Millimetres first
Section sizes are usually given in millimetres, while lengths are often in metres or feet. Always write the unit to avoid confusion.
How common sizes are written
Angle: leg × leg × thickness. Flat: width × thickness. Plate: thickness, then length × width. Round bar: diameter. Hollow section: outside size × wall thickness.
Estimating weight
Mild steel has a density of about 7.85 g/cm³ (7,850 kg/m³).
Plate or sheet: length (m) × width (m) × thickness (mm) × 7.85 = approximate kg.
Round bar: diameter (mm)² ÷ 162 = approximate kg per metre.
These are estimates; actual section weights vary with manufacturing tolerances.