Room Area Calculator — Square Feet, Metre & AC Tonnage

Calculate room area in square feet/metres plus AC tonnage and HVAC requirements.

About this calculator

A room area & volume calculator works out the floor area, wall area, and air volume of a room — the three measurements needed for every interior finishing and HVAC decision. Floor area determines flooring and ceiling material; wall area drives paint, wallpaper, and tile estimates; air volume sizes air conditioning, heating, and ventilation requirements.

For an L-shaped or irregular room, divide the floor plan into rectangles, compute each separately, and sum them. For sloped ceilings, the wall area should be calculated for each wall as a trapezoid or triangle as appropriate. Always measure twice — a 5% measurement error on a single room is forgivable but multiplied across a house leads to expensive material under-ordering.

Formula

A_{floor} = L \times W ; \quad A_{walls} = 2(L + W) \times H ; \quad V = L \times W \times H

  • L, W — Length and width
  • H — Ceiling height
  • Doors/Windows — Subtract their area from total wall area for paint/tile estimates

Worked example

Inputs: Bedroom: 4 m long × 3.5 m wide × 3 m ceiling height

  1. Floor area = 4 × 3.5 = 14 m²
  2. Wall area = 2 × (4 + 3.5) × 3 = 45 m²
  3. Subtract one door (0.9 × 2.1 = 1.9 m²) and one window (1.5 × 1.2 = 1.8 m²) = 41.3 m² net wall
  4. Air volume = 14 × 3 = 42 m³
  5. For AC sizing: ≈ 1 ton per 100 ft² floor (≈ 9.3 m²), so this room needs a 1.5-ton AC

Frequently asked questions

How do I calculate area of an L-shaped room?

Draw the floor plan and divide it into two rectangles. Compute the area of each separately and add them. For complex shapes with curves, use the trapezoidal rule or just over-estimate by treating curves as inscribed rectangles. Modern laser distance meters with floor-plan apps make this trivial — they auto-compute irregular areas as you walk the perimeter.

Should I include ceiling area in painting estimates?

Yes — if you are painting the ceiling, add the floor area (which equals the ceiling area for flat ceilings) to the total paintable surface. For ceilings with cornices or coffers, multiply by 1.1 to account for the extra surface. Ceilings typically need only 1 coat of paint if the underlying primer is good, while walls usually need 2.

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