Rebar Calculator
Turn a slab's dimensions and grid spacing into bar counts, linear feet, and 20-ft lengths to buy.
Slab dimensions
Spacing and bar size are structural decisions set by your plans, engineer, or local code — this calculator turns a specified grid into a shopping quantity; it does not tell you what grid your slab needs.
Results
For a 20 ft × 10 ft slab on a 12" grid, you'll need 11 bars one way and 21 the other — about 24 twenty-foot rebar lengths including a 10% lap-splice allowance.
20-ft rebar lengths to buy
24 bars
That's a typical residential quantity — a patio, driveway, or garage slab grid.
Plan on delivery or a full-size truck, and buy chairs and tie wire in the same order: the grid only works held at mid-slab height, and supports are the item most often forgotten until the morning of the pour.
Grid (bars each direction): 11 × 21 bars
Total length incl. 10% lap allowance: 473 linear ft
How this works
The calculator lays a two-way grid over your rectangular slab at the spacing you select. Bar counts are pure geometry: bars running along the slab's length are distributed across its width, so their count is the width divided by the spacing — rounded up, because a partial space at the far side still needs a bar bounding it — plus one, since a row of N spaces is bounded by N + 1 gridlines (the fence-post principle; both edges get a bar). The perpendicular direction is computed the same way from the length. Each bar's length is the slab dimension it runs along, giving total linear footage. That footage is then increased by a 10% lap-splice allowance: stock bars are finite (20 ft in this model, the standard US retail length), so runs longer than one bar are joined by overlapping the bars — codes size these laps in bar diameters, commonly around 40 diameters for tension laps per standard concrete reinforcement guidance, and a flat 10% on total footage is the established estimating convention for slab grids. Finally, the total with laps is divided by the 20-ft stock length and rounded up to whole bars, since that's what the supplier sells. Deliberately out of scope: structural design (spacing and bar size must come from your code, plans, or engineer — this tool quantifies a specified grid, it does not validate one), supports and tie wire, edge cover deductions (the full-dimension bar lengths slightly overstate each bar, a conservative simplification), and concrete volume, which is an independent calculation. All computation happens in canonical SI units; the unit toggle converts the display losslessly Lap-splice convention (~40 bar diameters; ~10% takeoff allowance): CRSI reinforcing steel placement guidance.
Worked example
A 20 ft × 10 ft driveway slab on a 12" grid: across the 10-ft width, ceil(10 ÷ 1) + 1 = 11 bars run the 20-ft length (220 linear ft); across the 20-ft length, ceil(20 ÷ 1) + 1 = 21 bars run the 10-ft width (210 linear ft). Total: 430 linear feet. Adding the 10% lap-splice allowance gives 473 ft, and dividing by the 20-ft stock length gives 23.65 — so you buy 24 bars. In #4 bar that's roughly 316 lb of steel, which is worth knowing before deciding how to get it home.
A second example: a square shed slab on a 16-inch grid
A 24 ft × 24 ft shop slab on a 16" grid: 24 ft is 288 inches, and 288 ÷ 16 = 18 spaces exactly, so each direction needs 19 bars. By symmetry both directions are identical: 19 bars × 24 ft × 2 directions = 912 linear feet. With the 10% lap allowance that's 1,003 ft, and at 20 ft per stock bar the buy quantity is 51 bars. Note what the exact division did: because 24 ft is a whole multiple of 16", no partial space exists — shifting the slab to 25 ft would add a rounded-up space and two more bars per direction.
Common mistakes
Treating this estimate as structural advice
The calculator converts a specified spacing into a purchase quantity — it cannot tell you whether your slab needs #3 at 18" or #4 at 12", which depends on thickness, loads, and soil. Get the spacing and bar size from your local code, plans, or an engineer first, then use this tool for the takeoff. Reversing that order is the most consequential mistake on this page.
Forgetting the plus-one edge bar in a hand calculation
Dividing 20 ft by 12" spacing and buying 20 bar runs leaves the far edge of the grid unbounded — the fence-post error. Every direction needs spaces-plus-one bars. On a large slab the missing edge bars are a real shortage discovered mid-pour, when it's least fixable.
Ignoring lap splices when dimensions exceed the stock bar length
A 30-ft run isn't one-and-a-half 20-ft bars — the joint needs an overlap of roughly 40 bar diameters, tied with wire, so it consumes more than the run's nominal length. Buying exact linear footage with no lap allowance guarantees running short on any slab with a dimension over 20 ft.
Letting the grid sit on the ground during the pour
Rebar at the bottom of the slab does almost nothing for crack control. Support the grid on chairs or dobies so it ends up in the slab's middle third, and walk boards — not the grid — during the pour. This is a placement mistake rather than a quantity mistake, but it wastes every dollar the quantity represents.
Skipping tie wire and supports in the budget
Every intersection (or every other one, on tight grids) gets tied, and the grid needs supports every few feet both ways. Tie wire, chairs, and the tying tool are cheap individually but add up on a big slab — and none of them are in this calculator's bar count.
Not checking clearance and cover at the edges
Bars run to the slab edge corrode and spall the concrete ('rust jacking'). Hold the grid back for the required cover — commonly 1.5"–3" depending on exposure — and don't let bar ends project into expansion-joint gaps against existing structures.
Confusing rebar with the concrete estimate
Rebar spacing has no effect on concrete volume, and slab thickness has no effect on bar count — they're independent takeoffs from the same dimensions. Run the concrete volume separately (see the linked Concrete Calculator) rather than assuming one estimate covers the other.
When this calculator's model stops applying
- Slabs specified with different spacing in each direction: run the tool once per spacing and combine the directional counts manually, as described in the FAQ — a single run with one spacing will be wrong in one direction.
- Non-rectangular slabs (L-shapes, notches, curved edges): break the shape into rectangles and run each separately; bars crossing between rectangles can sometimes be shared, so summing runs is slightly conservative.
- Two-layer (top and bottom mat) reinforcement, specified for structural slabs and thick foundations: double the grid count and add the chairs/standees that hold the top mat — this tool models a single mat only.
- Footings, grade beams, walls, and columns: linear elements with stirrups or ties follow different counting rules entirely; only the rectangular slab grid is modeled here.
- Slabs longer than about 40 ft in one dimension: multiple laps per run start compounding, and the flat 10% allowance loses accuracy — count laps per run explicitly (runs ÷ stock length, rounded up, minus one lap per run) for long slabs.
- Fiber-reinforced or post-tensioned slabs: fiber dosing replaces or supplements mesh in some designs, and post-tensioning uses tendons, not a passive bar grid — neither maps to this calculator's math.
- Metric-market stock lengths: the 20-ft stock bar is a US convention; 6 m or 12 m stock changes the buy count and lap frequency even for the same grid.
FAQ
What spacing should I choose — 12", 16", or 18"?
Spacing is a structural decision, not a preference: it depends on slab thickness, loading, soil conditions, and local code. As orientation, residential slabs-on-grade commonly use #3 or #4 bar on 12"–18" centers, with 12" for driveways and loaded slabs and 16"–18" for lighter-duty patios and shed floors — but your local building department or the project's engineer sets the actual requirement. This calculator turns whatever spacing you're told to use into a shopping quantity; it does not tell you what spacing is structurally adequate.
Why is the bar count 'dimension ÷ spacing, rounded up, plus one'?
Think of a fence: spaces between posts number one fewer than the posts. A 10-ft width at 12" spacing has 10 spaces, which takes 11 bars — one at each edge plus one at every interior gridline. When the division isn't exact (say 10.5 ft at 12"), the partial space still needs a bar bounding it, so the space count rounds up before adding the final edge bar. That's the ceil(dimension ÷ spacing) + 1 formula.
What is a lap splice and why does the calculator add 10%?
Stock rebar comes in fixed lengths (20 ft in this model). Where a slab dimension exceeds a stock bar, two bars are joined by overlapping them and tying the overlap — a lap splice. Codes size laps in bar diameters (a common tension-lap figure is around 40 diameters, roughly 20" for #4 bar), and the standard estimating shortcut for slab grids is a flat ~10% allowance on total linear footage, which is what this calculator applies. For a slab whose dimensions are all under 20 ft, the allowance is conservative headroom for placement adjustments.
What bar size (gauge) does this assume?
None — bar counts and linear footage are pure geometry, identical for #3, #4, or #5 bar. Bar size matters for the lap length (bigger bars need longer laps), the weight of your order, and of course the structural adequacy. Residential slabs typically use #3 (3/8") or #4 (1/2"); confirm the specified size before ordering, since suppliers price and stock by size.
Should the rebar grid stop short of the slab edge?
Yes — reinforcement needs concrete cover to protect it from corrosion. For concrete cast against earth, the standard cover requirement is 3"; for formed edges exposed to weather it's commonly 1.5"–2". In practice the grid is held back about 2–3" from each edge. This calculator uses the full slab dimensions for bar lengths, which slightly overstates each bar — deliberate, since the excess is a fraction of the lap allowance and rounding already in the estimate.
Do I need rebar chairs or supports?
Yes, and they're not in this count. Rebar only works if it sits in the correct plane of the slab (typically the middle, or slightly below for slab-on-grade), which requires chairs, dobies, or brick supports at roughly 3–4 ft intervals both ways. Pulling the grid up by hand while pouring ('hooking') is unreliable. Budget supports and tie wire as separate line items.
Is welded wire mesh a substitute for a rebar grid?
For light-duty slabs, often yes — welded wire reinforcement (WWR) in sheets or rolls serves the same crack-control role and is faster to place. Heavier slabs, driveways with vehicle loads, and anything engineered typically specify rebar. If your plans or local code allow either, compare cost: mesh usually wins on labor, rebar on material flexibility. This calculator models rebar grids only.
Does this calculator handle footings, walls, or columns?
No — it models a two-way grid in a rectangular slab. Footings use longitudinal bars with stirrups, walls use vertical and horizontal curtains, and columns use verticals with ties, all counted differently. For a block wall's vertical reinforcement, the count is driven by the grouted-cell spacing, not slab-grid math.
Can I cut rebar myself, and does cutting change the estimate?
Yes — rebar cuts with an angle grinder, a portable bandsaw, or a rebar cutter/bender, and cutting is assumed: the calculator buys whole 20-ft stock bars and your grid uses pieces of them. The lap allowance absorbs the fact that offcuts don't always chain perfectly into the next run. If your supplier offers cut-to-length service, order bar lengths matching your grid runs plus laps, and you can trim the waste below what stock-bar buying implies.
How much does the rebar for a typical slab weigh?
Standard weights per foot: #3 bar is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft. Our first worked example's 473 linear feet in #4 bar is about 316 lb — significant for transport. Twenty-four 20-ft bars won't go in a sedan; plan on delivery, a truck, or having bars cut in half (with extra laps accounted for) at the supplier.
Does the calculator work in metric?
Yes — the unit toggle converts dimensions and results between feet and metres losslessly, computing internally in canonical SI units. The spacing options are labeled in inches with centimetre equivalents (12" ≈ 30 cm, 16" ≈ 40 cm, 18" ≈ 45 cm), and the stock bar is the US-market 20-ft length; metric markets sell 6 m and 12 m bars, so adjust your interpretation of the buy count accordingly.
Do the two grid directions have to use the same spacing?
Structurally, not always — engineered slabs sometimes specify different spacing each way. This calculator applies one spacing to both directions, which matches how residential slab-on-grade grids are typically specified. If your plans call for different spacings, run the tool twice (once per direction's spacing), take the bars-along-length count from one run and bars-along-width from the other, and sum the linear footage manually.