Convert between built (gross) and usable (net) floor area, or work out how much you can build on a plot.
Enter the area
Enter a known area and the efficiency ratio, or switch to the buildable mode for a plot.
Usable is about 80% of built area; the exact ratio varies by walls, balconies and shared space. Buildable area follows local coverage and plot ratios.
Built area 100 m² · Usable area 80 m²Floor area converter. Convert between built (gross) and usable (net) area, or work out the buildable area of a plot. Instant in-browser calculation, no account. Last updated: 13 July 2026.
⚖︎ Results are for informational purposes and do not constitute tax advice. For specific situations, consult a licensed accountant or the relevant tax authority.
Usable area is the built area times an efficiency ratio (usable ÷ built) — typically around 0.80, the rest being walls and shared space. Buildable area = plot × the plot ratio:
A 100 m² built flat at 80% efficiency has ≈ 80 m² usable area. A 500 m² plot at a 1.5 plot ratio allows ≈ 750 m² of building.
Usable (net) area is the space you can actually use inside the walls. Built (gross) area also includes the walls and a share of common areas, so it is larger — usually by about 20–25%.
As a rule of thumb, usable is about 80% of built (efficiency 0.80), so built = usable ÷ 0.80 ≈ usable × 1.25. The exact figure depends on wall thickness, balconies and how much shared area is allocated.
Switch to the buildable mode and enter the plot area, the coverage ratio for the footprint and the plot ratio (KAKS / floor-area ratio). Buildable area = plot × plot ratio; footprint = plot × coverage.
UK practice uses GIA (Gross Internal Area) for homes and NIA (Net Internal Area, ~80–85% of GIA) for commercial lettings, plus GEA externally. This tool models the gross-to-net relationship with an efficiency ratio.