Free tool Concrete Metric and imperial

Rebar Calculator: How Much Rebar Does Your Slab Need?

Enter the slab size, grid spacing and bar size. You get the bar count each way, total length with overlap, weight, and how many 20 ft bars to order.

Short version: Count bars both ways: (width / spacing + 1) bars run the length, (length / spacing + 1) bars run the width. Add about 10% for lap splices and waste. 10M bar weighs 0.785 kg per metre, 15M weighs 1.570 kg per metre.

Calculate your rebar

How the calculator works

  • Bars are laid in a grid at the spacing you pick, one at each edge.
  • Overlap and waste covers lap splices where bars join and offcuts.
  • Weights use standard Canadian metric bar sizes: 10M 0.785 kg/m, 15M 1.570 kg/m, 20M 2.355 kg/m.
  • Your drawings or engineer set the bar size, spacing, cover and chairs. Follow them over any calculator.

Need the concrete too? Use the concrete calculator. Footings: concrete footings cost calculator.

Questions people ask

How much rebar do I need for a 24 x 24 slab?

At 16 in (400 mm) spacing each way, a 24 ft by 24 ft slab needs about 19 bars each way, roughly 300 m of bar with 10% for overlap and waste. Use the calculator for your exact size.

What size rebar is used for a slab in Ontario?

10M and 15M are common for residential slabs and garage floors, but the bar size and spacing should come from your drawings or engineer.

How long are rebar bars in Canada?

Suppliers commonly sell 20 ft (6.1 m) lengths, and some stock 6 m or longer bars.

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Updated November 2026. Quantities are estimates; follow your drawings, engineer and supplier.