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Rebar Calculator

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.

Slab length (m or ft)Slab length in metres or feet (see unit). Slab width (m or ft)Slab width in metres or feet. Length unit (optional)Unit of the length inputs (m or ft). Lengths are reported in both units. Spacing unit (optional)Unit of spacing and edge_cover. Bar spacing (mm or in) (optional)Centre-to-centre spacing of the bars, the same in both directions; default 300 mm or 12 in. Edge cover (mm or in) (optional)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). Bar size (optional)Metric bar diameter or US bar number, with its nominal mass per length. Stock bar length (m or ft) (optional)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. Lap length (bar diameters) (optional)Splice lap length as a multiple of the bar diameter; 40 diameters is a common minimum for tension laps in slabs (check the design).

When to use

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.

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).

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.

Inputs

Parameter	Type	Unit	Required	Description
`slab_length`	number	m or ft	yes	Slab length in metres or feet (see unit). Range: > 0, ≤ 1000
`slab_width`	number	m or ft	yes	Slab width in metres or feet. Range: > 0, ≤ 1000
`unit`	enum: m \| ft		default m	Unit of the length inputs (m or ft). Lengths are reported in both units.
`spacing_unit`	enum: mm \| in		default mm	Unit of spacing and edge_cover.
`spacing`	number	mm or in	no	Centre-to-centre spacing of the bars, the same in both directions; default 300 mm or 12 in. Range: > 0, ≤ 2000
`edge_cover`	number	mm or in	no	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). Range: ≥ 0, ≤ 500
`bar_size`	enum: 10_mm \| 12_mm \| 16_mm \| 20_mm \| no_3 \| no_4 \| no_5 \| no_6		default 12_mm	Metric bar diameter or US bar number, with its nominal mass per length.
`bar_stock_length`	number	m or ft	no	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. Range: > 0, ≤ 30
`lap_multiplier`	number		default 40	Splice lap length as a multiple of the bar diameter; 40 diameters is a common minimum for tension laps in slabs (check the design). Range: ≥ 0, ≤ 100

Outputs

Output	Type	Unit	Description
`bars_lengthwise`	integer		floor((slab_width − 2 × cover) / spacing) + 1, each slab_length − 2 × cover long.
`bars_widthwise`	integer		floor((slab_length − 2 × cover) / spacing) + 1, each slab_width − 2 × cover long.
`total_bars`	integer		bars_lengthwise + bars_widthwise.
`bar_length_lengthwise_m`	number	m	slab_length − 2 × cover.
`bar_length_widthwise_m`	number	m	slab_width − 2 × cover.
`total_length_m`	number	m	Sum of all run lengths, before laps.
`total_length_ft`	number	ft	Same in feet.
`total_weight_kg`	number	kg	total_length × unit mass, before laps.
`total_weight_lb`	number	lb	Same in pounds.
`lap_length_m`	number	m	lap_multiplier × bar diameter.
`splices`	integer		Total lap splices needed because runs exceed the stock length.
`stock_pieces`	integer		Per run, the smallest n with n × stock ≥ run + (n − 1) × lap, summed over all runs (offcuts are not reused).
`total_length_with_laps_m`	number	m	total_length + splices × lap_length.
`grid_intersections`	integer		bars_lengthwise × bars_widthwise; tie every intersection or at least every second one.
`chairs_estimate`	integer		ceil(slab area / 0.8 m²), i.e. supports about 0.9 m (3 ft) apart each way.

Example

10 × 6 m slab, 12 mm bars at 300 mm, 75 mm cover, 6 m stock, 40 d laps: {"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} → {"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}

20 × 12 ft slab, #4 bars at 12 in, 3 in cover, 20 ft stock: {"slab_length":20,"slab_width":12,"unit":"ft","spacing_unit":"in","spacing":12,"edge_cover":3,"bar_size":"no_4","bar_stock_length":20} → {"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}

GET 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

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/rebar (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/rebar · Markdown: https://tttkmbb.com/construction/rebar.md · JSON definition: https://tttkmbb.com/construction/rebar.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="rebar"
OpenAPI operationId: estimate_rebar
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Rebar (size tables) (reference)
ICC – 2021 International Residential Code, Chapter 4 Foundations (R403 footings) (standard)

FAQ

Which direction is 'lengthwise'?

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.

Why 40 bar diameters of lap?

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.

Rebar or welded mesh?

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.

Related calculators

Concrete Slab Calculator — Concrete volume of the slab.
Concrete Footing Calculator — Concrete for footings the bars sit in.
Steel Weight Calculator — Weight of other steel sections.