Understanding your result
The headline is the trim enamel for the windows and coats you chose, including the allowance, in quarts when it is under a gallon. The sub-line is the can list. The three figures show the area, the paint for one coat and the cans. The breakdown shows the area per window for the size class, the sash factor, the total area and the usual coverage, coats and allowance lines.
The quantities are small and the hours are not. Six windows take three quarts of paint and, by the production rates on this site, most of two days with two coats and the drying between them. The painting time calculator counts windows at about 45 minutes each per coat; the paint is the cheap part.
If you are painting the baseboard and the doors in the same room, run the trim and baseboard calculator and the door paint calculator and add the three quantities before you buy. Together they often cross the point where a gallon is cheaper than the quarts. Exterior window trim is in the exterior trim calculator, with exterior paint and its coverage.
How we calculate this
The trim area per window is derived from the parts you paint. A standard 3 × 5 ft window has about 156 in of casing around the sides and head; at 3.5 in wide that is 3.8 sq ft. The stool (the inside sill) and the apron under it add about 1.5 sq ft, and the jamb, the 4.5 in return between casing and sash, about 4.9 sq ft. Together just over 10 sq ft, rounded up to 12 to allow for a wider casing or a deeper jamb. The small and large classes scale that by the perimeter. The sashes, if painted, add about half again: the rails and stiles of two sashes are 3 in wide around a 32 × 28 in opening each, roughly 5 to 6 sq ft on a standard window, plus muntins on divided-light sashes.
Coverage is the enamel’s figure from the can, typically 350 to 400 sq ft per gallon; the coverage rates table has the published figures. The trim and doors guide shows how to measure a window when yours is far from these sizes, and the standard sizes page lists the opening sizes the classes are built on.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Small window trim | 8 sq ft | Derived: 3.5 in casing, stool, apron and 4.5 in jamb of a 2 × 3 ft window |
| Standard window trim | 12 sq ft | Derived for a 3 × 5 ft window, rounded up for wider casings |
| Large or picture window trim | 18 sq ft | Derived for a 5 × 6 ft window |
| Sash factor | 1.5 | Rails, stiles and muntins of two sashes on a standard double-hung window, about half the frame area |
| Coverage | 400 sq ft per gallon | Trim enamel data sheets (Benjamin Moore Advance, Sherwin-Williams Emerald Urethane, Behr Urethane Alkyd): 350–400 sq ft per gallon |
| Coats | 2 | Enamel needs two coats for an even sheen |
| Allowance | 15% | Brush loss on narrow profiles; a round figure |
Assumptions last reviewed October 7, 2026.
The calculator does not model bare or stripped wood, which needs primer first, nor the extra coat that a change from dark stained sashes to white often takes. It does not include glazing putty, which is painted after it has cured, or the exterior face of the sash, which takes exterior paint. The drying guide explains why sashes should be left open on blocks until the enamel has cured, not merely dried.
Two worked examples
Six standard windows with sashes
A floor of a house: six 3 × 5 ft double-hung windows, frames and sashes, two coats, 15% allowance.
- Per window: 12 × 1.5 = 18 sq ft
- All windows: 6 × 18 = 108 sq ft
- Per coat: 108 ÷ 400 = 0.27 gallons (1.08 quarts)
- Needed: 0.27 × 2 × 1.15 = 0.62 gallons, or 2.48 quarts
- Buy: 3 quarts, about half a quart left
Three quarts cost nearly as much as a gallon. If the doors or the baseboard are on the same list, buy the gallon; if not, three quarts and the leftover becomes the touch-up jar.
Fourteen large windows, frames only
A house with big picture windows and vinyl sashes that are not painted, so the frames only, two coats.
- Per window: 18 × 1 = 18 sq ft
- All windows: 14 × 18 = 252 sq ft
- Per coat: 252 ÷ 400 = 0.63 gallons
- Needed: 0.63 × 2 × 1.15 = 1.45 gallons
- Buy: 1 gallon + 2 quarts (1.5 gallons)
Two small windows with frames only and one coat, for comparison, are 16 sq ft and 0.16 quarts: a quart with most of it left, which is why the note on the result suggests a quart rather than a sample pot, since enamel is not sold smaller.
Where to find your inputs
Number of windows. Count openings in the rooms you are painting. A bay window is usually three windows; a pair of windows side by side with one casing around both is one large window.
Size class. Measure the glass opening roughly. Up to about 2 × 3 ft is small, around 3 × 5 ft is standard, anything from 5 × 6 ft up is large. For very tall windows, count two standard windows each.
Sashes. Painted if they are wood and you are painting them; not painted if they are vinyl, fibreglass or aluminium, or if only the casing is being refreshed.
Coverage. From the enamel can, lower end of the range.
Common mistakes
- Painting the sash shut. Move the sashes every hour while the enamel dries and leave them open overnight on a block.
- Using wall paint. It never fully hardens on a sash that rubs its frame. Use trim enamel.
- Guessing a gallon for every job. Six windows are under three quarts. A gallon is for a whole house of windows or for windows plus doors plus baseboard.
- Skipping primer on bare or filled wood. Enamel over bare wood flashes and peels. Spot-prime, or prime the whole sash if it was stripped.
- Painting over failed glazing putty. Re-glaze, let the putty skin over (a week or more for oil putty), then paint.
- Forgetting the exterior face. The outside of a wood sash takes exterior paint and is in the exterior trim calculator.
Questions people ask
- How much paint for six windows?
- Six standard 3 × 5 ft windows with their sashes are about 108 sq ft of trim. Two coats of enamel at 400 sq ft per gallon with a 15% allowance come to 2.48 quarts, so buy three quarts, or a gallon if the doors and baseboard are on the list too.
- How many square feet is a window frame?
- The calculator uses 8 sq ft for a small window (about 2 × 3 ft), 12 for a standard one (3 × 5 ft) and 18 for a large or picture window (5 × 6 ft), counting the casing, jamb, stool and apron. Sashes add 50%, because the rails, stiles and muntins of a double-hung window are nearly as much surface as the frame around them.
- Is a quart enough for one window?
- Easily. A standard window with sashes is 18 sq ft, about 0.05 gallons per coat, so one quart covers two coats on five windows. Trim enamel is not sold smaller than a quart, so a quart is the minimum buy anyway.
- Do I paint the sashes?
- If they are wood and the finish is failing, yes, and the sash is the part that protects the glazing. Vinyl and aluminium-clad windows are not painted. Painting a sash means masking or scraping the glass and keeping the sash moving while it dries so it does not seal shut.
- Which paint for window frames?
- The same trim and door enamel as the rest of the trim, in satin or semi-gloss, waterborne alkyd if you want the smoothest brush finish. Exterior sashes take an exterior trim paint; the exterior trim calculator covers those.
- Why a 15% allowance?
- Window trim is all narrow profiles and corners, and the brush loses more paint to the tape, the edges and the cup than a roller loses on a wall. Fifteen percent is a round figure; the quantity is so small that the allowance is usually less than the rounding to the next quart.