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Room Area Calculator
Create a dependable measurement baseline before estimating paint, flooring, drywall, trim, or wall coverings.
Preparing calculator...
How to calculate it
- Measure finished interior length and width.
- Measure floor-to-ceiling height.
- Combine major door and window openings for net wall area.
Formula and assumptions
What the calculator is doing
Floor and flat ceiling area = length × width; perimeter = 2 × (length + width); net wall area = perimeter × height − openings.
- The formulas assume a rectangular room with vertical walls and a flat ceiling.
- Openings affect net wall area only; they do not change floor, ceiling, or perimeter.
- L-shapes, bays, slopes, and partial-height walls should be divided into measurable rectangles or triangles.
Before checkout
Buying checklist
- Sketch the room and label every measured edge with units.
- Measure at more than one point when walls are not parallel.
- Separate recesses, bays, closets, and sloped surfaces.
- Photograph product labels and keep the measurement sheet with the estimate.
Avoid these errors
Common mistakes
Writing numbers without units
A bare “120” can later be mistaken for inches, feet, square feet, or centimetres.
Forcing an irregular room into one rectangle
Divide the plan into simple shapes and add them; large bounding rectangles overstate materials.
Rounding every measurement early
Keep field precision through the area calculation, then round the final purchase quantity.
Frequently asked questions
How do I handle an L-shaped room?+
Split the room into two rectangles, calculate each one, and add the floor and ceiling areas.
Are floor and ceiling areas always equal?+
They are equal for a flat ceiling over a rectangular room. Sloped, vaulted, or stepped ceilings need separate measurements.