Choose your measurement method first, break the plan into simple shapes, calculate each included area consistently and list exclusions separately.
1. Decide what “floor area” means for this comparison
Before multiplying any dimensions, decide whether you want usable internal area, an area measured to wall centrelines, or gross area measured to the outside face of external walls. This choice changes the boundary of every room and determines whether hallways and other circulation areas, internal walls, garages, balconies and storage are included.
If you are comparing a calculated number with an advertised number, find out how the advertised number was defined. Two mathematically correct totals can still disagree because they measure different things.
2. Confirm the plan scale and dimensions
Use printed room dimensions where available. Check the unit, decimal point and multiplication symbol carefully: 3.8 m × 3.1 m is very different from 3.8 m × 8.1 m. Do not assume a screenshot is displayed at a standard scale.
If a scale bar is present, verify it against at least one printed dimension. If the plan shows neither dimensions nor a reliable scale, pixel measurements alone cannot be converted into square metres.
- Prefer larger rectangular rooms with clear opposing walls.
- Avoid using a dimension that includes a bay, wardrobe or angled recess unless the label clearly does so.
- Check whether separate levels use the same drawing scale.
3. Break the outline into simple shapes
Trace the boundary of the area you want to include and split it into rectangles first. Use triangles or trapezoids only where the plan genuinely has angled edges. Calculate each shape separately, then add them together. Subtract voids, light wells and explicitly excluded spaces.
For example, an 8 m × 6 m main rectangle, plus a 3 m × 2 m extension, minus a 2 m × 1 m void gives 48 + 6 − 2 = 52 m². Keep the working visible so another person can reproduce it.
rectangle = length × widthtriangle = base × perpendicular height ÷ 2total area = sum of included areas − sum of excluded areas4. Calculate each level and outdoor space separately
Calculate each storey separately. Stairs and voids open to the level below need a consistent rule; otherwise the same physical space can be counted twice. List balconies, decks, garages and detached studios separately, even if you later combine them for a particular comparison.
A useful worksheet has columns for the space, dimensions, calculated area, whether it is included and the source of the measurements. This makes it easy to produce both an internal-only total and a broader total without remeasuring the plan.
5. Check the result
Add the room areas and compare the result with the area calculated from the outer outline. The room sum will normally be lower if hallways, walls and unlabelled spaces are omitted, but a very large gap often reveals a transcription error or an inconsistent boundary convention.
FloorSQ automates scale calibration from the image and calculates the outer-outline area, but the same review principles apply. Use the visual result to spot spaces that have been included incorrectly and errors in the boundary rather than skipping these checks.