What actually determines how much paint you need
Three numbers decide a paint order: the area, the number of coats, and the spread rate. Area and coats are easy — you measure one and choose the other. The spread rate is where estimates go wrong, because the figure on the can describes an ideal surface and yours almost never is.
A gallon of interior latex is typically labelled at 350 to 400 square feet. That number assumes a smooth, sealed, previously painted wall, rolled at a normal film build. Put the same gallon on unprimed drywall and the paper drinks it; put it on a knockdown texture and the added surface area alone eats a quarter of the coverage before absorption is considered. Spray it and you lose material to overspray and to the higher film thickness a sprayer lays down.
This calculator therefore treats the spread rate as an input you are meant to think about, and shows a sensitivity table so you can see what your uncertainty is worth in cans. It also separates the four products a room really needs: wall paint, ceiling paint (flat, usually a different can), trim paint (semi-gloss or satin, usually a different base) and primer. Rounding them together would hide the fact that you are going home with four partly used gallons.
The formula, term by term
The core relation is G = A × c ÷ s and every term is doing real work.
A, the net area. Walls are the perimeter times the height: 2(L + W) × H. Deduct the doors and windows at full area. A 3-0 × 6-8 door is 20 ft², a 3 × 5 window is 15 ft². Painters argue about whether to deduct small openings at all — the practical convention is to deduct anything larger than about 15 ft² and ignore the rest, on the grounds that cutting in around a small window costs more paint in edge work than the opening saves.
c, the coats. Two is the standard specification and the one you should assume unless you are refreshing an identical colour in an identical sheen. Deep reds, strong yellows and any dark-to-light change need three, because the pigment loads that make those colours vivid are exactly the ones with poor hiding power.
s, the spread rate. Square feet per gallon at the recommended film thickness. It is a property of the product and the substrate. Halving it doubles your order, which is why the table below sweeps it rather than assuming it.
Trim is an area too. Trim is quoted in lineal feet but painted by the square foot, so multiply the run by the developed width — the girth of the painted profile, measured around the shape rather than across it. A 3½ in baseboard with its top edge and a small shoe develops roughly 6 in per lineal foot. Fifty-six feet of that is 28 ft², which is a quart, not a gallon.
Primer covers everything once. Primer goes on walls, ceiling and trim, so its area is the sum of all three, times the number of primer coats, divided by the primer's own spread rate — which is usually lower than the finish paint's because primer is formulated to seal rather than to build a film.
Worked example: a 16 × 12 ft bedroom with an 8 ft ceiling
Two coats of eggshell on the walls at 350 ft² per gallon, the same paint on the ceiling, one coat of primer at 300 ft² per gallon, 56 lf of 6 in trim, one door and two windows totalling 60 ft², paint at $45 a gallon.
- Perimeter. 2 × (16 + 12) = 56 ft.
- Gross wall area. 56 × 8 = 448 ft².
- Net wall area. 448 − 60 = 388 ft².
- Ceiling area. 16 × 12 = 192 ft².
- Trim area. 56 lf × 6 in ÷ 12 = 28 ft².
- Wall paint. 388 × 2 ÷ 350 = 2.22 gal.
- Ceiling paint. 192 × 2 ÷ 350 = 1.10 gal.
- Trim paint. 28 × 2 ÷ 350 = 0.16 gal — one quart.
- Primer. (388 + 192 + 28) × 1 ÷ 300 = 608 ÷ 300 = 2.03 gal.
- Cans to buy. 3 wall + 2 ceiling + 1 trim + 3 primer = 9 gallons, or $405.
Look hard at the primer line. It is the largest single quantity in the job, because primer covers every surface once at a lower spread rate, while the finish paints each cover only their own surface. If the walls were already painted and sound, dropping primer to zero coats takes the order from nine cans to six and the cost from $405 to $270.
Look at the rounding too. The exact requirement is 2.22 + 1.10 + 0.16 + 2.03 = 5.51 gallons, but you buy nine cans. That 3.5 gallon gap is not waste in the estimate — it is the unavoidable consequence of four products that cannot share a can. Buying the ceiling paint in a 5 gallon pail because you are doing three bedrooms is how professionals close that gap.
How to read the result
Start with the gallons-per-100-square-feet check. At two coats and 350 ft² per gallon, you need 0.57 gallons per 100 ft² of surface. If your result is far from that ratio, something is off in the dimensions or the coat count.
Next, read the sensitivity table rather than the headline. It sweeps the spread rate from 250 to 450 ft² per gallon and shows what each assumption costs in whole cans. If the can count is the same across the whole sweep, your estimate is robust and you can order confidently. If it changes by two or more cans, the spread rate is doing all the work and you should look up the real figure for your product and substrate before you buy.
Then check whether you are near a rounding boundary. An exact requirement of 3.02 gallons and one of 3.98 gallons both cost four cans, but the first leaves you with almost a full gallon on the shelf. If you land just over a whole number, it is often worth adjusting: a slightly better roller cover, a primer that seals rather than one that builds, or accepting that the closet gets the same colour.
Finally, remember that a 5 gallon pail is normally cheaper per gallon than five separate cans and eliminates any batch-to-batch shift in a custom tint. Once one colour crosses about five gallons, price the pail.
Typical spread rates by surface
| Surface | Typical spread rate | Gallons per 1,000 ft², one coat |
|---|---|---|
| Smooth, primed, previously painted drywall | 400 ft²/gal | 2.50 |
| New drywall, unprimed | 300 ft²/gal | 3.33 |
| Light orange-peel texture | 325 ft²/gal | 3.08 |
| Heavy knockdown texture | 250 ft²/gal | 4.00 |
| Smooth wood trim, brushed | 400 ft²/gal | 2.50 |
| Rough-sawn or cedar siding | 200 ft²/gal | 5.00 |
| Stucco | 150 ft²/gal | 6.67 |
| Bare concrete block, first coat | 125 ft²/gal | 8.00 |
Rough and porous surfaces cost you twice — extra true surface area and extra absorption — which is why block and stucco run at a third of the rate of smooth drywall.
Buy the primer decision, not the primer
Primer is worth a coat when the substrate is new, patched, stained, glossy, or a drastically different colour from the finish. It is not worth a coat over a sound, matte, similar-coloured painted wall — a coat of the finish paint does the same job and gets you closer to a finished surface.
The exception is a strong colour change. A tinted primer in a shade close to the finish colour will save you a whole finish coat on a dark-to-light change, and primer is normally cheaper per gallon than a premium finish paint. In that case the primer coat pays for itself directly.
Common mistakes in a paint estimate
- Taking the spread rate from the can without adjusting for the surface. The printed figure assumes a smooth sealed substrate. Texture and porosity can cut it by half.
- Forgetting the second coat. Doubling the coats doubles the paint. It is the largest single error in amateur estimates.
- Rounding all products together. Wall, ceiling, trim and primer are separate cans. Adding the decimals and rounding once understates the order every time.
- Quoting trim in lineal feet and buying by the gallon. Convert to area using the developed width around the profile, not the face width.
- Deducting every small opening. Cutting in around a small window costs more paint in edge work than the opening saves. Deduct openings above roughly 15 ft² and ignore the rest.
- Ignoring the substrate on exteriors. Rough-sawn siding and stucco run at a fraction of the rate of smooth drywall — a two-coat exterior can need three times the paint per square foot of an interior.
- Buying five separate gallons of a custom tint. Batch-to-batch variation is real. Above roughly five gallons, buy the pail or box the cans together before you start.
What this calculator does not model
It assumes a rectangular room with flat walls and a flat ceiling, one wall colour, one ceiling colour, and brush or roller application. It does not model spray transfer efficiency, which consumes measurably more material than rolling because some atomised paint never lands and the wet film is thicker — for spray work, reduce the spread rate you enter rather than adding a percentage afterwards.
It does not account for accent walls in a second colour, wallpaper removal, skim coating, gable ends, dormers, or the sloped area of a vaulted ceiling. For a vault, work out the true sloped area first with the roof area calculator and enter it as your ceiling dimensions.
It prices everything at one gallon price, which is a simplification: primer, ceiling flat and trim enamel rarely cost the same. Use the gallon counts and price them individually if the budget matters. For a whole-project material and labour figure, the cost per square foot calculator and the construction labor hours calculator take it from there.
Where paint sits in the sequence
Paint is the last wet trade in a room, and its quantity depends on decisions made two trades earlier. The texture applied over the drywall sets your spread rate; the finish level agreed with the drywall contractor sets whether the wall is smooth enough to take a sheen without telegraphing every joint. If you are still at the board stage, the drywall sheet calculator and the drywall mud calculator use the same room dimensions you have just entered here.
On the exterior, siding is measured the same way — perimeter times height, minus openings — but the material and the coating are estimated together, because the substrate profile changes both. The vinyl siding calculator handles the cladding side of that takeoff.
One last piece of context: for a repaint, the cost of paint is usually a small share of the job compared with labour and preparation. Getting the gallons right matters because a shortfall stops the work and a colour re-tint is never exact — not because the material is the expensive part.