The job
Modelled, not recorded. Every figure here is the answer the Rebar Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The slab is the floor of a detached two-car garage, 24 ft (7.32 m) by 24 ft (7.32 m), cast on a compacted base. The drawing calls for a single layer of reinforcing bar in a square grid at 18 in (typical residential), the spacing the calculator lists as typical for residential slabs, tied at the crossings and held up on chairs so it ends up within the slab rather than on the ground under it.
That instruction is the drawing's, not the calculator's. Whether a garage floor is reinforced with bar, with welded wire, with fibres or not at all, and at what size and spacing, is a design decision for whoever specifies the slab and the local code. The calculator takes the spacing it is given and counts the bar; it does not choose either.
The lap allowance stays at the calculator's 15 per cent. A run longer than one stock length has to be made of two bars overlapped and tied, and the overlap is bar that does no extra work in the slab but has to be bought; the calculator's second source ties the default to common lap-length practice.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Slab Length | 24 ft (7.32 m) | The slab's outside dimension along one side, the same figure the concrete is ordered from. |
| Slab Width | 24 ft (7.32 m) | The other side; a square slab gives the same number of bars each way, which makes the grid easy to check by eye. |
| Grid Spacing (on-center) | 18 in (typical residential) (450 mm (~18 in, typical residential)) | Left at the calculator's own default. |
| Lap-Splice Allowance (%) | 15 | Left at the calculator's own default. |
The working, step by step
1. Bars running lengthwise
17 bars
Bars that run the length of the slab are spaced across its width: the width divided by the spacing, plus one bar to close the last gap, is 17 bars. Each one is as long as the slab, because the calculator does not deduct the concrete cover at the edges; that keeps the count on the safe side.
2. Bars running widthwise
17 bars
The bars that cross them are counted the same way along the length: 17 bars. On a square slab the two directions are identical, and the grid is the same pattern turned through a right angle.
3. Total bar length (incl. 15% lap splice)
938.4 ft (286 m)
Every bar's length is added up and the lap allowance put on top: 938.4 ft of bar in all. The calculator then divides that total by one stock length and rounds up, which is how it reaches the count of sticks in the headline.
Estimated rebar needed
47 x 20 ft sticks
The order is 47 x 20 ft sticks, the stock length most US yards sell, according to the calculator's first source. Bar is cut and bent on site from those sticks, so the number is a total of steel rather than a cutting list: the offcut from one run becomes the short piece in another. The same slab read in a metric market is counted in that market's longer mill length, 24 x 12 m sticks, which is a different number of bars for the same amount of steel. Ask for the bar size on the drawing; the calculator's count is the same whatever the size, but the weight on the truck is not.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated rebar needed | 47 x 20 ft sticks | Straight stock lengths of reinforcing bar in the size the drawing specifies, with the lap allowance included. |
The allowance, and what it does here
The allowance on this calculator is the lap, 15 per cent of the bar length, and it stands in for the waste factor other calculators carry. It covers the overlaps where a run is made of more than one stick and some of the offcut that results. It does not cover bars at the slab's thickened edge, dowels into a footing, extra bars at re-entrant corners or around a floor drain; those are details on the drawing, and each one is added to the order by hand.
The common mistake
Buying by the slab's area instead of by the grid
A rule of so many feet of bar per square foot sounds handy, but a grid does not scale that way: the number of bars is the width divided by the spacing plus one, so a narrow slab carries proportionally more bar than a wide one, and halving the spacing roughly doubles the bar rather than adding a fixed share. The other common slip is forgetting the lap, which on runs longer than one stick is a real share of the steel. Count the grid each way, as the calculator does, and keep the lap in.
Change the job, and see what moves
The same slab, on a closer grid
Grid Spacing (on-center): 12 in (heavier-duty slabs) (300 mm (~12 in, heavier-duty slabs)) → 69 x 20 ft sticks
Tighten the grid to 12 in (heavier-duty slabs), the spacing the calculator lists for heavier-duty slabs, and the order becomes 69 x 20 ft sticks. The bars each way rise from 17 bars to 25 bars; the spacing is the input that moves this order most, and it is set by the drawing, not by preference.
The same slab, with bars long enough to need no splice
Lap-Splice Allowance (%): 0 → 41 x 20 ft sticks
Take the lap allowance to zero, which would be right only if every run fitted inside one stick, and the count drops to 41 x 20 ft sticks. On this slab every run is longer than a stick, so the zero figure is wrong here: it is shown to make the size of the lap visible, not as an option.
What to run next
- Rebar Chair and Bolster Count Calculator
The grid sits on chairs that hold it at the right height in the slab, counted from the same area and a chair spacing.
- Rebar Lap Splice Length Calculator (ACI 318)
Works out the lap length itself for a bar size and a concrete, which is what the percentage here stands in for.
- Concrete Calculator
The same slab's length, width and thickness give the concrete it is cast in.
Sources
The method and every constant behind these figures belong to the Rebar Calculator, version 1.2.0, last verified 2026-09-17. It rests on:
- Rebar is most commonly sold in 20 ft lengths in North America and in 12 m mill lengths in metric markets (6 m being the common short length); grid spacing for residential slabs is commonly 12-18 in (300-450 mm) on-center in both directions
- A lap-splice allowance, 15% by default, is added to account for the overlap required where rebar sticks join end-to-end across a run longer than one stock length, per common ACI 318 lap-length practice
Frequently asked questions
- How much rebar for a 24 by 24 ft garage slab?
- On a grid at 18 in (typical residential) with the calculator's 15 per cent lap allowance, 47 x 20 ft sticks: 17 bars each way, 938.4 ft of bar in all.
- Does a garage slab need rebar?
- That is a design question for the slab's specifier and the local code, not something this calculator decides. It counts the bar a drawing calls for, at the spacing the drawing gives.
- How much more rebar does a closer grid take?
- At 12 in (heavier-duty slabs) the same slab takes 69 x 20 ft sticks instead of 47 x 20 ft sticks. The number of bars rises with the spacing in both directions at once.