Rebar Calculator
Buy 30 bars of rebar.
| Figure | Value |
|---|---|
| Slab area | 400 ft² |
| Bars running lengthwise spaced across the width | 15 |
| Bars running widthwise spaced along the length | 15 |
| Total linear length | 585 ft |
| 20 ft bars to buy | 30 |
| Tie intersections every crossing gets one | 225 (1 roll of tie wire) |
| Weight 0.668 lb per foot | 391 lb |
This is a two-way mat: bars at your spacing in both directions, held back from the slab edge by the concrete cover. Bar counts include the closing bar at each far edge — the one an off-by-one drops. Cover matters structurally: 3 in is standard for concrete cast directly against the ground, since steel that is too close to the surface rusts, expands, and spalls the concrete off. Bar length here is the cut length for a single run; lap splices are not included, and a bar that must be spliced needs an overlap of about 40 bar diameters — 20 in for #4 — so add that where your slab is longer than your stock. Rebar must also sit in the middle of the slab on chairs or bolsters, not on the ground: steel lying on the subgrade does nothing at all. Structural slabs, footings, and anything carrying a building need an engineer's design rather than a rule of thumb.
Enter your slab size and grid spacing to get bar counts each way, total linear feet, stock bars, tie count, and weight.
How to use this calculator
- Enter the slab length and width.
- Pick the bar size — #4 is the residential default — and the grid spacing.
- Set the edge cover: 3 inches for a slab cast against the ground.
- Choose the stock length you can buy. 20 ft is standard; check what fits in your vehicle.
The formula
Effective span = Dimension − (2 × cover) · Bars = floor(Span ÷ Spacing) + 1
Total length = (Bars lengthwise × Effective length) + (Bars widthwise × Effective width) · Ties = Bars lengthwise × Bars widthwise
Worked example — a 20 × 20 ft slab
#4 bar at 16 in on centre, 3 in cover, 20 ft stock.
- Effective mat: 20 − 0.5 = 19.5 ft each way
- Bars each way: floor(19.5 ft ÷ 16 in) + 1 = 14 + 1 = 15
- Total length: (15 × 19.5) × 2 = 585 linear ft
- Stock bars: 585 ÷ 20 = 29.25 → 30 bars
- Ties: 15 × 15 = 225 intersections
- Weight: 585 × 0.668 = 391 lb
Two practical notes. Those 30 bars weigh close to 400 pounds — plan the delivery. And the calculation gives cut lengths for single 19.5 ft runs; since that fits inside 20 ft stock, no splicing is needed here. On a larger slab you would need lap splices, which this does not add.
Bar sizes and weights
The bar number is its diameter in eighths of an inch — #4 is 4/8, or half an inch. Simple once you know it, opaque until you do.
| Size | Diameter | Weight per ft | 20 ft stick | Typical use |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb | Sidewalks, thin slabs |
| #4 | 1/2 in | 0.668 lb | 13.4 lb | Driveways, patios, most residential slabs |
| #5 | 5/8 in | 1.043 lb | 20.9 lb | Heavier slabs, footings, walls |
| #6 | 3/4 in | 1.502 lb | 30.0 lb | Structural work, columns |
Cover: the detail that decides how long the slab lasts
Concrete cover is the depth of concrete between the steel and the nearest surface, and it is not a rounding allowance. Steel needs concrete around it for two reasons: the concrete's alkalinity keeps the steel passive and unrusted, and the thickness keeps moisture and chlorides away from it.
When cover is too thin, the steel rusts. Rust occupies several times the volume of the steel it came from, so it exerts enormous pressure outward and spalls the face of the concrete off — which then exposes more steel, which rusts faster. Almost every crumbling concrete edge you have seen is this failure. The required minimums are:
| Condition | Minimum cover |
|---|---|
| Cast against and permanently exposed to earth | 3 in |
| Formed, then exposed to earth or weather (#6 and larger) | 2 in |
| Formed, then exposed to earth or weather (#5 and smaller) | 1½ in |
| Interior, not exposed to weather | ¾ in |
Rebar on the ground does nothing
This is the single most common way a reinforced slab fails to be reinforced. Steel resists tension, and concrete cracks where it is in tension. In a slab on grade that is generally the middle to upperportion, not the bottom face sitting on the subgrade. A mat lying on the ground is below the neutral axis, surrounded by the wettest, dirtiest concrete in the pour, and contributing essentially nothing.
Use chairs or bolsters — plastic or wire supports that hold the mat at a set height — spaced every 3 to 4 feet so the bars do not sag between them. The old trick of hooking the mesh and pulling it up during the pour rarely works, cannot be checked afterwards, and is why so many slabs have reinforcement in exactly the wrong place.
Splices, and what this calculator does not add
When a run is longer than your stock bar, the bars overlap and are tied together. The lap needs to be long enough to transfer the force from one bar to the next — roughly 40 bar diameters as a rule of thumb, so 20 inches for #4. Splices should be staggered rather than all landing on the same line, which would create a weak plane across the slab.
This calculator gives cut lengths for single runs. If your slab is longer than your stock, add the lap length for every splice. Also not included: dowels tying the slab to footings or adjacent pours, corner bars, and the extra steel around openings.
Common mistakes to avoid
- Laying the mat on the ground. Use chairs. Steel in the wrong place is steel wasted.
- Skimping on cover. Three inches against earth, and keep the mat back from the edges.
- Forgetting the closing bar. A 19.5 ft span at 16 in on centre takes 15 bars, not 14.
- Ignoring splice laps on a slab longer than the stock length.
- Assuming rebar prevents cracking. It does not — concrete cracks regardless. Reinforcement holds the cracks tight so they stay small and the slab keeps working. Control joints are what decide where it cracks.
Structural slabs, footings, retaining walls, and anything supporting a building need an engineer's design. This estimates material for a standard slab-on-grade mat.
How we calculate this
A two-way mat: bars at the given spacing in both directions, held back from each edge by the concrete cover. The effective mat is (length − 2 × cover) by (width − 2 × cover). Bars running lengthwise are spaced across the width and vice versa, and each count includes the closing bar at the far edge — floor(span ÷ spacing) + 1, with a tolerance so an exact division does not silently drop a bar. Total length is the sum of both directions, divided by your stock length and rounded up. Ties are the grid intersections. Weight uses the published unit weights: 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, and 1.502 for #6.
Sources
Frequently asked questions
How much rebar do I need for a 20x20 slab?
With #4 bar at 16 inches on centre and 3 inches of edge cover, you need 15 bars each way over a 19.5 by 19.5 foot mat — 585 linear feet total, which is 30 bars of 20-foot stock. That is 225 tie intersections and about 391 pounds of steel.
What size rebar for a concrete slab?
#4 bar (1/2 inch) at 16 to 18 inches on centre is the usual choice for a residential slab, driveway, or patio. #3 (3/8 inch) is enough for a thin sidewalk, and #5 (5/8 inch) is used where loads are heavier or the slab is structural. Anything carrying a building should be sized by an engineer, not a rule of thumb.
How far apart should rebar be spaced?
12 to 18 inches on centre for most residential slabs, with 16 inches being the common default. Tighter spacing distributes cracking better; wider spacing saves steel but lets cracks open further. A useful check is that spacing should not exceed three times the slab thickness — so a 4-inch slab wants bars no more than 12 inches apart if you want real crack control.
How much rebar overlap is needed at a splice?
About 40 bar diameters is the common rule of thumb, so 20 inches for #4 bar, 15 inches for #3, and 25 inches for #5. Splices must be tied, and they should be staggered rather than all landing in the same line. This calculator gives cut lengths for single runs and does not add splice laps — add them yourself where a run exceeds your stock length.
How high off the ground should rebar sit?
In the middle of the slab, or slightly above the middle for a slab in bending. Rebar lying on the subgrade does essentially nothing — steel only works where the concrete is in tension, and the bottom face of a slab sitting on the ground is not it. Use chairs or bolsters to hold the mat at the right height; the trick of pulling the mesh up as you pour rarely works and is impossible to verify afterwards.
How much concrete cover does rebar need?
3 inches for concrete cast directly against the ground, 1½ inches for formed surfaces exposed to weather, and ¾ inch for interior surfaces. Cover is not a detail — steel too close to the surface takes in moisture, rusts, expands to several times its original volume, and spalls the concrete face off. Most crumbling concrete edges are a cover failure.
How much does rebar weigh?
#3 bar is 0.376 pounds per foot, #4 is 0.668, #5 is 1.043, and #6 is 1.502. The bar number is its diameter in eighths of an inch, so #4 is 4/8 or 1/2 inch. A 20-foot stick of #4 weighs about 13 pounds, which is why 30 of them is a job for a truck rather than a car boot.
Can I use wire mesh instead of rebar?
For a light residential slab, welded wire mesh is a common and acceptable alternative, and it is cheaper and faster to place. But it suffers badly from the positioning problem — flat sheet mesh is easier to keep at the right height than rolled mesh, and mesh trampled to the bottom of a pour provides nothing. Rebar tied into a mat holds its shape and its position, which is why it is preferred anywhere loads matter.