Using the rule of thumb, what is the percentage ratio between total exterior wall surface and floor area?

• A. 85%
• B. 70%
• C. 60%
• D. 50%

Answer: (A)

The rule of thumb here uses a quick way to estimate exterior wall area compared to floor area. Exterior wall area is roughly the wall height times the building’s perimeter, while floor area is length times width. For a typical one-story rectangular tilt‑up, with common dimensions and a ceiling height around 10 ft, the perimeter times height relative to floor area tends to land near 0.85.

For example, if a building footprint is 60 ft by 40 ft, the floor area is 2,400 ft². With 10 ft tall exterior walls, the wall area is about 2 × (60 + 40) × 10 = 2,000 ft². The ratio 2,000 ÷ 2,400 ≈ 0.83, which is close to 85%. Because this is a rough estimate intended for quick planning, 85% is the best fit for the rule of thumb.