The job
Modelled, not recorded. Every figure here is the answer the Paint Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The room is 18 ft (5.49 m) by 14 ft (4.27 m), with walls 9 ft (2.74 m) high: the kind of living room with a hallway door, a door through to the dining room and a run of three windows along the front. The walls are being changed from one mid-tone to another, which is the most common reason anyone paints a room that is not new.
The coverage is entered from the label, 350 ft²/gal, the figure US interior wall paints commonly print for a smooth, sound wall. That number is the paint maker's, not the room's, and it is the input the variants below test hardest. The calculator applies it to every coat alike.
Doors and windows are taken off at 21 sq ft for each door and 15 sq ft for each window, the opening allowances professional estimators use and the calculator's first source names, entered in square feet so the table reads in the unit the room was measured in. Ceilings and trim are separate jobs with their own paint, and are left out here.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Surface to Paint | A room's four walls (length, width, height) | Left at the calculator's own default. |
| Room Length | 18 ft (5.49 m) | The long wall, measured along the baseboard from corner to corner. |
| Room Width | 14 ft (4.27 m) | The short wall; together with the length it sets the perimeter the calculator multiplies by the height. |
| Wall Height | 9 ft (2.74 m) | Floor to ceiling. A nine foot wall is a foot taller than the common eight-foot ceiling, and a foot of height on a room this long is a lot of wall. |
| Number of Doors | 2 | Two door openings in the painted walls: the hallway and the dining room. |
| Number of Windows | 3 | Three windows along the front wall. |
| Area of One Door | 21 sq ft (1.95 m²) | The estimator's standard allowance for one interior door with its casing, in square feet. |
| Area of One Window | 15 sq ft (1.39 m²) | The estimator's standard allowance for one window, in square feet. |
| Number of Coats | 2 | Left at the calculator's own default. |
| Paint Coverage | 350 ft²/gal (8.59 m²/L) | Read from the can: the label's spread rate for a smooth, previously painted wall. |
The working, step by step
1. Gross wall area
576 sq ft (53.51 m²)
The perimeter multiplied by the height gives 576 sq ft, the four walls as if they had no openings. On a room this size the height is doing a lot of the work: every extra inch of wall is an inch along the whole perimeter, which is why tall rooms surprise people at the store.
2. Doors + windows area (subtracted)
87 sq ft (8.08 m²)
Five openings at their standard allowances come to 87 sq ft. It is a real share of the wall here, about a sixth, and it is why a living room with a lot of glass takes less paint than its floor size suggests.
3. Paintable area
489 sq ft (45.43 m²)
What is left, 489 sq ft, is one coat's worth of wall. It is the number to keep for the next time: the same room repainted in five years has the same paintable area, whatever colour goes on it.
4. Total area to coat
978 sq ft (90.86 m²)
The paintable area times the number of coats gives 978 sq ft of rolling. The calculator then divides that by the coverage from the label, which is how it arrives at the gallons in the headline; there is no waste factor, because the spread rate is meant to be the paint's real yield.
Estimated paint needed
2.794 gal
10.58 L
The answer is 2.794 gal, which is 10.58 L for anyone reading a metric can. Paint is sold in fixed can sizes, so the order is the smallest combination of cans at or above that figure, and the leftover in the last can is the touch-up paint for the next scuffed wall. Buying all of it from one batch, or boxing the cans together before starting, keeps the colour even from wall to wall.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated paint needed | 2.794 gal (10.58 L) | Wall paint for both coats, in the can sizes your store sells; the ceiling and the trim are bought separately. |
The allowance, and what it does here
The paint calculator has no allowance box, on purpose: it divides by the spread rate, and the spread rate already assumes normal application. What actually eats paint on a repaint is the roller and the tray, a few cut-in passes around the five openings, and a wall that is rougher than the label assumed. The first two are small and are covered by rounding up to the next can. The third is not small at all, and the rough-wall case below shows how far it moves the answer.
The common mistake
Planning two coats of a light colour over a dark one
Two coats is the calculator's default and the right figure for a change between similar colours. Going from a deep red or a navy to a pale neutral is a different job: the old colour shows through the first two coats of most paints, and the usual result is a third coat bought in a hurry from a different batch. If the change is large, prime the wall first in a tinted primer or count a third coat from the start, and check the hiding with the calculator built for painting over dark colours before the order is placed.
Change the job, and see what moves
The same room, one coat over a close colour
Number of Coats: 1 → 1.397 gal (5.289 L)
If the new colour is close to the old one and the wall is in good order, a single coat can do, and the order halves to 1.397 gal. The calculator scales the coats linearly, so this is exactly half; whether one coat hides is a property of the paint and the two colours, so it is a decision to test on a patch, not to assume.
The same room, on a textured wall that drinks paint
Paint Coverage: 250 ft²/gal (6.14 m²/L) → 3.91 gal (14.8 L)
The same two coats on a knockdown or orange-peel texture, entered at a spread rate of 250 ft²/gal, take 3.91 gal. A textured wall has more surface than its measured area and holds more paint in its hollows, so a lower spread rate is the honest way to enter it: here the texture adds most of what a whole extra coat would.
What to run next
- Paint Coverage Over Dark Colours Calculator
When the colour change is large, this works out the coats or the primer needed to hide the old colour.
- Ceiling Paint Calculator
The ceiling of the same room has its own area and is painted in a flat ceiling white, counted separately.
- Door & Window Casing Calculator
The casings around the two doors and three windows are trim, measured by length and painted in their own finish.
Sources
The method and every constant behind these figures belong to the Paint Calculator, version 1.2.1, last verified 2026-09-17. It rests on:
- Standard interior door/window opening allowances used by professional estimators (~21 sq ft door, ~15 sq ft window)
- Paint coverage rate varies by product — always check your specific paint can's label for its stated coverage per liter/gallon
Frequently asked questions
- How much paint for an 18 by 14 ft room with 9 ft walls?
- With two doors and three windows taken off and 2 coats at 350 ft²/gal, the walls take 2.794 gal. That is 978 sq ft of coats over 489 sq ft of wall.
- Should I subtract the doors and windows when estimating paint?
- Yes, at a realistic size for each. On this room the five openings take off 87 sq ft, which is a large part of a gallon on a two-coat job.
- How much more paint does a textured wall need?
- Enter a lower spread rate and the calculator shows it: at 250 ft²/gal the same room takes 3.91 gal instead of 2.794 gal.