How it is calculated
One bar: A = π·Ø² / 4. Bars at spacing s: a = A · 100 / s in cm²/m (s in cm). Weight: steel density 7850 kg/m³, which gives the well-known 0.00617 · ز kg/m (Ø in mm) – for example 0.888 kg/m for Ø12.
Spacings below 5 cm are marked as too dense – concrete could not flow between the bars. The required area itself comes from the design of the element (bending, minimum reinforcement, crack control). Also check the minimum and maximum spacing and the concrete cover required by the code.
For learning and quick checks only. Real structures must be designed by a licensed engineer according to the valid standards.