Wall paint is estimated from area. Trim is measured in linear feet, and the conversion to gallons is where estimates go wrong — usually by buying a gallon where a quart would do, or a quart where a gallon was needed.
Linear feet to square feet
Divide by the profile's developed width — the distance a brush travels across the face, including the shaped edges.
| Trim | Developed width | Linear ft per ft² |
|---|---|---|
| 2 1/4 in casing | ~2.7 in | 5.3 |
| 3 1/4 in baseboard | ~3.9 in | 3.6 |
| 5 1/4 in baseboard | ~6.0 in | 2.4 |
| 4 1/2 in crown | ~5.3 in | 2.7 |
| Chair rail | ~3.2 in | 4.5 |
| Window stool and apron | ~5.5 in | 2.6 |
Developed width exceeds nominal width because a profile is not flat — the curves and steps add surface a flat measurement misses.
A worked room
A 12 × 14 ft room, 52 linear feet of perimeter, one door, two windows, baseboard and crown throughout.
| Item | Linear ft | ft² |
|---|---|---|
| Baseboard, 3 1/4 in, less door width | 49 | 13.6 |
| Crown, 4 1/2 in | 52 | 19.3 |
| Door casing, both sides | 34 | 6.4 |
| Window casing, two windows | 34 | 6.4 |
| Window stools and aprons | 8 | 3.1 |
| Total trim | 48.8 ft² |
At 400 ft² per gallon, one coat is 0.12 gallons — about 16 fluid ounces. Two coats is a quart with a little left.
A quart does a typical room's trim. A gallon does about four rooms.
Doors are the heavy item
A door is not trim in quantity terms. It is a wall.
| Door | Area, both faces + edges |
|---|---|
| Flush slab, 30 × 80 | 35 ft² |
| Six-panel, 30 × 80 | 44 ft² |
| Six-panel, 36 × 80 | 52 ft² |
| Bifold pair | 40 ft² |
A panelled door has more surface than a slab because the profiles and reveals add area — roughly 25% more.
Two coats on one six-panel door is 0.22 gallons, nearly a quart. Budget a quart per door, and note that a single door consumes more paint than all the baseboard in the room.
Add the jamb and stop — about 5 ft² per opening — if they are being painted, which they should be if the door is.
Two coats, always
Trim takes handling, cleaning and impact. One coat of enamel over an existing finish rarely levels properly and rarely covers a color change at the profile edges, where the film is thinnest.
Two thin coats also level better than one heavy one, which is the whole point of the product.
Which product
Not wall paint. Trim wants a waterborne alkyd or an acrylic enamel — these flow out like an oil paint, level brush marks, and cure hard over several days while cleaning up in water.
Sheen is semi-gloss in most houses, satin where a softer look is wanted. Flat trim is a deliberate specification and it does not survive a kitchen. The sheen ladder sets out the trade between washability and how much it shows.
Cure time is not dry time
Waterborne enamel is dry to touch in an hour and fully cured in 14 to 30 days.
Closing a door onto a freshly painted jamb at day three is how paint comes off in sheets, and stacking books against a new shelf does the same. Talc on the jamb stops the sticking while it cures, and it is worth doing on every door.
Order of work
Ceiling, then walls, then trim is the common advice, and for a room with existing trim it is backwards.
Paint the trim first, then cut the walls to it. A brush runs along the edge of trim easily because the trim gives it a hard edge to follow; cutting trim paint against a finished wall gives it nothing. Professionals work in this order and mask almost nothing.
Prep is where trim jobs are won
Fill nail holes with a lightweight filler and sand flush. Caulk every joint where trim meets wall — this is the step that makes trim look installed rather than nailed on, and it is the one most often skipped.
Then sand the existing gloss. A deglosser or a bonding primer will substitute, and either works over a glossy surface — but neither works over a surface that is also greasy. On kitchen trim, degreasing comes before everything.
Buying
For a whole house: one gallon per four rooms of trim, plus a quart per door, plus 20% for the second coat being thinner than you expect. Trim enamel is the most expensive paint in the store per gallon and the smallest quantity in the job, so buying a little over is cheap insurance against a mid-job color batch change.