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Rebar calculator

73 bars of #4 × 20'= 847 lb
Bars along the length
21 @ 31' 7" (1 lap each)
Bars along the width
31 @ 19' 6"
Laps
21 × 25"
Total bar length
1,268 LF
#4 weight
847 lb (0.42 tons)
20' bars to buy
73 bars
#4

These sketches are here to help you order material and price the work quickly. They are not engineered or shop drawings — always build from your approved project drawings.

Bar size
Mats
Concrete strength (f'c)

ACI Class B lap for #4 in 4,000 psi concrete. It follows the bar size; type your own to change it. Source

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Defaults: ACI Class B lap for the bar and concrete you pick, 3" edge cover, 20' stock bars. Use the numbers on your drawings when they're given.

Stock bar length
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  1. Lap
    lap = 1.3 × 60,000 ÷ (25 × √f'c) × db, rounded up, ≥ 12"
    ⌈1.3 × 60,000 ÷ (25 × √ 4,000 ) × 1/2" ⌉ = 2' 1"
  2. Bars along the length: how many
    n = ⌈(width − 2 × cover) ÷ spacing⌉ + 1
    ⌈( 20' 0" − 2 × 3" ) ÷ 1' 0" ⌉ + 1 = 21
  3. Bars along the length: length of each
    ℓ = (length − 2 × cover) + laps × lap
    30' 0" − 2 × 3" + 1 × 2' 1" = 31' 7"
  4. Bars along the width: how many
    n = ⌈(length − 2 × cover) ÷ spacing⌉ + 1
    ⌈( 30' 0" − 2 × 3" ) ÷ 1' 0" ⌉ + 1 = 31
  5. Bars along the width: length of each
    ℓ = width − 2 × cover
    20' 0" − 2 × 3" = 19' 6"
  6. Total bar length
    Σ = nₓ × ℓₓ + nᵧ × ℓᵧ
    21 × 31' 7" + 31 × 19' 6" = 1,267' 9"
  7. Weight
    W = Σ × 0.668 lb/ft
    1,267' 9" × 0.668 lb/ft = 847 lb
  8. Stock bars to buy
    full stock bars + bars the cut pieces are packed into (longest first, offcuts reused)
    21 full + 52 for cut pieces = 73

Where the numbers come from

  • Rebar weights (ASTM A615): #3 0.376 · #4 0.668 · #5 1.043 · #6 1.502 · #7 2.044 · #8 2.670 · #9 3.400 · #10 4.303 · #11 5.313 lb/ftASTM A615 Table 1 nominal weights, as listed on Harris Supply Solutions' "Concrete Reinforcing Bar Products" sheet. Each equals the bar area × 3.40 lb/ft per in² (steel at about 490 lb/ft³): #4 = π × 0.5² ÷ 4 = 0.1963 in² × 3.40 = 0.668 lb/ft.Source
  • Edge cover: 3"ACI 318-19 Table 20.5.1.3.1: 3" for concrete cast against and permanently in contact with ground. Formed edges exposed to earth or weather need 1-1/2" (#5 and smaller) or 2" (#6 and larger). Follow your drawings.Source
  • Lap splice (ACI 318-19 Class B): 4,000 psi, Grade 60, uncoated: #3 19" · #4 25" · #5 31" · #6 37" · #7 54" · #8 62" · #9 70" · #10 79" · #11 87"Class B tension lap = 1.3 × ℓd (§25.5.2.1), ℓd from Table 25.4.2.3: fy ÷ (25 λ √f'c) × db for #6 and smaller, ÷ 20 for #7 and larger, rounded up, at least 12". Assumes bars not cast with more than 12" of concrete below them, uncoated, cover ≥ db and spacing ≥ 3 db — usual for a slab grid. Lightweight concrete: λ = 0.75 (longer lap). Same as Table 1.11 of the Dayton Superior Rebar Splicing Handbook (Other bars, Case 1, Class B) at 3,000 and 4,000 psi; at 5,000 psi #4, #6, #8 and #10 come out 1" longer here because they are rounded up. Use the lap on your structural drawings when it is given.Source

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Works out a rebar grid for a slab: how many bars each way, how long, the laps, the weight and how many stock bars to buy.