{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"rebar"},"result":{"entity_type":"calculator","id":"rebar","calculator_id":"rebar","canonical_url":"https://tttkmbb.com/construction/rebar","name":"Rebar Calculator","title":"Rebar Calculator – Bars, Total Length, Weight, Stock Lengths with Laps and Chairs for a Slab Grid","category":"construction","category_name":"Construction & Home","tool_name":"estimate_rebar","featured_mcp_tool":false,"description":"Lays out a two-way rebar grid in a rectangular slab from the bar spacing and edge cover (bars per direction = floor((span − 2 × cover) / spacing) + 1), then reports the bar counts, run lengths, total length and weight from the bar's unit mass, the stock lengths needed when runs exceed the stock length (lap = 40 bar diameters per splice), the grid intersections to tie and an estimate of bar chairs. Planning estimate only.","use_when":"You need to order reinforcing bar (metric 10–20 mm or US #3–#6) for a slab, footing pad or driveway with a single-layer grid.","do_not_use_when":"The design requires two layers, bent bars, stirrups or a specific bar schedule from the engineer (use the schedule), or you want welded mesh (divide the area by the sheet size instead). Bar size and spacing are structural decisions, not outputs of this calculator.","inputs":[{"name":"slab_length","label":"Slab length","type":"number","unit":"m or ft","required":true,"max":1000,"exclusive_min":0,"description":"Slab length in metres or feet (see unit).","example":10},{"name":"slab_width","label":"Slab width","type":"number","unit":"m or ft","required":true,"max":1000,"exclusive_min":0,"description":"Slab width in metres or feet.","example":6},{"name":"unit","label":"Length unit","type":"enum","required":false,"default":"m","values":[{"value":"m","label":"Metres (m)","aliases":["metre","meter","metres","meters","metric"]},{"value":"ft","label":"Feet (ft)","aliases":["feet","foot","imperial"]}],"description":"Unit of the length inputs (m or ft). Lengths are reported in both units.","example":"m"},{"name":"spacing_unit","label":"Spacing unit","type":"enum","required":false,"default":"mm","values":[{"value":"mm","label":"Millimetres","aliases":["millimetre","millimeter"]},{"value":"in","label":"Inches","aliases":["inch","inches"]}],"description":"Unit of spacing and edge_cover.","example":"mm"},{"name":"spacing","label":"Bar spacing","type":"number","unit":"mm or in","required":false,"max":2000,"exclusive_min":0,"description":"Centre-to-centre spacing of the bars, the same in both directions; default 300 mm or 12 in.","example":300},{"name":"edge_cover","label":"Edge cover","type":"number","unit":"mm or in","required":false,"min":0,"max":500,"description":"Distance from the slab edge to the end of the bars and to the first bar; default 75 mm or 3 in (concrete cast against earth).","example":75},{"name":"bar_size","label":"Bar size","type":"enum","required":false,"default":"12_mm","values":[{"value":"10_mm","label":"10 mm (0.617 kg/m)","aliases":["10","10mm","n10","y10"]},{"value":"12_mm","label":"12 mm (0.888 kg/m)","aliases":["12","12mm","n12","y12"]},{"value":"16_mm","label":"16 mm (1.578 kg/m)","aliases":["16","16mm","n16","y16"]},{"value":"20_mm","label":"20 mm (2.466 kg/m)","aliases":["20","20mm","n20","y20"]},{"value":"no_3","label":"#3, 3/8 in (0.376 lb/ft)","aliases":["#3","3"]},{"value":"no_4","label":"#4, 1/2 in (0.668 lb/ft)","aliases":["#4","4"]},{"value":"no_5","label":"#5, 5/8 in (1.043 lb/ft)","aliases":["#5","5"]},{"value":"no_6","label":"#6, 3/4 in (1.502 lb/ft)","aliases":["#6","6"]}],"description":"Metric bar diameter or US bar number, with its nominal mass per length.","example":"12_mm"},{"name":"bar_stock_length","label":"Stock bar length","type":"number","unit":"m or ft","required":false,"max":30,"exclusive_min":0,"description":"Length of the bars as supplied (same unit as the slab); default 6 m or 20 ft. Common: 6, 9, 12 m; 20, 30, 40, 60 ft.","example":6},{"name":"lap_multiplier","label":"Lap length (bar diameters)","type":"number","required":false,"default":40,"min":0,"max":100,"description":"Splice lap length as a multiple of the bar diameter; 40 diameters is a common minimum for tension laps in slabs (check the design).","example":40}],"outputs":[{"name":"bars_lengthwise","label":"Bars running lengthwise","type":"integer","decimals":4,"description":"floor((slab_width − 2 × cover) / spacing) + 1, each slab_length − 2 × cover long."},{"name":"bars_widthwise","label":"Bars running widthwise","type":"integer","decimals":4,"description":"floor((slab_length − 2 × cover) / spacing) + 1, each slab_width − 2 × cover long."},{"name":"total_bars","label":"Total bars (runs)","type":"integer","decimals":4,"description":"bars_lengthwise + bars_widthwise."},{"name":"bar_length_lengthwise_m","label":"Length of each lengthwise bar","type":"number","unit":"m","decimals":3,"description":"slab_length − 2 × cover."},{"name":"bar_length_widthwise_m","label":"Length of each widthwise bar","type":"number","unit":"m","decimals":3,"description":"slab_width − 2 × cover."},{"name":"total_length_m","label":"Total bar length","type":"number","unit":"m","decimals":2,"description":"Sum of all run lengths, before laps."},{"name":"total_length_ft","label":"Total bar length","type":"number","unit":"ft","decimals":1,"description":"Same in feet."},{"name":"total_weight_kg","label":"Total weight","type":"number","unit":"kg","decimals":2,"description":"total_length × unit mass, before laps."},{"name":"total_weight_lb","label":"Total weight","type":"number","unit":"lb","decimals":1,"description":"Same in pounds."},{"name":"lap_length_m","label":"Lap length per splice","type":"number","unit":"m","decimals":3,"description":"lap_multiplier × bar diameter."},{"name":"splices","label":"Splices","type":"integer","decimals":4,"description":"Total lap splices needed because runs exceed the stock length."},{"name":"stock_pieces","label":"Stock bars to order","type":"integer","decimals":4,"description":"Per run, the smallest n with n × stock ≥ run + (n − 1) × lap, summed over all runs (offcuts are not reused)."},{"name":"total_length_with_laps_m","label":"Total length incl. laps","type":"number","unit":"m","decimals":2,"description":"total_length + splices × lap_length."},{"name":"grid_intersections","label":"Grid intersections","type":"integer","decimals":4,"description":"bars_lengthwise × bars_widthwise; tie every intersection or at least every second one."},{"name":"chairs_estimate","label":"Bar chairs (approx.)","type":"integer","decimals":4,"description":"ceil(slab area / 0.8 m²), i.e. supports about 0.9 m (3 ft) apart each way."}],"input_schema":{"type":"object","properties":{"slab_length":{"description":"Slab length in metres or feet (see unit). Unit: m or ft.","type":"number","maximum":1000,"exclusiveMinimum":0,"examples":[10],"x-unit":"m or ft"},"slab_width":{"description":"Slab width in metres or feet. Unit: m or ft.","type":"number","maximum":1000,"exclusiveMinimum":0,"examples":[6],"x-unit":"m or ft"},"unit":{"description":"Unit of the length inputs (m or ft). Lengths are reported in both units.","type":"string","enum":["m","ft"],"default":"m","examples":["m"]},"spacing_unit":{"description":"Unit of spacing and edge_cover.","type":"string","enum":["mm","in"],"default":"mm","examples":["mm"]},"spacing":{"description":"Centre-to-centre spacing of the bars, the same in both directions; default 300 mm or 12 in. Unit: mm or in.","type":"number","maximum":2000,"exclusiveMinimum":0,"examples":[300],"x-unit":"mm or in"},"edge_cover":{"description":"Distance from the slab edge to the end of the bars and to the first bar; default 75 mm or 3 in (concrete cast against earth). Unit: mm or in.","type":"number","minimum":0,"maximum":500,"examples":[75],"x-unit":"mm or in"},"bar_size":{"description":"Metric bar diameter or US bar number, with its nominal mass per length.","type":"string","enum":["10_mm","12_mm","16_mm","20_mm","no_3","no_4","no_5","no_6"],"default":"12_mm","examples":["12_mm"]},"bar_stock_length":{"description":"Length of the bars as supplied (same unit as the slab); default 6 m or 20 ft. Common: 6, 9, 12 m; 20, 30, 40, 60 ft. Unit: m or ft.","type":"number","maximum":30,"exclusiveMinimum":0,"examples":[6],"x-unit":"m or ft"},"lap_multiplier":{"description":"Splice lap length as a multiple of the bar diameter; 40 diameters is a common minimum for tension laps in slabs (check the design).","type":"number","minimum":0,"maximum":100,"default":40,"examples":[40]}},"additionalProperties":false,"required":["slab_length","slab_width"]},"output_schema":{"type":"object","properties":{"bars_lengthwise":{"description":"floor((slab_width − 2 × cover) / spacing) + 1, each slab_length − 2 × cover long.","type":"integer"},"bars_widthwise":{"description":"floor((slab_length − 2 × cover) / spacing) + 1, each slab_width − 2 × cover long.","type":"integer"},"total_bars":{"description":"bars_lengthwise + bars_widthwise.","type":"integer"},"bar_length_lengthwise_m":{"description":"slab_length − 2 × cover. Unit: m.","type":"number","x-unit":"m"},"bar_length_widthwise_m":{"description":"slab_width − 2 × cover. Unit: m.","type":"number","x-unit":"m"},"total_length_m":{"description":"Sum of all run lengths, before laps. Unit: m.","type":"number","x-unit":"m"},"total_length_ft":{"description":"Same in feet. Unit: ft.","type":"number","x-unit":"ft"},"total_weight_kg":{"description":"total_length × unit mass, before laps. Unit: kg.","type":"number","x-unit":"kg"},"total_weight_lb":{"description":"Same in pounds. Unit: lb.","type":"number","x-unit":"lb"},"lap_length_m":{"description":"lap_multiplier × bar diameter. Unit: m.","type":"number","x-unit":"m"},"splices":{"description":"Total lap splices needed because runs exceed the stock length.","type":"integer"},"stock_pieces":{"description":"Per run, the smallest n with n × stock ≥ run + (n − 1) × lap, summed over all runs (offcuts are not reused).","type":"integer"},"total_length_with_laps_m":{"description":"total_length + splices × lap_length. Unit: m.","type":"number","x-unit":"m"},"grid_intersections":{"description":"bars_lengthwise × bars_widthwise; tie every intersection or at least every second one.","type":"integer"},"chairs_estimate":{"description":"ceil(slab area / 0.8 m²), i.e. supports about 0.9 m (3 ft) apart each way.","type":"integer"}}},"formula":"bars_lengthwise = floor((W − 2c) / s) + 1 with length L − 2c; bars_widthwise = floor((L − 2c) / s) + 1 with length W − 2c; total_length = Σ bars × length; weight = total_length × kg/m (10 mm 0.617, 12 mm 0.888, 16 mm 1.578, 20 mm 2.466; #3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft); lap = lap_multiplier × diameter; pieces per run = ceil((run − lap) / (stock − lap)); chairs = ceil(area / 0.8 m²).","method":"Single-layer two-way grid with the first and last bars one cover distance from the edges, unit masses from the metric (0.006165 × d² kg/m) and ASTM A615 bar tables, and laps added only where a run is longer than the stock; dowels, bent bars, tie wire (about 0.3 m per intersection) and the second layer of thick slabs are not included. Planning estimate only; check quantities against the supplier's coverage figures and local code before ordering.","sources":[{"name":"Wikipedia – Rebar (size tables)","url":"https://en.wikipedia.org/wiki/Rebar","type":"reference","retrieved_at":"2026-09-24"},{"name":"ICC – 2021 International Residential Code, Chapter 4 Foundations (R403 footings)","url":"https://codes.iccsafe.org/content/IRC2021P2/chapter-4-foundations","type":"standard","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"10 × 6 m slab, 12 mm bars at 300 mm, 75 mm cover, 6 m stock, 40 d laps","inputs":{"slab_length":10,"slab_width":6,"unit":"m","spacing_unit":"mm","spacing":300,"edge_cover":75,"bar_size":"12_mm","bar_stock_length":6,"lap_multiplier":40},"expected":{"bars_lengthwise":20,"bars_widthwise":33,"total_bars":53,"bar_length_lengthwise_m":9.85,"bar_length_widthwise_m":5.85,"total_length_m":390.05,"total_length_ft":1279.7,"total_weight_kg":346.36,"total_weight_lb":763.6,"lap_length_m":0.48,"splices":20,"stock_pieces":73,"total_length_with_laps_m":399.65,"grid_intersections":660,"chairs_estimate":75},"url":"https://tttkmbb.com/api/v1/calculate/rebar?slab_length=10&slab_width=6&unit=m&spacing_unit=mm&spacing=300&edge_cover=75&bar_size=12_mm&bar_stock_length=6&lap_multiplier=40"},{"name":"20 × 12 ft slab, #4 bars at 12 in, 3 in cover, 20 ft stock","inputs":{"slab_length":20,"slab_width":12,"unit":"ft","spacing_unit":"in","spacing":12,"edge_cover":3,"bar_size":"no_4","bar_stock_length":20},"expected":{"bars_lengthwise":12,"bars_widthwise":20,"total_bars":32,"bar_length_lengthwise_m":5.944,"bar_length_widthwise_m":3.505,"total_length_ft":464,"total_length_m":141.43,"total_weight_lb":309.95,"total_weight_kg":140.59,"splices":0,"stock_pieces":32,"grid_intersections":240,"chairs_estimate":28},"url":"https://tttkmbb.com/api/v1/calculate/rebar?slab_length=20&slab_width=12&unit=ft&spacing_unit=in&spacing=12&edge_cover=3&bar_size=no_4&bar_stock_length=20"}],"faq":[{"q":"Which direction is 'lengthwise'?","a":"Bars running parallel to slab_length; they are spaced across the width, so their count comes from the width and their length from slab_length minus cover at both ends."},{"q":"Why 40 bar diameters of lap?","a":"40 d is a common minimum tension lap for deformed bars in slabs (480 mm for 12 mm bars, 20 in for #4); design codes give longer laps for high-strength steel, top bars or low-grade concrete, so use the engineer's figure when there is one."},{"q":"Rebar or welded mesh?","a":"For thin residential slabs (100–150 mm) welded mesh sheets are usually quicker; rebar grids suit thicker slabs, footings and slabs with loads. This calculator counts bars only."}],"tags":["rebar calculator","how much rebar","rebar weight","rebar spacing","rebar for slab","reinforcing bar"],"related":[{"calculator_id":"concrete-slab","reason":"Concrete volume of the slab."},{"calculator_id":"concrete-footing","reason":"Concrete for footings the bars sit in."},{"calculator_id":"steel-weight","reason":"Weight of other steel sections."}],"links":{"html":"https://tttkmbb.com/construction/rebar","markdown":"https://tttkmbb.com/construction/rebar.md","json":"https://tttkmbb.com/construction/rebar.json","api":"https://tttkmbb.com/api/v1/calculate/rebar","schema":"https://tttkmbb.com/api/v1/calculators/rebar","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:45:08Z","next_actions":[{"tool":"run_calculator","calculator_id":"rebar","reason":"Run Rebar Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/construction/rebar.md"}}