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How to use this calculator#
- Measure the perimeter, not the floorWall area is the distance right around the room multiplied by the ceiling height. A 12 × 14 ft room has a perimeter of 2 × (12 + 14) = 52 ft, so at 8 ft it has 416 sq ft of wall. The 168 sq ft floor figure is irrelevant to the walls.
- Subtract the openingsTake off 21 sq ft for a standard door and 15 sq ft for an average window. Skip this on a small room with one door and the answer barely moves; skip it on a room with patio doors and a bay and you buy a whole extra gallon.
- Set the coverage to the surface you are painting350–400 sq ft per gallon is for smooth, sealed, previously painted walls. Bare plaster, new drywall, render and masonry are porous and drop to 150–250 sq ft per gallon, and they need a primer or sealer coat first.
- Choose the number of coats honestlyTwo is the realistic default. One coat only works over a nearly identical colour in the same sheen. Deep reds, strong yellows and any light-over-dark change need three, or a tinted primer to cut one coat out.
- Round up to whole tins and buy them together2.09 gallons means three gallons, or two gallons plus a quart. Buy all your colour in one purchase so the tins share a batch, and box them — pour every tin into one bucket and stir — before starting a large room.
The formula#
Paint quantity from wall area
Paint = ((2 × (L + W) × H) − openings) × coats ÷ coverage
- L, W
- Room length and width, in feet or metres
- H
- Floor-to-ceiling height
- 2 × (L + W)
- The perimeter — the total distance around all four walls
- openings
- Total area of doors and windows to subtract (about 21 sq ft per door, 15 sq ft per window)
- coats
- Number of finish coats, normally 2
- coverage
- Spread rate per tin: about 350 sq ft per US gallon, or 10 m² per litre, on smooth sealed surfaces
The ceiling is a separate sum entirely: it is length × width, the floor area, not the perimeter. Adding the two together in one figure is fine for a total, but ceiling paint is usually a different product bought in its own tin. Coverage figures on a tin are best-case laboratory numbers on a smooth, sealed, non-absorbent surface — real walls almost always come in lower.
How much paint you actually need#
A US gallon covers roughly 350-400 sq ft on smooth, previously painted drywall; a litre covers about 10-13 square metres. Bare plaster, new drywall, textured surfaces and masonry drink far more — drop to 250 sq ft per gallon and prime first. Two coats is the realistic default: one coat works only over a very similar colour, while any real colour change needs two and deep reds often need three.
A worked example#
A 12 by 14 ft room with 8 ft ceilings has a 52 ft wall perimeter, so 52 times 8 gives 416 sq ft of wall. Subtract one door at about 21 sq ft and two windows at 15 sq ft each and you are left with 365 sq ft. Two coats means 730 sq ft of coverage; at 350 sq ft per gallon that is 2.09 gallons, so buy three gallons or two gallons plus a quart.
The mistake that leaves you short#
People round down, skip the allowance for the second coat, then run out mid-wall and buy a fresh tin from a different batch. The sheen and shade rarely match and the join shows in raking light. Round up to whole tins, buy all of your colour at once, and box it — pour every tin into one bucket and stir — on large rooms. Ceilings need a separate sum: length times width, not perimeter.
Worked examples#
A standard bedroom, two coats
A 12 × 14 ft room with 8 ft ceilings, one door and two windows, in a mid-tone emulsion.
- Perimeter = 2 × (12 + 14) = 52 ft
- Wall area = 52 × 8 = 416 sq ft
- Subtract one door (21) and two windows (2 × 15 = 30): 416 − 51 = 365 sq ft
- Two coats = 365 × 2 = 730 sq ft of coverage needed
- At 350 sq ft per gallon: 730 ÷ 350 = 2.09 gallons (7.90 litres)
2.09 gallons. Buy 3 gallons, or 2 gallons plus a quart, and keep the remainder for touch-ups.
The same room in metric, on bare plaster
3.66 m × 4.27 m with a 2.44 m ceiling, freshly plastered so far more absorbent.
- Perimeter = 2 × (3.66 + 4.27) = 15.86 m
- Wall area = 15.86 × 2.44 = 38.70 m²
- Subtract door 1.95 m² and two windows at 1.39 m²: 38.70 − 4.73 = 33.97 m²
- Bare plaster covers about 6.5 m² per litre, and needs a mist coat plus two finish coats — three coats in total
- 33.97 × 3 ÷ 6.5 = 15.68 litres
About 15.7 litres — nearly double the 7.9 litres the same room needs once it is sealed. Porous surfaces are the reason paint jobs run over budget.
Reference tables#
| Surface | Sq ft per US gallon | m² per litre | Coats normally needed |
|---|---|---|---|
| Smooth, previously painted drywall | 350–400 | 8.6–9.8 | 2 |
| New or primed drywall | 300–350 | 7.4–8.6 | 2 + primer |
| Bare plaster or fresh skim coat | 250–300 | 6.1–7.4 | 2 + mist coat |
| Lightly textured (orange peel) | 275–325 | 6.7–8.0 | 2 |
| Heavy texture or knockdown | 200–250 | 4.9–6.1 | 2 |
| Smooth wood trim and doors | 350–400 | 8.6–9.8 | 2 |
| Bare or rough-sawn wood | 200–250 | 4.9–6.1 | 2 + primer |
| Smooth concrete or render | 250–300 | 6.1–7.4 | 2 |
| Bare brick, block or rough masonry | 150–200 | 3.7–4.9 | 2 + sealer |
| Ceiling, previously painted flat | 350–400 | 8.6–9.8 | 1–2 |
Conversion: 350 sq ft per US gallon = 8.59 m² per litre. Multiply sq ft/gal by 0.02454 to get m²/L.
| Room | Ceiling | Wall area | After openings | 2 coats (gal) | 2 coats (L) | Buy |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 8 ft | 320 sq ft | 269 sq ft | 1.54 | 5.8 | 2 gallons |
| 12 × 12 ft | 8 ft | 384 sq ft | 333 sq ft | 1.90 | 7.2 | 2 gallons |
| 12 × 14 ft | 8 ft | 416 sq ft | 365 sq ft | 2.09 | 7.9 | 2 gal + 1 qt |
| 14 × 16 ft | 9 ft | 540 sq ft | 489 sq ft | 2.79 | 10.6 | 3 gallons |
| 15 × 20 ft | 9 ft | 630 sq ft | 579 sq ft | 3.31 | 12.5 | 4 gallons |
| 20 × 20 ft | 9 ft | 720 sq ft | 669 sq ft | 3.82 | 14.5 | 4 gallons |
Ceilings are extra: add length × width. A 12 × 14 ft ceiling is 168 sq ft, about a gallon for two coats.
| Opening | Typical size | Deduct (sq ft) | Deduct (m²) |
|---|---|---|---|
| Interior door | 2 ft 6 in × 6 ft 8 in | 16.7 | 1.55 |
| Entry door | 3 ft × 6 ft 8 in | 20.0 | 1.86 |
| Door (rule-of-thumb figure) | 3 ft × 7 ft | 21.0 | 1.95 |
| Small window | 2 ft × 3 ft | 6.0 | 0.56 |
| Standard window | 3 ft × 4 ft | 12.0 | 1.11 |
| Large window | 3 ft × 5 ft | 15.0 | 1.39 |
| Sliding patio door | 6 ft × 6 ft 8 in | 40.0 | 3.72 |
| Single garage door | 8 ft × 7 ft | 56.0 | 5.20 |
If doors and windows total less than about 10% of the wall area, many decorators skip the deduction and let it absorb the waste allowance instead.
Common mistakes#
- Buying for one coat and discovering you need twoOne coat covers only over a near-identical colour in the same sheen. Any real colour change needs two, and a light shade over a dark one often needs three. Budgeting one coat halves the paint you buy and guarantees a second shopping trip mid-job.
- Using floor area instead of wall areaA 12 × 14 ft room has 168 sq ft of floor and 416 sq ft of wall. Ordering against the floor figure buys 40% of the paint you need. Walls are perimeter × height; floors and ceilings are length × width.
- Ignoring how thirsty the surface isFresh plaster, new drywall, filler patches and bare masonry pull 150–250 sq ft from a gallon against 350–400 on a sealed wall. Skipping the primer or mist coat does not save money — the topcoat simply soaks in and you use more of the expensive tin.
- Topping up from a different batchTinted paint varies slightly between batches. Running out and buying a fresh tin later leaves a shade and sheen join that shows up in raking light across a large wall. Buy the whole job at once, and box multiple tins together before you start.
Frequently asked questions#
How much paint do I need for a 12x12 room?
With 8 ft ceilings that is about 384 sq ft of wall, or roughly 340 sq ft once a door and window are taken off. Two coats needs around 2 gallons, or about 7.5 litres.
Does this include the ceiling?
Only if you add it. Ceiling area is length times width — add that figure to your wall total before calculating tins, and remember ceiling paint is usually a different product.
How much does one litre of paint cover?
Around 10-13 square metres per coat on a smooth, sealed surface. Textured, porous or bare surfaces can pull coverage down to 6-8 square metres per litre.
Do I need primer as well?
Prime bare plaster, new drywall, filled patches and any dramatic colour change. Primer coverage is similar to topcoat, so budget for it as one extra coat.
Key terms#
- Spread rate (coverage)
- The area one unit of paint covers in a single coat, quoted as square feet per gallon or square metres per litre. Manufacturer figures are laboratory best cases on smooth sealed substrates.
- Mist coat
- A first coat of emulsion thinned with roughly 10–20% water, used to seal fresh plaster so the topcoat sits on the surface instead of soaking in. Skipping it causes patchy sheen and peeling.
- Primer
- A preparatory coat that seals porous surfaces, blocks stains and gives the topcoat something to grip. Tinted primer close to the finish colour can save a whole topcoat on a dramatic colour change.
- Boxing
- Pouring several tins of the same colour into one large container and stirring them together, so any batch-to-batch variation is spread evenly across the whole room rather than showing as a line.
- Sheen
- How much light the dried film reflects, from flat or matt through eggshell and satin to semi-gloss and gloss. Higher sheens cover slightly less area per litre and show substrate imperfections more clearly.
- Cutting in
- Brushing a band of paint along edges, corners and against trim where a roller cannot reach. It uses roughly 10% of the total paint on a typical room and is worth allowing for when you round up.
Sources#
- American Coatings Association — paint and coatings resources — American Coatings Association
- Lead Renovation, Repair and Painting Program — US Environmental Protection Agency
- Special Publication 811: Guide for the Use of the International System of Units (SI) — NIST
Figures last checked .
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