{
  "id": "rebar",
  "version": "c1232e814a93",
  "status": "published",
  "name": "Rebar Calculator",
  "question": "How much rebar do I need?",
  "summary": "Works out the rebar grid for a concrete slab: bars each way from the size, spacing and edge cover, laps where a run is longer than a stock bar, waste, the number of stock bars to buy, and their weight.",
  "category": "construction",
  "subcategory": "concrete",
  "url": "https://www.acalculator.org/construction/rebar-calculator",
  "markdown": "https://www.acalculator.org/construction/rebar-calculator.md",
  "kind": "function",
  "method": "Bars along the length = ⌊(width − 2 × cover) ÷ spacing⌋ + 1, each (length − 2 × cover) long, and the same the other way; a run longer than a stock bar gets ⌈(run − stock) ÷ (stock − lap)⌉ laps of 40 bar diameters; total = (runs + laps) × (1 + waste); stock bars = ⌈total ÷ stock length⌉; weight = feet × lb/ft.",
  "assumptions": [
    "One layer of bars at the same spacing both ways, as in most slabs on the ground.",
    "Laps of 40 bar diameters and 10% waste are Hercules Chair’s rules of thumb, not a design. Your plans, engineer and local code set the bar size, spacing, cover and lap length.",
    "Weights per foot are nominal; the real weight varies slightly by maker."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "l": {
        "title": "Slab length",
        "description": "The long side of the slab.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.1,
        "maximum": 1000
      },
      "w": {
        "title": "Slab width",
        "description": "The short side of the slab.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.1,
        "maximum": 1000
      },
      "s": {
        "title": "Spacing",
        "description": "The distance between bars, centre to centre, the same both ways.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.025,
        "maximum": 3
      },
      "c": {
        "title": "Edge cover",
        "description": "How far the bar ends stay back from each edge of the slab.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0,
        "maximum": 1
      },
      "bar": {
        "title": "Bar size",
        "description": "The US bar size, with its diameter in inches and the metric size it matches.",
        "type": "string",
        "enum": [
          "3",
          "4",
          "5",
          "6",
          "7",
          "8"
        ]
      },
      "stock": {
        "title": "Stock bar length",
        "description": "The length of the bars you buy (20 ft is the most common).",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 1,
        "maximum": 30
      },
      "waste": {
        "title": "Waste allowance",
        "description": "Extra steel for cutting offcuts and mistakes, in percent.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 50
      }
    }
  },
  "outputs": {
    "bars": {
      "label": "Stock bars to buy",
      "description": "The total length with waste ÷ the stock bar length, rounded up.",
      "format": "integer"
    },
    "alongLength": {
      "label": "Bars along the length",
      "description": "Bars that run the long way, across the width.",
      "format": "integer"
    },
    "alongWidth": {
      "label": "Bars along the width",
      "description": "Bars that run the short way, along the length.",
      "format": "integer"
    },
    "feet": {
      "label": "Total length (ft)",
      "description": "All the steel with laps and waste, in feet.",
      "format": "number"
    },
    "meters": {
      "label": "Total length (m)",
      "description": "All the steel with laps and waste, in metres.",
      "format": "number"
    },
    "pounds": {
      "label": "Weight (lb)",
      "description": "Total length in feet × the bar’s weight per foot.",
      "format": "number"
    },
    "kilograms": {
      "label": "Weight (kg)",
      "description": "The same weight in kilograms.",
      "format": "number"
    },
    "laps": {
      "label": "Lap splices",
      "description": "How many overlaps join bars in runs longer than a stock bar.",
      "format": "integer"
    },
    "lapInches": {
      "label": "Lap length (in)",
      "description": "40 bar diameters.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "l": 6.096,
      "w": 3.6576000000000004,
      "s": 0.4572,
      "c": 0.0508,
      "bar": "4",
      "stock": 6.096,
      "waste": 10
    },
    "outputs": {
      "bars": 18,
      "alongLength": 8,
      "alongWidth": 14,
      "feet": 352.73333333333335,
      "meters": 107.51312,
      "pounds": 235.62586666666667,
      "kilograms": 106.87809529463733,
      "laps": 0,
      "lapInches": 20
    },
    "text": "Buy 18 stock bars (352.7 ft of steel, about 236 lb)."
  },
  "examples": [
    {
      "given": {
        "l": 6.096,
        "w": 3.6576000000000004,
        "s": 0.4572,
        "c": 0.0508,
        "bar": "4",
        "stock": 6.096,
        "waste": 10
      },
      "expect": {
        "alongLength": 8,
        "alongWidth": 14,
        "laps": 0,
        "feet": 352.73333333333335,
        "meters": 107.51312,
        "bars": 18,
        "pounds": 235.62586666666667,
        "kilograms": 106.87809529463733,
        "lapInches": 20
      },
      "source": "hand calculation in content.mdx: runs 236 in and 140 in; 8 × 236 + 14 × 140 = 3,848 in = 320.667 ft; × 1.1 = 352.733 ft; ⌈352.733 ÷ 20⌉ = 18; × 0.668 lb/ft = 235.6 lb; Hercules Chair, How to Calculate Rebar for a Slab (lap at least 40 bar diameters; 10% waste; 18–24 in spacing for flat slabs; 2 in edge cover), https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/; Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html"
    },
    {
      "given": {
        "l": 12.192,
        "w": 3.048,
        "s": 0.30479999999999996,
        "c": 0.07619999999999999,
        "bar": "4",
        "stock": 6.096,
        "waste": 10
      },
      "expect": {
        "alongLength": 10,
        "alongWidth": 40,
        "laps": 20,
        "feet": 889.1666666666666,
        "bars": 45,
        "pounds": 593.9633333333334
      },
      "source": "hand calculation in content.mdx: run 474 in, ⌈(474 − 240) ÷ (240 − 20)⌉ = 2 laps; Hercules Chair, How to Calculate Rebar for a Slab (lap at least 40 bar diameters; 10% waste; 18–24 in spacing for flat slabs; 2 in edge cover), https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/; Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html"
    },
    {
      "given": {
        "l": 6,
        "w": 4,
        "s": 0.3,
        "c": 0.05,
        "bar": "5",
        "stock": 6,
        "waste": 5
      },
      "expect": {
        "alongLength": 14,
        "alongWidth": 20,
        "meters": 168.63,
        "bars": 29,
        "kilograms": 261.7398964937116
      },
      "source": "hand calculation in content.mdx: 14 × 5.9 + 20 × 3.9 = 160.6 m; × 1.05 = 168.63 m; ⌈168.63 ÷ 6⌉ = 29; Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html (#5 1.043 lb/ft)"
    }
  ],
  "sources": [
    "Harris Supply Solutions, Steel Rebar Sizes: US bar sizes #3 to #8 with nominal diameter and weight per foot; standard lengths of 20 and 30 ft. https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html (retrieved 2026-10-02)",
    "Hercules Chair (T. J. Harris Co.), How to Calculate Rebar for a Slab the Right Way: lap at least 40 times the bar diameter; 10% waste; 18 to 24 in spacing for flat slabs, 12 in for footings; 2 in cover for slab bottoms and walls. https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/ (retrieved 2026-10-02)",
    "NIST Special Publication 811, Appendix B (1 ft = 0.3048 m, 1 in = 2.54 cm, 1 lb = 0.45359237 kg exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)"
  ],
  "related": [
    "concrete",
    "concrete-block",
    "steel-weight",
    "cubic-yards"
  ],
  "changelog": []
}