How many bars, how many feet and how much weight a slab grid needs — including the plus-one people forget.
Rebar Calculator
Inputs
18 in is common for residential slabs. 12 in for heavier loading.
Overlap where bars join. Typically 40 × bar diameter.
Results
Total rebar
816.00 ft
bars = (span ÷ spacing) + 1 each way; total ft × lb/ft = weight
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Rebar in a slab is a grid, and the quantity is a counting problem: how many bars fit each way at your spacing, times the span they cross. The one detail people get wrong is the plus-one — a 24 ft span at 18 inch spacing takes 17 bars, not 16, because you need one at each edge.
From total length, weight follows directly from the bar size, and weight is what suppliers price on.
n = ⌊span ÷ spacing⌋ + 1total = n_across × length + n_along × widthlb = total ft × lb per ft for the bar sizesticks = total ft ÷ 20 (standard bar length)typically 40 × bar diameterSpacing counts gaps, not bars. A 24 ft span divided by 1.5 ft spacing gives 16 gaps — which needs 17 bars to create them. Forget it on both directions of a large slab and you are short by a meaningful number of sticks.
US rebar sizes are eighths of an inch. A #3 is ⅜ inch, #4 is ½ inch, #5 is ⅝ inch. Residential slabs typically use #3 or #4; footings and structural work go up from there.
Rebar comes in 20 ft sticks. Any run longer than that needs a joint, and the two bars must overlap enough to transfer load between them — conventionally 40 times the bar diameter, so 20 inches for a #4. Butted bars transfer nothing.
Rebar works in the middle third of the slab thickness, held there on chairs before the pour. Bars lying on the base do nothing at all — the steel needs concrete on both sides to work in tension. Pulling bars up mid-pour is unreliable and widely done.
A standard residential garage slab with a #4 grid.
816 ft — 41 sticks, 545 lb
Without the plus-one you would count 16 bars each way and order 768 ft — 48 ft short, which is more than two sticks.
Lighter reinforcement in a non-structural slab.
220 ft — 11 sticks, 82.7 lb
At 24 inch spacing with #3 bar this is light reinforcement, which suits a patio. A driveway would want #4 at 18 inches.
Heavier grid for a slab carrying equipment.
1,660 ft — 83 sticks, 1,731 lb
Nearly a ton of steel. The 40 ft runs also need lap splices, since bars come in 20 ft lengths — add 40 × the bar diameter at each joint.
| Size | Diameter | Weight per ft | Typical use |
|---|---|---|---|
| #3 | ⅜ in | 0.376 lb | Patios, light slabs |
| #4 | ½ in | 0.668 lb | Driveways, garage slabs |
| #5 | ⅝ in | 1.043 lb | Heavy slabs, footings |
| #6 | ¾ in | 1.502 lb | Structural footings, walls |
| #7 | ⅞ in | 2.044 lb | Structural |
| #8 | 1 in | 2.670 lb | Structural |
Non-structural guidance. Anything carrying load needs an engineer.
| Application | Bar size | Spacing |
|---|---|---|
| Patio, walkway | #3 | 24 in |
| Garage slab | #4 | 18 in |
| Driveway | #4 | 18 in |
| Heavy vehicle slab | #5 | 12 in |
| Strip footing | #4 or #5 | 2–3 bars continuous |
Conventional 40 × diameter. Structural work may require more.
| Bar size | Diameter | Lap length |
|---|---|---|
| #3 | 0.375 in | 15 in |
| #4 | 0.500 in | 20 in |
| #5 | 0.625 in | 25 in |
| #6 | 0.750 in | 30 in |
Rebar is a small share of a slab — usually 5–8% — and one of the worst places to economise, because retrofitting it is impossible once the concrete is down.
Typically $0.70–$1.10 per pound in 2026, or roughly $9–$15 for a 20 ft #4 stick. Steel prices move, so quotes older than a few months are worth rechecking.
Bar chairs run $0.30–$0.80 each and you need one every few feet in both directions. Tie wire is a few dollars a roll. Small numbers, but they are what keep the steel in the middle third where it works.
Welded wire mesh is cheaper per square foot and adequate for patios and light slabs. Rebar is stiffer, easier to keep at the right height, and better where vehicles are involved. Mesh that ends up lying on the base is a common and invisible failure.
Fibres mixed into the concrete cost $8–$15 per cubic yard and control shrinkage cracking well, but they do not replace bar for structural purposes. They are a complement, not a substitute.
Spacing counts gaps, not bars. A 24 ft span at 18 inches needs 17 bars, not 16. Applied to both directions on a large slab this is several sticks short.
Steel on the base does nothing. It has to sit in the middle third of the slab thickness, on chairs, with concrete surrounding it. This is the single most common reinforcement failure and it is completely invisible once poured.
Two bars end to end transfer no load. They must overlap — conventionally 40 times the bar diameter, so 20 inches for a #4 — and be tied.
"Hooking" mesh or bar up as the concrete goes in is widely done and unreliable. Some of it ends up at the right height and some does not, and nobody can tell which afterwards.
This is a quantity takeoff. Bar size and spacing for anything carrying load — footings, retaining walls, suspended slabs — must come from an engineer and a code check.
Count bars as (span ÷ spacing) + 1 in each direction, then multiply by the span they cross. A 24 × 24 ft slab at 18 inch spacing needs 17 bars each way — 816 feet in total, or 41 twenty-foot sticks.
Because spacing counts the gaps between bars, not the bars themselves. A 24 ft span at 18 inches has 16 gaps, which requires 17 bars to create them. Forgetting this leaves you short on both directions.
#3 for patios and walkways, #4 for driveways and garage slabs, #5 for slabs carrying heavy vehicles. US sizes are eighths of an inch, so a #4 is half an inch.
18 inches for typical residential slabs, 24 inches for light patios, and 12 inches where loading is heavy. Anything structural should be specified by an engineer.
#3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043 and #6 is 1.502. Suppliers price by weight, so total footage times these figures gives you the number to quote on.
Conventionally 40 times the bar diameter — 20 inches for a #4, 25 for a #5. Bars butted end to end transfer no load at all, so the overlap is what makes a long run continuous.
In the middle third of the thickness, held on chairs before the pour. Bars lying on the base do nothing — steel needs concrete on both sides to work in tension. This is the most common and most invisible reinforcement failure.
For patios and light slabs, mesh is adequate and cheaper. Rebar is stiffer and much easier to keep at the correct height, which matters more than the material difference — mesh that ends up on the base is doing nothing.
Concrete Slab Cost per Square Foot — A standard broom-finish slab installs at $6–$12 per square foot in 2026. This guide shows where in that range your project lands, and what the per-square-foot number hides.
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It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.