RebarMath Tip

Rebar calculator

Enter the slab size and bar spacing and RebarMath lays out the grid: how many bars run each way and how long they are after edge cover, where runs longer than a stock bar need a lap splice, and how many 10, 20 or 40 ft bars to buy with waste. It also gives the weight, the number of tie points and roughly how many chairs to hold the mat up. The example is a 30 × 20 ft slab with #4 bars at 12 in on center: 64 bars of 20 ft.

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    Switch to "Footing" for continuous footings, or "Weight & lap" for unit weights and lap lengths.

    How the rebar calculator works

    RebarMath lays out a grid. In each direction the number of bars is the clear span (the slab dimension minus edge cover on both sides) divided by the spacing, rounded down, plus one; each bar is as long as the other dimension minus cover. A run longer than a stock bar is made of lapped pieces: each extra piece adds one lap of 40 bar diameters (20 in for #4) by default. The total length plus waste, divided by the stock length, gives the number of bars to order, and weight uses the standard ASTM A615 unit weights. Footings use continuous bars along the length plus optional cross bars.

    Rebar sizes, weights and laps

    BarDiameterWeight per ft20 ft barLap at 40 db
    #30.375 in0.376 lb7.52 lb15 in
    #40.5 in0.668 lb13.36 lb20 in
    #50.625 in1.043 lb20.86 lb25 in
    #60.75 in1.502 lb30.04 lb30 in
    #70.875 in2.044 lb40.88 lb35 in
    #81 in2.67 lb53.4 lb40 in

    The 40-diameter lap is a common rule of thumb for slabs on grade; structural laps under ACI 318 depend on concrete strength, bar position, coating and cover and can be much longer, and plans or local codes govern. Bar counts here do not optimize cutting, so check your cut list before ordering. Results are estimates for planning, not engineering.

    Frequently asked questions

    How much rebar do I need for a slab?

    Count the bars each way (span minus cover, divided by spacing, plus one) and multiply by their lengths. A 30 × 20 ft slab with #4 at 12 in needs about 1,208 ft, or 64 bars of 20 ft with 5% waste.

    What size rebar for a 4 inch slab?

    #3 or #4 bars at 12–18 in on center are common for 4 in slabs on grade; driveways and garages often use #4 at 12–16 in.

    How much cover does rebar need?

    Commonly about 3 in from the edges and soil, with the bars in the middle to upper third of the slab thickness.

    Is RebarMath free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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