The Standard Door and Window Deductions Every Room Calculator Uses — And When Your Room Breaks Them

2026-09-14

The Paint Calculator and the Wallpaper Calculator on this site solve visibly different problems — one tells you how much paint to buy, the other how many rolls — but they share a detail most users never notice: both start wall area from the same formula (perimeter × height) and subtract the same standard deductions for doors and windows. That's not a coincidence or a shortcut; it's a deliberate, shared constant, and understanding it tells you when to trust the default and when to override it.

The shared numbers: about 1.6 m² per door, 1.5 m² per window

Both calculators deduct roughly 1.6 m² of wall area for a standard interior door and roughly 1.5 m² for a standard window, before dividing the remaining paintable/wallpaperable area by coverage-per-liter or per-roll. These aren't independently chosen numbers that happen to match — they're the same reference figures for a reason: a standard door and a standard window really do occupy roughly that much wall area across most residential construction, so both calculators can share the constant without either one being wrong for its own purpose.

When "standard" stops describing your actual door or window

The deduction is a genuine average, which means it's wrong in both directions for anything non-standard. A narrow bathroom door is smaller than 1.6 m² of wall it removes; a wide sliding patio door removes far more. A small bathroom window is well under 1.5 m²; a bay window, a picture window, or a floor-to-ceiling French window can remove several times that much wall area — and if you have several of them in one room, the standard deduction can meaningfully overstate how much wall you actually have to cover, leading you to buy more paint or more rolls than the room needs.

What to do about it

Neither calculator currently lets you enter a custom door/window size — the deduction is a reasonable default, not a promise about your specific room. For a room with mostly standard doors and windows, trust the number. For a room with one or two clearly oversized or undersized openings, the practical fix is the same for both tools: measure the actual opening, work out the difference from the standard deduction by hand, and adjust the wall-area input (or just buy accordingly) rather than assuming the shared constant covers every room equally well.

• The Paint Calculator and Wallpaper Calculator on this site deliberately share the same door (≈1.6 m²) and window (≈1.5 m²) deduction constants — an intentional shared reference value, not a coincidence.
• The deduction is a genuine average for standard residential openings — it understates the true deduction for oversized doors/windows and overstates it for undersized ones.
• A room with several non-standard openings (sliding doors, bay windows, floor-to-ceiling glass) is exactly the case where it's worth measuring by hand instead of trusting the default.