Rebar Calculator
For slab and pad reinforcement grids. Enter the slab size and grid spacing and get bar counts, total length, stock to buy and the weight you will be tying.
Your results
Worked example
A 5 m by 4 m slab on a 300 mm grid, 10M bars
Bars running the 5 m direction, spaced across the 4 m width: ceil(4 ÷ 0.3) + 1 = 15 bars × 5 m = 75 m. Bars the other way: ceil(5 ÷ 0.3) + 1 = 18 bars × 4 m = 72 m. Total 147 m; with the 5 percent allowance, 154.35 m, which is ceil(154.35 ÷ 6) = 26 stock bars and 154.35 × 0.785 = 121.2 kg of steel.
How this calculator works
For each direction: bars = ceil(span ÷ spacing) + 1 (the +1 is the bar on the near edge), and direction length = bars × bar length. Epsilon-safe ceiling means a span that divides the spacing exactly does not gain a phantom space. The allowance covers laps (typically 40 bar diameters or as specified) and cutting waste; engineered slabs follow the drawings, not this page.
Weights per metre: 10M 0.785 kg, 15M 1.57 kg, 20M 2.355 kg, 25M 3.925 kg; imperial bars #3 0.560 kg, #4 0.994 kg, #5 1.552 kg, #6 2.235 kg.
Common questions
How far apart should slab rebar be?
Residential slabs on grade commonly use 10M bars at 300 to 400 mm each way, or welded wire mesh, but the real answer comes from your building department and the load the slab carries. Driveways and structural slabs often need more. This calculator does quantities for the spacing you enter; it does not design the reinforcement.
How much lap do I need where bars join?
A common rule is 40 times the bar diameter (400 mm for 10M) for lap splices in residential work, but specifications vary. The percentage allowance field lets you carry the lap length your drawings call for.
Can I use this for a wall or footing grid?
The grid math is the same for any rectangular two-way mat: enter the panel size and spacing. Footings with two parallel bars and stirrups are a different layout; count those bars directly.
Estimates only, for quantity planning with the spacing you enter. Structural design, spacing and laps follow your drawings, engineer and local building code.