Rebar Calculator
Work out how many rebar bars a slab or footing needs, with lap splices, total linear feet, weight by bar size and cost.
How to use this calculator
- 1Enter the slab or footing dimensions and the bar spacing your drawings call for, in each direction.
- 2Set the edge cover — 3 in for concrete cast against ground — and pick the bar size.
- 3Read the bar count, total linear feet including lap splices, and the weight for ordering.
How the calculation works
n = ⌊(S − 2c) ÷ s⌋ + 1 total steel = Σ n × (ℓ + splices × lap) lap = k × d- n
- Bars in one direction, counting both end bars
- S
- Slab dimension across which the bars are spaced, in inches
- c
- Edge cover — concrete between the steel and the edge, in inches
- s
- Bar spacing on centre, in inches
- ℓ
- Length of one run of bar, after cover is deducted
- k × d
- Lap splice length: a multiple k of the bar diameter d, commonly 40d
The "+1" is the fencepost rule. Bars sit at both ends of the run as well as between, so a 19 ft grid at 12 in centres takes 20 bars, not 19. Every direction gets its own count.
Cover is subtracted from both edges before the count, because steel that reaches the edge of a slab rusts and spalls. Concrete cast against the ground needs 3 in.
A lap splice is the overlap where two bars share load. It is normally specified as a multiple of bar diameter — 40d is a common tension-lap rule of thumb, so a #4 bar laps 40 × 0.5 = 20 in. That steel is real and must be bought.
Weights are ASTM A615 nominal values, which are reproducible from the geometry: 0.2836 lb/in³ × 12 in × πd²/4 gives 0.668 lb/ft for a #4 against the published 0.668.
Worked example
A 20 × 12 ft slab with #4 bar at 12 in centres
- 1.Cover of 3 in on each edge leaves a steel grid of 19.5 × 11.5 ft.
- 2.Bars running lengthwise are spaced across the 11.5 ft width: 11.5 × 12 ÷ 12 = 11.5, floor 11, plus 1 = 12 bars.
- 3.Bars running widthwise are spaced along the 19.5 ft length: 19.5 × 12 ÷ 12 = 19.5, floor 19, plus 1 = 20 bars.
- 4.Grid steel: 12 × 19.5 + 20 × 11.5 = 234 + 230 = 464 linear ft.
- 5.Every run is under the 20 ft stock length, so there are no splices.
- 6.Add 5% waste: 464 × 1.05 = 487 ft. At 0.668 lb/ft that is 326 lb.
Result: 32 bars, 487 linear ft, 326 lb
A 40 ft long footing, where splices start to matter
- 1.Cover leaves a 39.5 × 1.5 ft steel grid.
- 2.Each lengthwise bar runs 39.5 ft, which is longer than a 20 ft stock bar, so it needs 2 pieces and 1 lap.
- 3.A #5 bar is 0.625 in, so a 40-diameter lap is 40 × 0.625 = 25 in, or 2.08 ft.
- 4.Each 39.5 ft run therefore consumes 39.5 + 2.08 = 41.58 ft of steel.
- 5.The lap steel is invisible in the drawing dimensions but must be bought and paid for.
Result: 2 lengthwise runs spliced once each
The two counts people get wrong
Estimating rebar by dividing the span by the spacing is wrong twice over, and the two errors push in opposite directions so they do not cancel.
The first is the fencepost problem. Bars sit at both ends of a run, not just between the gaps, so a 19 ft grid at 12 in centres takes 20 bars. Miss it once in each direction on a large slab and you are two bars short before you start.
The second is lap splices. Rebar is stocked in finite lengths — 20 ft is the common one — so any run longer than stock is made from two or more bars overlapping. That overlap is typically 40 bar diameters, and it is steel you buy but never see in the plan dimensions. On a long footing the laps can add several percent to the order.
Why cover matters more than it looks
Concrete protects steel chemically as well as physically: its high alkalinity keeps a passive oxide film on the bar. Cover is the depth of that protection, and it is a durability requirement rather than a construction convenience.
Steel too close to a surface rusts. Rust occupies several times the volume of the steel it replaces, so it cracks and spalls the concrete off, exposing more steel — a failure that accelerates once it starts. This is why concrete cast against the ground needs 3 in of cover while a formed interior surface can use 3/4 in.
Cover is why the steel grid is always smaller than the slab, and why the bar count comes from the reduced dimension.
What rebar is actually doing
Concrete is strong in compression and weak in tension — roughly a tenth as strong. Steel is the opposite. Reinforced concrete puts the steel where the tension is, which for a simply supported slab is near the bottom, and over a support is near the top.
That is why placement matters as much as quantity. Rebar lying on the ground at the bottom of a pour does nothing useful and is worse than no rebar at all, since it interrupts the concrete without adding tensile capacity. Chairs and bar supports hold the grid at its design height, and they are cheap relative to the consequence.
This calculator prices a grid you specify. It cannot tell you whether that grid is right for your loads, soil or span — that is a structural design question.
What this assumes, and where it stops
Assumptions
- A single flat orthogonal mat of bar, with one layer in each direction.
- Cover is equal on all four edges.
- Lap splices are the specified multiple of bar diameter, and every splice occurs within a run rather than being staggered into extra pieces.
- ASTM A615 nominal weights, which carry a mill tolerance of up to 6% on actual delivered weight.
Limitations
- Sizes a grid you specify — it is not a structural design and cannot tell you whether the spacing or bar size is adequate for your loads.
- Assumes one mat. Slabs with top and bottom mats, or beams and thickened edges, need each element estimated separately.
- Does not include chairs, tie wire, dowels, corner bars, additional steel around openings, or column and beam cages.
- Splices are calculated as though bars are cut from stock in whole pieces per run, which is normal practice but not the only sequencing a crew might use.
Common questions
How much rebar does a concrete slab need?
For a residential slab, #4 bar at 12 to 18 inch centres each way is common, but the right answer comes from a structural design that accounts for loads, soil bearing and slab thickness. Some slabs on well-prepared subgrade use welded wire mesh or fibre instead. Treat any default as a starting point for a conversation with an engineer.
How long should a rebar lap splice be?
A common rule of thumb is 40 bar diameters for a tension lap, so a #4 bar laps 20 inches and a #5 bar 25 inches. The code figure depends on bar size, concrete strength, bar spacing, coating and whether the splice is in tension or compression, so local code or your engineer governs. Never simply butt two bars together.
How much does rebar weigh?
ASTM A615 nominal weights are 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, 1.502 for #6, 2.044 for #7 and 2.670 for #8. Bar numbers are eighths of an inch of diameter, so a #4 is half an inch. Actual mill weight may vary by up to 6% under the standard tolerance.
Why does the calculator subtract cover from the slab size?
Because rebar must not reach the edge of the concrete. Steel near a surface rusts, and rust expands enough to crack the concrete off, which exposes more steel and accelerates the failure. Concrete cast against the ground needs 3 inches of cover, so the steel grid is 6 inches smaller than the slab in each direction.
Sources
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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