Rebar Calculator
Bar sizing is a design decision that belongs to your engineer or your approved drawings. Counting what that design costs you in bars, laps, ties and pounds is a takeoff, and that is what this does.
Give this rebar calculator a slab size, a spacing, a bar size and your stock length. It returns bar counts in each direction, the lap allowance where a run exceeds a stock bar, stock bars to buy, tie intersections and approximate weight for ordering. It lays out the grid you specify; it does not design it.
One takeoff, line by line: 20 × 10 slab, #4 at 18 in on center
- Running lengthwise7 bars at 19.50 ft each, or 136.5 linear ft
- Running crosswise14 bars at 9.50 ft each, or 133.0 linear ft
- Lap allowanceNone: both runs are inside a 20 ft stock bar
- Stock bars to buy269.5 linear ft at 20 ft each, so 14 bars
- Ties and weight98 intersections, about 180 lb of steel
The 3 in clear cover is what shrinks the grid: a 20 ft slab gives 19.5 ft of bar, and a 10 ft width fits 7 lines rather than 8. Ignore the cover and the count comes out one bar high in each direction, which is the most common way a slab takeoff goes wrong before anybody reaches for a lap table.
Bar counts, laps and weight from the grid you specify
Enter the slab dimensions, the rebar spacing on center, your clear cover, the bar size and the stock length your supplier delivers. Turn the grid off if you are running bar in one direction only. The result breaks the takeoff down by direction so you can check it against the plan before you call the yard.
Three inputs that decide the whole count
The slab size is the number everyone gets right. These are the three that move the answer most, and two of them come off the drawings rather than out of a calculator.
Clear cover from the edge
Concrete cast against and permanently exposed to earth is normally detailed with 3 in of cover. Cover is taken off both ends of every dimension, so it reduces both the bar length and the number of lines that fit. Reduce it only if the approved drawings say so, not to make a count come out even.
Spacing on center
Bar count in each direction is the clear dimension divided by the spacing, plus one, because the grid starts and ends with a bar. That plus one is why an 18 in grid on a 10 ft slab gives 7 lines rather than 6, and why halving the spacing does not simply double the steel bill.
Stock length and lap splices
A lap splice only appears where a run is longer than a stock bar, and they are added before the weight is calculated rather than after. On a 40 ft run made of 20 ft sticks, each bar line picks up one lap, and the calculator adds that length to the linear feet you are ordering.
The tool lays out the grid. It does not design it
Bar size, spacing and cover come from your engineer or your approved drawings, and on a slab on grade they are often driven by soil conditions rather than by load. This calculator takes those three as given and does the arithmetic behind them. Where the count is used for ordering rather than for placing, remember that the stock-bar figure assumes offcuts get used elsewhere in the mat: if every bar is cut from a fresh stick, buy the per-direction counts in the table instead. Joints are laid out independently of the mat, because reinforcement running continuously through a control joint is exactly what stops that joint working.
Three things that are quoted wrong more often than right
Reinforcement in residential slabs attracts more confident misinformation than almost anything else on a jobsite. These three are quoted from the 2021 International Residential Code and from ASTM directly, and the first one surprises most people.
“Where provided”
IRC 2021 R506.2.4 opens with those two words, which tells you the code does not generally mandate reinforcement in a slab on ground. R506 sets thickness, base course and vapor retarder. Reinforcement becomes a requirement through local amendment, expansive soils, or a structural slab.
Upper one-third
R506.2.4 requires reinforcement to be supported so it stays from the center to the upper one-third of the slab for the whole placement. Not mid-depth, and certainly not on the subgrade. The word doing the work is supported: chairs hold position under a crew, hooking mesh up does not.
40 bar diameters
This is a field rule of thumb, not a code minimum. Real lap length depends on bar size, grade, concrete strength, cover and confinement. The tool uses 40 diameters so the linear-feet figure is not optimistic, but the governing number is whatever the drawings specify.
| Bar size, Grade 60 deformed bar to ASTM A615 | Nominal diameter | Nominal weight | Lap at 40 diameters |
| #3 | 0.375 in | 0.376 lb/ft | 15 in |
| #4 | 0.500 in | 0.668 lb/ft | 20 in |
| #5 | 0.625 in | 1.043 lb/ft | 25 in |
| #6 | 0.750 in | 1.502 lb/ft | 30 in |
Multiply total linear feet, laps included, by the unit weight to get the order weight. The lap column is the rule-of-thumb figure the calculator applies, shown here so you can compare it against the lap schedule on your drawings before you accept the linear-foot total. Deformed bar to ASTM A615 and welded wire reinforcement to ASTM A1064 are different products with different placement problems, and only one of them holds its height under a crew.
- IRC 2021 R506.2.4 Reinforcement support — where provided in slabs-on-ground, reinforcement shall be supported to remain from the center to the upper one-third of the slab for the duration of the placement.
- IRC 2021 R506.1 General — slab-on-ground floors designed per Section R506 or ACI 332, minimum 3½ in thick, with expansive soils referred to R403.1.8.
- IRC 2021 R506.2.3 Vapor retarder — 10-mil ASTM E1745 Class A with 6 in lapped joints, which is the layer chairs have to sit on without puncturing it.
- ASTM A615 / A615M — the standard specification for deformed and plain carbon-steel bars for concrete reinforcement, which is where the bar designations, nominal diameters and nominal unit weights in the table above come from.
- ASTM A1064 / A1064M — the standard for carbon-steel wire and welded wire reinforcement, the specification behind the mesh alternative discussed above.
Code text read at ICC Digital Codes and figures checked 11 September 2026. We publish no contraction-joint spacing rule as sourced data: the widely repeated figure of 24 to 36 times the slab thickness could not be tied to a specific ACI document and section, so it is left out rather than reprinted. Nominal diameters and unit weights are taken from the ASTM standard itself rather than from a secondary bar chart, and CRSI's detailing material remains the trade reference for placing and supporting the mat. The 40-diameter lap splice is labelled as a field rule of thumb everywhere it appears on this page, because it is not a code minimum, and this tool does not select bar size, spacing or cover.
Rebar questions, answered
Is rebar required in a slab on grade by the IRC?
Generally no. IRC 2021 R506 sets slab thickness, base course and vapor retarder but does not mandate reinforcement in a non-structural slab on ground, and R506.2.4 begins with the words “where provided”. Reinforcement becomes required through a local amendment, in expansive-soil areas, or where the slab is structural. Check with the AHJ before you price it either way.
How many pieces of rebar for a slab 20 by 10 at 18 inch spacing?
Seven bars lengthwise and 14 crosswise with 3 in of clear cover, using #4 bar. The clear dimensions become 19.5 ft and 9.5 ft, so the grid is 269.5 linear feet, which is 14 stock bars at 20 ft and roughly 180 lb of steel with 98 tie intersections. Change the cover and every one of those numbers moves.
Is a rebar lap length of 40 bar diameters a code requirement?
No. It is the common field rule of thumb, around 20 in for a #4 and 25 in for a #5. Actual lap length depends on bar size, grade, concrete strength, cover and confinement, which is why there is no universal number. Use it for a takeoff, and use the lap schedule on your approved drawings for what goes in the ground.
Rebar vs wire mesh for a driveway slab?
Neither prevents cracking; both control crack width after it happens. Welded wire reinforcement to ASTM A1064 is cheaper but frequently ends up on the subgrade instead of held at height. Deformed bar to ASTM A615 on chairs stays where it was set under a crew, which makes it easier to defend as a spec for vehicle loads.
What is the rebar weight per foot for #4 and #5 bar?
Nominal unit weights under ASTM A615 are 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5 and 1.502 for #6. Multiply total linear feet by the unit weight, and add laps before you multiply rather than after. On a 40 ft run built from 20 ft stock, one lap per bar line adds around 1.7 ft of #4 to every line.
Where in the slab should the rebar sit?
Between the center of the slab and the upper one-third, and it has to stay there through the whole placement. That is IRC 2021 R506.2.4, and the operative requirement is that it be supported rather than positioned once and hoped for. Chairs at a spacing the crew cannot walk out of position are what satisfies it in practice.
Accurate steel counts protect one job at a time
Getting the mat right saves the margin on the slab you are already pouring. Getting the phone to ring is what decides whether there is another slab behind it, and no takeoff tool touches that.
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Zayn founded Zainfy in 2023 and has spent six years in performance marketing for home remodeling and construction companies across the United States. He built this tool set because the contractors he works with were losing more money to pricing and takeoff math than to any advertising decision.
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