This rectangle uses two dimensions in the same unit. Because its sides are perpendicular, length times width directly counts the one-square-metre tiles that fit over the surface.
Substitute the measured sides
A = length × width = 8 × 5 = 40 m². Both inputs are exact integers, so the result is the exact integer 40 with no rounding.
Convert the finished area
Forty square metres equals exactly 400,000 cm² and exactly 0.004 ha. Converting the lengths first would also work, but each length factor must then be applied before multiplication.
Area alone does not estimate materials
The 40 m² result does not add waste, openings, slope, coverage, package size, price, or labor. Use a domain-specific estimator only after confirming which real surface and assumptions belong in the project.