, not the trim. If the trim is painted the same colour, its area is small; if it is a different product, add it separately.
- For several rooms in the same colour, pool the purchase so rounding happens once — see buying paint for several rooms.
Scope and limits
Masking, repair compound, brushes and labour are outside the quantity and are added as manual items in the planner. Coverage on porous, textured or dark substrates can be much lower than the label figure.
How to calculate paint
- Measure room length, width and wall height, or enter a known net area.
- Tick the walls (and ceiling) you are painting and enter the number of doors and windows to deduct.
- Choose your paint or enter its coverage per coat and container size from the label.
- Set the number of coats and a loss allowance.
- Read the estimated volume and the number of cans to buy. Carry the room into the planner to add primer and a second colour.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
Three walls at 9 ft with a door and a window give 287 sq ft of net wall area. At two coats, 350 sq ft per gallon and 10% loss, that is 287 × 2 ÷ 350 × 1.10 = 1.80 gallons — two 1-gallon cans. Four walls with the same openings give 395 sq ft and 2.48 gallons — three cans, or one gallon plus a quart if your paint comes in quarts.
How many coats should I plan for?
Two coats is the usual planning figure for a colour change or new drywall; one coat may do for a same-colour refresh with a quality paint. Primer is a separate product with its own coverage and is calculated separately in the planner.
What coverage should I use?
Use the figure on the can — commonly 350–400 sq ft per gallon (around 10–12 m² per litre) for interior wall paint at one coat. Porous or textured substrates, dark-to-light changes and spraying all reduce effective coverage. The preset is a planning value; confirm it on the label.
Do I subtract doors and windows for paint?
Yes. Paint covers installed wall area, so the opening is excluded. If you are also painting the door or its trim, add that as its own surface rather than leaving the opening in the wall area.
Should paint use the drywall waste allowance?
No. Drywall waste comes from cutting sheets; paint covers the surface that is actually there. The paint loss allowance (roller and tray losses, touch-ups) is a separate, smaller figure, and it is separate from the number of coats.
Can I calculate ceiling paint too?
Tick "Ceiling" above to include it in the same product, or in the room planner add a second paint application (for example a flat ceiling white) with its own coverage — different products and colours never merge on the shopping list.