Calculators

Rebar Calculator

Bars, lap splices, weight and sticks to buy for a slab, footing or deck grid.

How it works

A grid is two sets of parallel bars. The number of bars in each direction is how many spacings fit across the other dimension, plus one for the starting bar, after pulling the steel back from each edge by the concrete cover. Bars that run longer than a stock stick are spliced by overlapping them, typically 40 bar diameters, so each splice adds material. Total footage is then rounded into whole sticks, with 10% extra for cut-offs.

bars = ⌊(width − 2 × cover) ÷ spacing⌋ + 1 length per bar = run + splices × (lap × bar diameter)
cover
Concrete between the steel and the outside face
spacing
Centre-to-centre distance between bars
lap
Overlap at a splice, in multiples of bar diameter
#4
Bar size in eighths of an inch: #4 is 1/2 in

Worked example

A 24 × 20 ft slab with #4 bars at 12 in on centre, 3 in cover, 20 ft sticks and 40× laps.

  1. 1Steel area after cover: 23.5 × 19.5 ft
  2. 2Lengthwise bars: ⌊19.5 ÷ 1⌋ + 1 = 20, each 23.5 ft, so one 20 in splice apiece: 20 × 25.17 = 503 ft
  3. 3Widthwise bars: ⌊23.5 ÷ 1⌋ + 1 = 24, each 19.5 ft, no splice: 24 × 19.5 = 468 ft
  4. 4Total: 971 ft, plus 10% = 1,068 ft, ÷ 20 = 54 sticks

Buy 54 sticks of #4 rebar, about 650 lb of steel.

Frequently asked questions

What rebar spacing is used for a concrete slab?

A 4-inch residential slab on grade commonly uses #3 bars at 18 inches or #4 bars at 12 to 18 inches each way, though many driveways and patios use wire mesh or fibre instead. Heavier-duty slabs use tighter spacing or larger bars. Spacing should never exceed three times the slab thickness or 18 inches under most codes.

How long should a lap splice be?

40 bar diameters is a common rule of thumb for Grade 60 bar in ordinary concrete, which is 15 in for #3, 20 in for #4 and 25 in for #5. The required length under ACI 318 depends on concrete strength, bar position and spacing, and can be longer. Your drawings will specify it for structural work.

How much concrete cover does rebar need?

ACI 318 calls for 3 inches where concrete is cast against and permanently exposed to earth, as in footings. It calls for 1½ to 2 inches for concrete exposed to weather or soil but formed, and ¾ inch for interior slabs not exposed to weather. Cover protects the steel from corrosion, so skimping on it shortens the life of the concrete.

How much does rebar weigh?

Per foot: #3 is 0.376 lb, #4 is 0.668 lb, #5 is 1.043 lb and #6 is 1.502 lb. A 20 ft stick of #4 is about 13.4 lb, which matters for delivery and handling. Weight is also how steel is often priced on large orders.

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