Rebar Calculator

Lay out a grid: bars one way every so many inches, crossing bars the other way at the same spacing. The count each way is the span divided by the spacing, plus one. A 16 × 13 ft slab (5 × 4 m) with #4 bars at 12 in (300 mm) needs about 17 bars one way and 14 the other — roughly 445 linear feet (135 m) of steel before laps. The calculator totals the bars, length and weight for you.

For a typical residential slab, 12 in (300 mm) on centre each way is common; patios and light slabs can go to 16–18 in (400–450 mm), while structural slabs and heavy loads use 6–8 in (150–200 mm). Closer spacing means more steel — the calculator recalculates the bar count and weight when you change it.

#3 (⅜ in / 10 mm) or #4 (½ in / 12 mm) suits most residential slabs and driveways; #4–#5 (12–16 mm) for heavier driveways and footings. Bigger bars carry more load but cost and weigh more. Pick the bar size and the calculator uses its weight per foot to give the total tonnage.

By bar size: #3 is about 0.38 lb/ft (0.56 kg/m), #4 about 0.67 lb/ft (0.99 kg/m), #5 about 1.04 lb/ft (1.55 kg/m). Multiply by the total length to get the weight — the calculator does this and shows the tonnage so you can order and price by weight.

Where two bars join, they lap by roughly 40 times the bar diameter — about 20 in (500 mm) for a #4 (½ in) bar — so the load transfers between them. That is why a lap and waste allowance is added: 10% covers a simple slab, more for a job with many splices. Always confirm the lap length for your bar size and local code.