Paintable area is the number the whole estimate rests on
Everything in a painting estimate — gallons, hours, price — is derived from one figure: the area that actually receives paint. Get that wrong and every downstream number is wrong by the same proportion, which is why this calculator spends most of its effort on the geometry.
Wall area starts as the room perimeter multiplied by the ceiling height. A 14 ft × 12 ft room has a perimeter of 2 × (14 + 12) = 52 ft, and at a 9 ft ceiling that is 468 ft² of gross wall. From that you deduct the openings. This calculator uses 20 ft² per doorway — a standard 3 ft × 6 ft 8 in door is 20 ft² almost exactly — and 15 ft² per window, about a 3 ft × 5 ft unit. Those are nominal figures, deliberately. Measuring every opening is more precise and rarely changes the gallon count, because paint is bought in whole cans; if your room has a wall of glass, enter it as several windows.
The ceiling is simply length × width. Trim is the awkward one: it is a length, not an area, and it has to become an area before it can consume paint. A run of baseboard with casing has roughly half a foot of painted surface developed across its face and top edge, so this calculator converts one running foot of trim into 0.5 ft² of paintable surface. That keeps the paint quantity honest. It does not keep the labour honest, which is the subject of the next section.
Why labour is priced two different ways
Paint quantity depends only on area. Labour does not — it depends on how the paint gets onto the surface, and rolling a wall and brushing a baseboard are different jobs at different speeds.
Walls and ceilings are priced against a rolling production rate in square feet per hour, per coat, including cutting in at the edges. That single figure absorbs a lot: cutting in around a busy room with many openings is slower than a plain box, an extension pole makes a ceiling faster than a wall, and a sprayed job is far quicker than a rolled one. Use your own timed rate if you have kept records; if you are quoting, your production rate is the number that decides whether you make money, so it deserves more attention than the paint price does.
Trim is priced against a running feet per hour rate instead, because its labour has almost nothing to do with its area. The worked figures make this stark: in the trim-heavy test case on this page, 100 ft of trim is only 50 ft² of surface — one seventh of the room's 370 ft² total — and yet takes 4.0 hours against the walls' 3.2. Divide it out and the trim is consuming 0.08 hours per square foot while the walls consume 0.01, a factor of eight. Any estimate that prices trim by area will lose money on every room with joinery in it.
Both rates are applied per coat, and the primer counts as a coat. That is the second place estimates go wrong: adding a primer coat does not add a small allowance, it adds a full pass over every surface at full labour. On the default room the primer pass is 551 ÷ 180 + 62 ÷ 50 = 4.30 of the 12.90 hours — a third of the labour — for $53 of material. Decide honestly whether you need it, and see the recoat schedule calculator for how long you have to wait between those passes, since the waiting is what turns a two-coat job into a two-day job.
Worked example: a 14 × 12 bedroom, walls, ceiling and trim
A 14 ft × 12 ft bedroom with a 9 ft ceiling, two doorways and three windows, 62 ft of baseboard and casing. Two finish coats plus a primer coat over patched walls. Paint at $45 a gallon, primer at $32, spread rate 350 ft²/gal, $60 of sundries. Labour at $55 an hour, rolling 180 ft²/hr and trimming 50 ft/hr, both per coat.
- Perimeter. 2 × (14 + 12) = 52 ft.
- Gross wall. 52 × 9 = 468 ft².
- Openings. 2 × 20 + 3 × 15 = 40 + 45 = 85 ft².
- Net wall. 468 − 85 = 383 ft².
- Ceiling. 14 × 12 = 168 ft². Trim. 62 × 0.5 = 31 ft².
- Total paintable area. 383 + 168 + 31 = 582 ft².
- Finish paint. 582 × 2 ÷ 350 = 1,164 ÷ 350 = 3.33 gal, costing 3.3257 × $45 = $149.66. Buy four gallons, or a five-gallon pail.
- Primer. 582 × 1 ÷ 350 = 1.66 gal, costing 1.6629 × $32 = $53.21.
- Materials. 149.66 + 53.21 + 60 = $262.87.
- Rolling hours. Walls and ceiling are 383 + 168 = 551 ft², over three passes: 551 × 3 ÷ 180 = 9.18 hr.
- Trim hours. 62 × 3 ÷ 50 = 3.72 hr. Total 9.18 + 3.72 = 12.90 hr.
- Labour. 12.9033 × $55 = $709.68.
- Total. 262.87 + 709.68 = $972.55, which is 972.55 ÷ 582 = $1.67 per ft² of painted surface.
Materials are 262.87 ÷ 972.55 = 27% of this job and labour is 73%. That ratio is why spending more per gallon on better paint is usually a good trade: an extra $15 a gallon adds about $50 here, under 6% of the total, while a paint that hides in two coats rather than three saves a full pass — 4.3 hours, or $236.
How to read the result, and what a quote should show
The figure to compare against a contractor's bid is the total, not the cost per square foot, because painters measure square footage inconsistently — some quote against floor area, some against wall area, some against the gross wall before openings. Ask which one a per-square-foot quote uses before comparing it to anything.
Check the hours figure against the calendar. Twelve or thirteen painter-hours is a day and a half for one person, but it is not a day and a half of elapsed time, because coats have to dry between passes. A three-pass job in a cool room can easily run three days even though the labour is under two. If the room's availability matters, price that separately with the downtime cost calculator.
Look at the split between materials and labour. On interior repaints, labour dominates on almost any job where someone is being paid, which has two practical consequences. First, buying cheap paint to save money is usually a false economy, because a paint that needs an extra coat costs a full pass of labour. Second, the highest-value thing you can do to a quote is reduce passes — accept one coat where the colour is unchanged, or skip primer where the substrate genuinely does not need it — not haggle the material line. For a materials-only DIY check, set the labour rate to zero and compare against the paint coverage calculator, which handles the gallon arithmetic on its own.
Paintable area and paint quantity for common rooms
| Room | Wall area after openings | Ceiling area | Total | Gallons, 2 coats |
|---|---|---|---|---|
| 10 × 10 ft | 275 ft² | 100 ft² | 375 ft² | 2.14 gal |
| 12 × 10 ft | 311 ft² | 120 ft² | 431 ft² | 2.46 gal |
| 14 × 12 ft | 383 ft² | 168 ft² | 551 ft² | 3.15 gal |
| 16 × 14 ft | 455 ft² | 224 ft² | 679 ft² | 3.88 gal |
| 20 × 15 ft | 545 ft² | 300 ft² | 845 ft² | 4.83 gal |
Every row here rounds up to the next whole gallon in practice, and four of the five sit close enough to a boundary that a slightly lower spread rate pushes them over it. That is the argument for buying by the pail on anything above about three gallons.
What this estimate leaves out
- Preparation beyond routine. Filling, sanding and caulking are inside a normal production rate; stripping wallpaper, skim-coating a damaged wall or removing textured ceiling is not, and each can exceed the painting itself.
- Furniture and protection. Moving a room's contents, masking floors and covering fittings is real time. On an occupied house it is often an hour at each end of the job.
- Access equipment. Above roughly 12 ft the labour rate stops being valid, because staging or a lift is needed and the working pace drops.
- Repairs found during prep. Nail pops, failed tape joints and water damage all surface once the room is bare. Hold a contingency, the way you would on any renovation — see the contingency calculator.
- Cutting in against unpainted ceilings. If the ceiling is not being painted, the wall-to-ceiling line has to be cut precisely rather than blended, which is slower than the same room done throughout.
- Second colours. Every additional colour means another cut-in line, another can and another cleanup. The area does not change; the hours do.
- Drying and recoat times. The hours here are working hours, not elapsed days. Cold or humid conditions extend the wait between coats considerably.
Buying whole cans without over-buying
The gallon figures here are exact, not rounded, so that the arithmetic stays transparent. In the shop you buy whole containers. Round each of paint and primer up to the next whole gallon separately — do not add them together first, because they are different products.
Above about three gallons, compare the per-gallon price of a five-gallon pail against four separate cans; the pail is usually cheaper and it also removes the risk of a colour mismatch between batches, which is a genuine problem with tinted paint. Whatever is left over should be labelled with the room and the date, because a touch-up two years later from a can of unknown age and unknown tint will show.
One warning about spread rates: the figure on the can is measured on a smooth, sealed, primed surface. Bare drywall, fresh filler and textured or porous surfaces all drink more, sometimes far more on the first coat. If you are painting anything that has not been sealed, either prime it or reduce the spread rate you enter here.
Quoting work versus pricing your own job
If you are quoting for a customer, this calculator is a cost model, not a price. Your price has to carry overhead, insurance, vehicle, non-billable time and profit, and the usual way to do that is to build them into the hourly rate rather than to add a percentage at the end. A charge-out rate that only covers a painter's wage produces jobs that look busy and lose money. The same discipline applies to the production rate: use the speed you actually achieve on a real occupied house with furniture in the way, not the speed you achieve on an empty new-build.
If you are pricing your own work, the labour figure is still worth calculating even at a rate of zero, because it tells you how long you will be living around it. A thirteen-hour room is a full weekend once you count masking, cleanup and the waits between coats, and knowing that before you start is the difference between a finished room and a half-painted one. Comparing DIY against a contractor puts a value on those hours explicitly.
Painting usually sits alongside other work. If the trim is going back after new flooring, run the flooring cost calculator first and paint the trim before the floor goes down. If walls are being repaired or replaced, the drywall sheet calculator gives the board count, and remember that new board needs a sealer coat that this calculator will only include if you tick primer. And where the wall covering is changing rather than being repainted, the wallpaper roll calculator uses the same paintable-area geometry.