Which floor area are we talking about?
Before calculating, decide what to measure. The names change between countries and uses, but the three concepts are the same everywhere:
| Floor area | What it includes | What it is used for |
|---|---|---|
| Gross | Everything within the outer face of the external walls, walls included | Building volumes, comparisons between buildings |
| Net (or walkable) | The inside of each room, measured at the inner face of the walls | Floors, finishes, how the space is lived in |
| Commercial | Net floor area plus a share of walls, balconies and accessories according to a convention | Listings and valuations |
A model that measures each room gives the net floor area per room and, from the wall thicknesses, the gross one. Commercial conventions are applied afterwards by the professional.
Method 1: by hand with the dimensions
For each rectangular room, multiply width by depth: a room of 4.00 × 3.20 m measures 12.80 m². Rooms with an L shape are split into rectangles; irregular shapes are approximated with triangles. Add up the rooms to get the net floor area.
It works for small flats with complete dimensions. It stops working when dimensions are missing, when the plan is not to scale, or when the layout changes and everything has to be redone.
Method 2: with a CAD
In a CAD program you draw a closed polyline along the inner face of the walls of each room and read its area. Precise, but every room is a separate polygon that you must draw, name and keep updated by hand: if a wall moves, two polygons must be redrawn and the table updated.
Method 3: with a model that measures every room
In ShotPlan you trace the walls on the CAD lines, place the openings and generate the rooms: each room is a polygon that knows its walls, and its floor area and perimeter are calculated from the model. Available The quantities per room are exported as JSON, with the gross and net wall surfaces too.
Every correction of the model (a wall state, a thickness) creates a numbered revision, so the quantities of two versions can be compared without redrawing anything. In testing In the demo project “Appartamento tipo” (69.30 m²), the five rooms add up to 69.30 m²: living and kitchen 32.03, bedroom 1 14.58, bedroom 2 10.62, hall 6.79, bathroom 5.29.
Three mistakes that cost money
- Plan not to scale. Printed or scanned plans are often resized. Always calibrate with a known measurement, for example a 90 cm door.
- Walls counted as rooms. Measuring at the axis of the walls instead of their inner face adds half the thickness to every side: on a flat this can mean several square metres.
- Numbers copied by hand. Every copy between plan, table and quote is a chance to make a mistake. Export the quantities from the model and let the table read them.
From square metres to costs
Floor areas per room, perimeters and wall surfaces are the quantities a price list is applied to: €/m² for floors, €/m for skirting boards, €/m² for painting. The quantities and costs guide shows the full path with a worked example.