Rebar Calculator Formula

Understand the math behind the rebar calculator. Each variable explained with a worked example.

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Formulas Used

Bars Along Length

bars_lengthwise = floor(slab_width / spacing_ft) + 1

Bars Along Width

bars_widthwise = floor(slab_length / spacing_ft) + 1

Total Bars (both directions)

total_bars = (floor(slab_width / spacing_ft) + 1) + (floor(slab_length / spacing_ft) + 1)

Total Linear Feet

total_linear_ft = (floor(slab_width / spacing_ft) + 1) * slab_length + (floor(slab_length / spacing_ft) + 1) * slab_width

Rebar Sticks to Buy

rebar_sticks = ceil(((floor(slab_width / spacing_ft) + 1) * slab_length + (floor(slab_length / spacing_ft) + 1) * slab_width) / rebar_length)

Variables

VariableDescriptionDefault
slab_lengthSlab Length(feet)20
slab_widthSlab Width(feet)20
spacingRebar Spacing(inches)18
rebar_lengthRebar Length(feet)20
spacing_ftDerived value= spacing / 12calculated

How It Works

How to Calculate Rebar Needed

Formula

Bars Along Length = floor(Width / Spacing) + 1

Bars Along Width = floor(Length / Spacing) + 1

Rebar is laid in a grid pattern (both directions). Standard rebar comes in 20-foot lengths. Overlap joints should be at least 24 bar diameters (about 18 inches for #4 rebar). Common sizes are #3 (3/8"), #4 (1/2"), and #5 (5/8").

Worked Example

A 20×20 ft slab with rebar on 18-inch centers, using 20 ft rebar.

slab_length = 20slab_width = 20spacing = 18rebar_length = 20
  1. 01Spacing in feet = 18 / 12 = 1.5 ft
  2. 02Bars along length = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
  3. 03Bars along width = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
  4. 04Total bars = 14 + 14 = 28 bars
  5. 05Total linear feet = 14 × 20 + 14 × 20 = 560 lin ft
  6. 06Rebar sticks = 560 / 20 = 28 sticks

Ready to run the numbers?

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