Interior projects
Trim, doors and windows: the quart maths
Trim is a small area and a lot of work. Here is how to turn linear feet and door counts into quarts, and why the answer is usually two or three quarts more than people think.
Small areas, separate product
Trim, doors and window frames are painted in a different product from the walls: a harder, glossier enamel, usually semi-gloss, that stands up to shoes, hands and cleaning. They are also measured differently. A wall is an area; trim is a length, and a door is a count. Turning those into an area is the whole of the trim arithmetic, and the quantities are small enough that quarts, not gallons, are the unit to think in.
Baseboard, crown and chair rail
Trim mouldings are measured in linear feet and converted to area with their face height:
Baseboard runs around the room perimeter, less about 3 ft at each doorway. A 12 × 12 ft room with one door has 45 ft of baseboard; at a 5 in face, that is 45 × 5 ÷ 12 = 18.75 sq ft. Crown moulding and chair rail are the perimeter again, at their own face height. The top edge of a baseboard and the profile of a crown add a little, which the allowance covers.
Door and window casings
Casings are fiddly to measure, so they are counted:
| Item | Area | What it includes |
|---|---|---|
| Door casing | 16 sq ft per door | Both faces of the casing and the jamb |
| Window casing | 10 sq ft per window | Casing, stool and apron |
| Window frame and sash, small / standard / large | 8 / 12 / 18 sq ft, × 1.5 with sashes | For painting the window itself |
These are site figures derived from standard door and window sizes, listed in the standard sizes reference.
Trim for one bedroom
45 ft of 5 in baseboard, one door casing, one window casing, two coats of enamel at 400 sq ft per gallon, 10% allowance.
- Baseboard: 45 × 5 ÷ 12 = 18.75 sq ft
- Casings: 16 + 10 = 26 sq ft
- Total: about 45 sq ft
- Enamel: 45 ÷ 400 × 2 × 1.10 = 0.98 quarts, buy 1 quart
A room with two door casings and two window casings and 48 ft of baseboard comes to 72 sq ft and 1.58 quarts, so two quarts. Both are from the trim paint calculator.
Doors
A standard interior door is 36 × 80 in, 20 sq ft a face. A flat (flush) door is 20; a raised or recessed panel door has more surface in its profiles, about 15% more; a French door with glass panes has less paintable area, about 40% less because the glass is not painted, and much more cutting in; a louvered door has far more, about 60% more. Both faces are painted, for colour consistency and so the door does not take up moisture unevenly, plus the three visible edges.
Four panel doors
36 × 80 in, both faces and edges, two coats at 400 sq ft per gallon, 10% allowance.
- Per door: 20 × 1.15 × 2 faces + about 2 sq ft of edges = 48 sq ft
- Four doors: 194 sq ft
- Enamel: 194 ÷ 400 × 2 × 1.10 = 1.06 gallons, buy 1 gallon + 1 quart
Eight doors take 2.13 gallons, so two gallons and a quart. From the door paint calculator.
Doors use more paint than people expect: four doors need more enamel than all the baseboard and casings in a typical bedroom. They also take time; at about 45 minutes per side per coat, four doors both sides at two coats is about 12 hours of work.
Windows
Painting the window itself, the frame and the sashes, is the slowest painting in the house per square foot. The quantity is small:
Six standard double-hung windows
Frame and sashes, two coats, 400 sq ft per gallon, 15% allowance.
- Per window: 12 sq ft of trim × 1.5 for the sashes = 18 sq ft
- Six windows: 108 sq ft
- Enamel: 108 ÷ 400 × 2 × 1.15 = 2.48 quarts, buy 3 quarts
From the window frame and sash calculator. The allowance is 15% because a brush on narrow muntins and sash edges wastes more than anything else.
Why allowances are bigger on trim
On walls, a 5% allowance covers the roller and the tray. On trim it is 10%, and on windows and cabinets 15%, because paint is brushed in narrow strips, the brush holds a lot relative to what goes on the surface, and a small can has proportionally more left in the bottom. The difference is a few ounces per job, which matters only because the quantities are so small that a few ounces decide whether you need one quart or two.
Quarts or a gallon
Enamel in quarts is expensive per gallon: at typical prices a quart costs well over a quarter of a gallon, and three quarts usually cost more than a gallon. So the rule of thumb is: if the trim, doors and windows for the whole job come to more than about half a gallon, buy a gallon; if less, buy quarts. A gallon of trim white also keeps for touch-ups across several rooms.
| Job | Enamel needed | Simple rounding | Usually cheaper |
|---|---|---|---|
| One bedroom’s trim | 0.98 qt | 1 quart | 1 quart |
| Six windows | 2.48 qt | 3 quarts | 1 gallon if three quarts cost more |
| Four panel doors | 1.06 gal | 1 gallon + 1 quart | the same, or 2 gallons on a whole-house job |
The can size optimizer checks your actual prices, and paint can sizes and the leftover problem explains the general rule.
Whole house
For a whole house, add it up by room rather than door by door: the whole-house estimator includes baseboard and casings for every room template. A house with two small rooms, three medium rooms, a large room, two bathrooms and a hallway has about 471 sq ft of trim and casings, 2.59 gallons of enamel at two coats, so two gallons and three quarts. Add doors separately if they are being painted.
Outside trim
Exterior trim follows the same arithmetic with different standard sizes: fascia and soffits are measured as length times width, door trim is about 8 sq ft per door and window trim about 6 sq ft per window, and corner boards are counted per linear foot of height. The coverage of exterior trim paint is usually lower than interior enamel, about 350 sq ft per gallon, and the quantities are much larger, because fascia and soffits run the full perimeter: a single-storey house with a 160 ft perimeter has about 500 sq ft of trim and soffit, and takes 3.16 gallons at two coats. The exterior trim, fascia and soffit calculator adds it up.
Technique notes
- Sand between coats. A light scuff with fine sandpaper after the first coat of enamel takes off dust nibs and gives the second coat a better bite.
- Paint doors flat if you can. Off their hinges on sawhorses, enamel levels without runs.
- Use a good angled brush. A 2 in angled sash brush is the trim painter’s main tool.
- Caulk before painting. Gaps between trim and wall filled with paintable caulk make the line look crisp.
- Paint trim last, or first, but consistently. Most painters do walls, then trim; see painting a room.
Common mistakes
- Using wall paint on trim. It scuffs and marks; use enamel.
- Forgetting the doors. They often need more paint than the rest of the trim combined.
- Buying many quarts. Over half a gallon in total, a gallon is usually cheaper.
- Painting one face of a door. Paint both, and the edges.
- Skipping the scuff-sand on old gloss. New enamel will not grip it; scuff it or use a bonding primer.