Paint Calculator
How much paint a room actually takes, once the doors and windows come off and the coats are counted — in whole gallons, in gallons plus quarts, and in litres. The door and window sizes are yours to change, because 21 and 15 square feet are typical rather than true.
The room you are painting
Use the perimeter for anything that is not a plain rectangle: an L-shaped room, a stairwell, a hallway with a return.
The long side of the floor.
The short side. Length and width give the perimeter, and the ceiling area if you want it.
Eight feet is the standard in most post-war housing. Measure it anyway — a nine or ten foot ceiling adds a quarter to the walls.
Two is the normal answer. Three if you are covering a dark colour with a light one, or painting a strong colour over white.
Count the openings, whether or not a door hangs in them.
Twenty-one is a 3 ft × 6 ft 8 in door with its casing — typical, not universal. A patio slider is nearer 40.
Leave out anything smaller than about a square foot; it is inside the rounding.
Fifteen is a 3 ft × 5 ft sash. A picture window is two or three times that, and worth measuring.
350 is the usual label figure, measured on a sealed, smooth wall. Bare drywall, fresh joint compound, and knockdown texture pull it toward 250.
The ceiling is length × width, so it needs both. Measure a perimeter instead and this question has no honest answer.
Trim is normally a different sheen in a different tin, so its area is kept on its own line below.
365 sq ft to cover at 2 coats needs 2.09 gallons — 7.90 litres — so buy 3 gallons, or 2 gallons + 1 quart if you would rather not carry the surplus.
- Wall area before deductions
- 416 sq ft
- Doors and windows
- −51 sq ft
- Net wall area
- 365 sq ft
- Total paintable area
- 365 sq ft
- In square metres
- 33.91 m²
- Paint per coat
- 1.04 gal
- Paint for 2 coats
- 2.09 gal
- In litres
- 7.90 L
| How you buy it | Tins | Paint in the tins |
|---|---|---|
| Whole gallons | 3 gallons | 3.00 gal |
| Gallons and quarts | 2 gallons + 1 quart | 2.25 gal |
| With a touch-up quart | 2 gallons + 2 quarts | 2.50 gal |
- A door is counted at 21 sq ft and a window at 15 sq ft. Both are typical, neither is universal — measure a slider or a picture window and change them.
- Every coat is charged in full. The second coat genuinely goes further than the first over a sealed surface, but not reliably enough to buy one tin fewer.
- Primer is a separate calculation. Bare drywall and fresh joint compound drink far more than 350 sq ft a gallon.
- Buy the touch-up quart with the rest of the order. A tin tinted later comes from a different batch and will not match on the wall, however carefully the formula is copied.
From a tape measure to a tin
The sum is short. Walk the room and get a perimeter, multiply by the ceiling height for gross wall area, take off the openings, multiply by the number of coats, and divide by the coverage the tin claims. Everything that makes paint calculators disagree is in the last two terms, not in the geometry.
P = 2 × (L + W) • A = P × H − (nd × ad + nw × aw) • G = A × c ÷ r- P, H
- Perimeter of the room in feet and the floor-to-ceiling height. Give a perimeter directly for anything that is not a rectangle
- nd, ad
- Number of doors and the square feet each one takes off, default 21
- nw, aw
- Number of windows and the square feet each one takes off, default 15
- c, r
- Coats, and the coverage in square feet a gallon printed on the tin
- × 3.785411784
- US gallons to litres, exact by definition: 231 cubic inches to the gallon, 25.4 mm to the inch
The ceiling is length × width and is therefore only offered when you give both. A perimeter is one number short of a footprint — 52 feet of wall is a 12 by 14 room with a 168 square foot ceiling, or a 25 by 1 corridor with a 25 square foot one — so in perimeter mode the question is not asked rather than answered badly.
Trim is estimated as a half-foot band of baseboard round the perimeter plus 4 square feet of casing per door and 3 per window, and is reported on its own line because it is normally a different sheen in a different tin.
A 12 by 14 bedroom, two coats
The perimeter is 2 × (12 + 14) = 52 feet. At an 8 foot ceiling that is 52 × 8 = 416 square feet of wall. One door at 21 and two windows at 15 take off 51, leaving 365 square feet to paint — 33.91 square metres.
At 350 square feet a gallon that is 1.04 gallons a coat and 2.09 gallons for two coats, or 7.90 litres. Paint is sold in whole tins, so the buy is three gallons, or two gallons and a quart if you would rather not have most of a gallon left over. The second option is 2.25 gallons of paint for a 2.09 gallon job.
Add the ceiling and the trim and the same room becomes 365 + 168 + 36 = 569 square feet, or 3.25 gallons over two coats: three gallons and two quarts. That is three separate products in practice — flat ceiling white, wall paint, and a semi-gloss enamel for the trim — which is why those areas stay on separate lines rather than collapsing into one number.
It really does go further. It does not go far enough to matter.
The first coat is being absorbed. Drywall paper, joint compound, bare wood and old chalky paint all pull binder and water out of the film, and some of what you roll on is not covering anything — it is being drunk. The second coat lands on a sealed surface and spreads further: fifteen to twenty-five percent is the usual range, wider on porous substrates and narrower over a sound existing paint film in a similar colour.
Run that through the bedroom above. The first coat needs 365 ÷ 350 = 1.04 gallons. If the second spreads twenty percent further — call it 420 square feet a gallon — it needs 0.87, so the honest total is about 1.91 gallons rather than 2.09. That is a saving of 0.18 gallons: less than three quarters of a quart, on a job where the smallest tin you can buy is a quart.
Which is precisely why this calculator does not model it. The saving would move the recommendation from two gallons and a quart to two gallons flat — and the quart it saves you is the quart you should be buying for touch-ups anyway. Calculators that taper the second coat are being technically accurate and practically useless, and they are wrong in the one direction that costs you a second trip to the store.
A different product and a different sum
Primer is not one of the coats counted here, and it should not be run through this calculator at the paint’s coverage figure. Its job is to seal a porous surface and give the topcoat something uniform to grip, which means it is being applied to exactly the conditions where 350 square feet a gallon is a fantasy. New drywall — paper facing and joint compound side by side, two different porosities on the same wall — routinely takes primer at 200 to 250 square feet a gallon, and the taped seams take more than the field.
Prime bare drywall, skimmed or patched areas, bare or knotty wood, anything glossy you have keyed with sandpaper, water stains, and any drastic colour change. Where the existing paint is sound and you are staying in the same family, a good self-priming topcoat over two coats is usually enough. When you do need primer, measure the same area and divide by the primer’s own stated coverage.
Buy the touch-up quart now, not later
Architectural paint is tinted to order: a base is dispensed and colorant is injected by a machine to a formula. Two tins of the same colour code, mixed weeks apart, will come from different lots of base and different lots of colorant, on a machine that has been recalibrated in between. In the tin they look identical. On a wall, in daylight, across a patched repair, they frequently do not — and the mismatch shows up as a faint rectangle that catches the eye from across the room.
The fix costs a few dollars and has to happen at the same time as the rest of the order: buy one more quart than the arithmetic says, from the same batch, and put it somewhere that will not freeze. That is what the third row of the buy table on this page is for. A well-sealed tin of latex is usable for years, and a repainted wall is not something you want to schedule because a radiator leaked.
If you are within a quart of a whole gallon, round up instead — the leftover is the same insurance and the price per unit is better. And write the colour code, the sheen, the base, and the room on the lid in marker before you put it away, because the label will be unreadable long before the paint is unusable.
Measuring the floor instead
This page measures walls, which is a perimeter and a height. If you need the floor — for flooring, tile, carpet or turf — that is a different measure-up and a different tool.
- Square footage calculator — adds the floor area of several rooms into one total, with a waste allowance for offcuts and a price per square foot.
- Concrete calculator — the same round-up logic applied to a material sold by the bag and by the cubic yard.
What this calculator assumes
- A door is 21 square feet and a window 15 by default. Those are typical figures for a 3 by 6 foot 8 inch door and a 3 by 5 sash with casing, not standards. Both are editable, and a patio slider or a picture window is worth measuring.
- Coverage defaults to 350 square feet a gallon, the usual label figure for a sealed, smooth surface at the stated film thickness. Bare drywall, fresh joint compound, and knockdown or sand texture all reduce it, sometimes below 250.
- Every coat is charged in full. The second coat genuinely spreads further over a sealed first coat, but on a normal room the saving is under a quart and it never changes the tin count in a direction you would want to act on.
- Primer is excluded entirely. It is a different product with a different, usually much lower, coverage figure and needs its own calculation.
- Trim is estimated as a half-foot baseboard band round the perimeter plus 4 square feet per door and 3 per window. Crown moulding, panelling, chair rail and window seats are not included.
- Gallons are US liquid gallons of 231 cubic inches, which is 3.785411784 litres exactly. An imperial gallon is 4.54609 litres and is not used anywhere on this page.
- Nothing here accounts for spillage, roller and tray loading, or the paint that stays in the brush. Those are small, and the tin you round up to covers them.
Paint calculator FAQ
How much paint do I need for a 12 by 14 foot room?
With an 8 foot ceiling, one door and two windows, about 2.1 gallons for two coats. The walls are 2 × (12 + 14) = 52 feet of perimeter, 52 × 8 = 416 square feet, less 21 square feet for the door and 15 for each window, which leaves 365 square feet of paintable wall. At the label figure of 350 square feet a gallon that is 1.04 gallons a coat, so 2.09 for two. You buy three whole gallons, or two gallons and a quart if you would rather not carry the surplus home.
Does the second coat really use less paint than the first?
Yes, and less than most guides imply. The first coat is going onto a surface that absorbs; the second is going onto paint, which does not, so the spread rate improves — commonly by fifteen to twenty-five percent, depending on the substrate and how big a colour change you are making. On the 12 by 14 room above that is the difference between 2.09 gallons and roughly 1.91. It is real, and it is worth less than one quart, and paint is not sold in fractions of a quart. The saving almost never changes what you put in the trolley, which is why this calculator charges every coat in full and tells you about the saving here instead.
Do I have to subtract the doors and windows?
On a normal room it is worth doing but it will not change the tin count much: one door and two windows is 51 square feet out of 416, about twelve percent, which on this job is a third of a gallon. On a room with a patio slider and a picture window it matters a great deal — those are nearer 40 and 40 square feet rather than 21 and 15, and leaving them in can buy you a whole extra gallon. Both figures are inputs here for exactly that reason. The 21 and 15 are typical, not universal: 21 is a 3 foot by 6 foot 8 inch door with its casing, 15 is a 3 by 5 sash.
How many square feet does a gallon of paint cover?
The label usually says 350 to 400 square feet, and that figure is measured on a smooth, sealed, primed surface at the manufacturer's stated film thickness. Real walls are rarely that. Bare drywall and fresh joint compound are porous and can pull the effective rate under 250; knockdown, orange-peel and sand textures increase the actual surface area well beyond the flat dimensions you measured; bare wood and masonry drink more still. A rough surface does not just take more paint, it takes more paint unevenly, which is the real reason it needs two coats.
Do I need primer, and does it count as a coat?
Primer is a separate product with a separate coverage figure and it should be calculated separately — running it through a paint calculator at 350 square feet a gallon will leave you short. Prime bare drywall, patched or skimmed areas, bare wood, anything glossy you have sanded, and any severe colour change, particularly over red or over a deep base. Where the wall is already painted, sound, and you are staying in the same colour family, two coats of a decent self-priming paint is usually enough. Primer is not one of the coats this calculator counts, and it should not be.
Should I buy an extra quart of paint?
Almost always, and buy it with the rest of the order rather than later. Architectural paint is tinted in batches, and two tins of the same colour code mixed weeks apart, on different machines, from different lots of base and colorant, will not match on a wall — close enough in the tin, visibly different in daylight across a patched repair. A quart is a few dollars now and a repainted wall later. If you are close to a whole gallon anyway, round up and keep the remainder sealed; a well-closed tin of latex stored somewhere that never freezes is usable for years.
How many litres is a gallon of paint?
3.785411784, exactly. The United States liquid gallon is defined as 231 cubic inches and the inch is defined as 25.4 millimetres, so the conversion is exact by definition rather than measured. Note that a US gallon is not an imperial gallon: the imperial gallon is 4.54609 litres, about twenty percent larger, so a tin sized in imperial gallons holds materially more paint. Every gallon on this page is the US one.
Can I use wall paint on the ceiling and the trim?
You can, but it is normally the wrong product. Ceilings are usually painted in a dead flat white designed not to show roller lap marks or the shadows that raking light throws across a large plane; trim is usually a harder semi-gloss or enamel that can be wiped and will survive a vacuum cleaner. That is why the ceiling and the trim are reported on their own lines here rather than folded into the wall total — if you are buying three products, you need three numbers, and the areas are the part a calculator can give you.
Sources and review notes
- NIST Special Publication 811, Guide for the Use of the International System of Units — the definitions behind every conversion on this page: 231 cubic inches to the US gallon, 25.4 mm to the inch, 0.3048 m to the foot
- Master Painters Institute — the body behind the Architectural Painting Specification Manual that North American painting specifications reference, and the reason professional specs state a required film thickness rather than a coverage figure
The geometry and the unit conversions here are exact. The figures that move are the ones this page makes editable: the coverage rate, which is a property of the coating and the surface rather than a constant, and the door and window deductions, which are typical sizes rather than standards. The trim estimate is a rule of thumb and nothing more — measure it if the trim is the expensive part of the job.