Deduct doors and windows from paint area

Measure each opening, apply width × height × quantity to walls only, and check when openings exceed the gross wall area.

Prepared with AI assistance by Luminesca · Updated 16 October 2026 · Examples are synthetic.

Door and window deductions are the step that turns a rectangular shell into a paintable wall area. The paint calculator does not invent an opening size. Each deduction is the rectangle you type: width × height × quantity, subtracted from walls only. This guide works that arithmetic with the same synthetic bedroom used elsewhere on the site, then shows the mistakes that still look tidy on a form.

Start from the gross shell, then list openings

A rectangular room’s gross wall area is 2 × (length + width) × height. Openings come next. They do not shrink the ceiling. Ceiling area is length × width whenever you assign a ceiling product; doors and windows never reduce it.

Load that example on the paint calculator and confirm the room row shows 31 m² before you change any product fields. Coverage and coats belong to a later step; see the coverage and coats guide once the area is stable.

Same size vs different sizes

Quantity exists so identical openings share one row. Two windows that are both 1.2 m by 1.0 m are one row with quantity 2. A door that is 0.9 m by 2.1 m needs its own row. Putting both sizes on one row multiplies the wrong rectangle.

With the test product (two coats, 10 m²/L, 10% allowance, 2.5 L cans, nothing owned) the need is 6.7562 L and the purchase is still three cans, 7.5 L. The built-in 4 m² opening produced 6.82 L and the same three cans. A more careful opening list can change the area without changing the can count, so read the area line and the litre line separately.

What not to borrow from memory

Do not substitute a remembered “standard door.” The calculator accepts the rectangle you type. Measure the opening you will not paint: the rough opening or the finished frame, consistently, for every opening on the list. A glazed door that you will paint as wall should not appear as a deduction. A window you will leave unpainted should.

Trim, architraves and reveals are not modelled. If you intend to paint them, keep them in the wall area by not deducting a larger opening than you will leave bare. If you will not paint them, your deduction should match the unpainted face, not a catalogue door code.

When openings exceed the walls

If the sum of opening rectangles is larger than the gross wall area, the estimate stops. It does not clamp the net area to zero and still offer cans. That would hide the typing error.

A subtler overflow is stacking quantity on an already large opening, or listing the same physical window twice under two names. The model only sees the sum of your rows.

Ceiling assignment is independent of openings

Assigning the test bedroom’s ceiling to the same wall product changes the room row from 31 m² to 43 m². The 4 m² opening is still walls-only. Theoretical volume becomes 43 × 2 ÷ 10 = 8.6 L; with 10% allowance, 9.46 L and four 2.5 L cans. Openings never “also clear the ceiling.”

A separate ceiling product with matching coverage still buys its own cans. Walls stay on the opening-adjusted area; the ceiling alone is 12 m². Package rounding can make two product entries cost more cans than one shared entry. That is a packaging effect, not an openings effect. For combining rooms and products before rounding, use the combine orders guide.

Units: feet into metre fields

Changing Measurements to “Feet / US gallons” redisplays stored metres. It does not reinterpret digits you already typed. Typing 4, 3 and 2.5 as feet into metre fields invents a room whose walls are about 10.76 times too large in area, because wall area scales with the square of the foot-to-metre factor. Opening rows typed the same wrong way will not cancel that error; they scale with it.

Enter every opening in the same system as the room shell. After switching the control, re-check width and height on each opening row, not only the room length fields.

Wallpaper and flooring are different models

Wallpaper ordering counts full-height strips. Ordinary doors and windows are not deducted there; short pieces above a door are not returned to the roll. Flooring uses floor area with no wall openings at all. Do not copy a paint opening list into those calculators expecting the same net area.

Checklist before you trust the can count

1. Confirm gross walls: 2 × (L + W) × H in one unit system.
2. List every unpainted opening as width × height × quantity; split different sizes onto different rows.
3. Confirm openings apply to walls only; ceiling assignment is a separate decision.
4. If the tool says openings exceed wall area, fix inputs — do not override with a remembered can count.
5. Only then enter coverage, coats, allowance and can size.

For measuring the shell and the ceiling decision in one pass, see measure a room for paint. When the net area matches your sketch, enter the openings on the paint calculator and recompute before you buy.

Sources and scope

For definitions of wallpaper match, repeat and packaging, see Brewster's product properties and Graham & Brown's wallpaper FAQ. Product instructions take precedence over generic examples. Unit conversions use the exact international foot (0.3048 m) and US liquid gallon (3.785411784 L); see NIST conversion factors.