Room Dimensions Calculator

Direct answer

For a rectangular room, floor area is length times width and volume is length times width times height. OpenStax gives the same length × width × height relationship for a rectangular prism and notes that the input measures must use the same unit.

Variables and formula

• Length, width and height: interior measurements in metres.
• Wall area: 2 × (length + width) × height before subtracting openings.

Floor area = Length × Width

Room volume = Length × Width × Height

Reproducible example

A 5 m by 4 m by 2.5 m room has 5 × 4 = 20 m² of floor area and 5 × 4 × 2.5 = 50 m³ of volume. Its wall area before openings is 2 × (5 + 4) × 2.5 = 45 m².

Interpretation

The result is a geometric baseline for a room that is approximately rectangular. Use the wall-area output as a measuring aid, then subtract openings separately for a materials estimate.

Assumptions and limitations

• The room has perpendicular walls and a uniform height.
• Alcoves, sloped ceilings, columns and openings are not modelled.
• Measurements are interior measurements in the same unit.

Common mistakes

• Calling cubic metres a floor-area unit.
• Including doors and windows when estimating paint without subtracting them.
• Measuring exterior dimensions for an interior project.

Assumptions

• The room is approximated as a rectangular prism.
• Wall area excludes neither doors nor windows and is not a materials quote.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• Measuring Volume; OpenStax Contemporary Mathematics; accessed Jul 27, 2026