A worked example: 4m × 3m slab, 150mm spacing, 12mm bars
A 4m by 3m slab with bars spaced 150mm apart in both directions and a 12mm bar diameter needs 21 bars running lengthwise and 28 running widthwise, for a total bar length of 168m and a total weight of 149.33 kg.
The formula: (dimension ÷ spacing) + 1 bars per direction, weight from diameter²
Bars in each direction = (the opposite dimension ÷ spacing, rounded up) + 1 — the "+1" accounts for a bar at both the start and end of the grid, not just every gap between bars. Weight per meter follows the IS 1786 standard for TMT reinforcement bars: diameter (mm) squared ÷ 162. Multiplying that per-meter weight by the total bar length gives the total weight.
A larger example: 6m × 5m slab, 200mm spacing, 16mm bars
A bigger 6m by 5m slab with wider 200mm spacing but a thicker 16mm bar needs 26 bars lengthwise and 31 widthwise, for 311m of total bar length and 491.46 kg of steel — more than triple the first example's weight, from both a larger grid and a thicker, heavier bar diameter.
Why spacing and diameter pull the weight in opposite directions
Wider spacing means fewer bars, which reduces total length and weight. But a bigger bar diameter increases weight per meter, since diameter enters the IS 1786 formula squared — doubling the diameter quadruples the weight per meter, not just doubles it. The two inputs work against each other, so neither one alone predicts the final weight.