Rebar Calculator
Estimate the reinforcing bar for a concrete slab: bars each way at a chosen grid spacing, total linear footage, grid intersections, and standard bars to buy.
Comment utiliser cette calculatrice
- 1Enter the slab's length and width.
- 2Enter the grid spacing (on centre) and the edge cover to the first bar.
- 3Enter the length of rebar you'll buy and a lap/waste allowance.
- 4Read the number of bars to buy, the linear footage, and the tie-point count.
Comment ça marche
Rebar grid
bars one way = ⌊(span − 2 × edge cover) ÷ spacing⌋ + 1 linear feet = (bars × their length) both directions total with laps = linear feet × (1 + overlap %) bars to buy = ⌈total ÷ bar length⌉
Reinforcing a concrete slab means laying a grid of steel bars in both directions so the concrete, which is strong in compression but weak in tension, gains tensile strength. Estimating the grid is a counting exercise. In each direction, the number of bars is the span minus the edge cover on both sides, divided by the on-centre spacing, plus one for the starting bar. Bars running lengthwise are spaced across the width and each run the slab's length; bars running widthwise are spaced along the length and each run the slab's width. Adding up the length of all the bars in both directions gives the total linear footage, and an allowance is added for the laps where bars overlap to splice into continuous runs. Dividing by the length of the bars you can buy, rounding up, gives the number to order. The bar size, steel grade, and spacing themselves are structural decisions dictated by the load and building code, not by this material count.
Exemple détaillé
A 20 ft by 10 ft slab with a 12-inch grid and 3-inch edge cover needs about 10 bars running lengthwise (each 20 ft) and 20 running widthwise (each 10 ft) — 400 linear feet. With a 10% lap allowance that is 440 feet, or 22 standard 20-foot bars, tied at roughly 200 grid intersections.
Rebar Calculator : le guide complet
Why concrete needs steel
Concrete is a paradoxical material: immensely strong when squeezed but surprisingly weak when stretched, cracking under tension at a fraction of its compressive strength. Most structural elements experience both forces — a slab or beam bends under load, compressing on one face and stretching on the other — so plain concrete would crack and fail on the tension side. Reinforcing bar, or rebar, solves this by embedding steel where the tension occurs. Steel is strong in tension, concrete strong in compression, and together they form reinforced concrete, one of the most important building materials ever developed.
The partnership works because the two materials bond and move together. Rebar's ridged surface grips the concrete, and, conveniently, steel and concrete expand at almost the same rate when heated, so temperature changes do not tear them apart. The concrete also protects the steel from corrosion, provided there is enough cover — the layer of concrete between the bar and the surface. This is why edge clearance and depth of cover matter: too little, and moisture reaches the steel, rusting it and eventually spalling the concrete. Estimating rebar is quantifying this hidden steel skeleton.
Spacing, size, and lap splices
The grid spacing and bar size together determine how much steel is in the slab, and both are structural decisions rather than matters of convenience. Closer spacing and larger bars put more steel in, increasing the slab's capacity to resist tension and cracking, at higher cost. Typical residential slabs use spacings around 12 to 18 inches with bars in the #3 to #5 range (three-eighths to five-eighths inch diameter), but the correct values come from the design load, the slab thickness, the soil beneath, and the local building code. Guessing here is unwise; the quantities this calculator produces assume a spacing you have already been given or specified.
Lap splices are the other detail that affects quantity. Rebar comes in fixed lengths, and when a slab is longer than the available bars, two bars must be overlapped and tied so they act as one continuous piece. Codes specify a minimum lap length — often around 40 times the bar diameter — to transfer the force across the joint. These overlaps consume extra steel that a bare linear-foot count would miss, which is why an overlap allowance is added. On large slabs the laps add up, so the allowance is not merely a rounding cushion but a genuine material requirement.
Placing and tying the grid
A rebar estimate produces the bars, but the grid also has to be assembled and positioned correctly, which the intersection count hints at. Where the bars cross, they are secured with tie wire to hold the grid together and keep it in shape while concrete is poured around it. The number of intersections indicates roughly how many ties to make; each is a quick twist of wire, but on a large grid they number in the hundreds and add up in time and material.
Just as important as tying is the height of the steel within the slab. Rebar must sit at the correct depth — often near the middle or lower third of the slab, depending on where tension is expected — not lying on the ground where it does nothing and will rust. Small supports called chairs or dobies hold the grid up to the right level before the pour, and the edge cover keeps the bars set in from the sides. A grid that is correctly sized but poorly placed loses much of its benefit, which is why the physical execution matters as much as the quantity. The calculator sizes the order; proper placement turns that steel into genuine reinforcement.
Questions fréquentes
How much rebar do I need for a slab?
Count bars each way: (span minus edge cover on both sides) ÷ spacing, plus one, in each direction. Multiply by their lengths for total linear feet, add ~10% for laps, and divide by the bar length sold. A 20×10 ft slab at 12-inch spacing needs about 22 twenty-foot bars.
What spacing should rebar be?
Commonly 12 to 18 inches on centre for residential slabs, but the correct spacing is a structural decision based on the load, slab thickness, soil, and building code — not a rule of thumb. Closer spacing and bigger bars add strength and cost. Use the spacing your design specifies.
Why do bars need to overlap?
Because rebar comes in fixed lengths, and slabs longer than a bar need two bars spliced. Overlapping them by a code-specified length — often about 40 bar diameters — lets the force transfer across the joint so they act as one continuous bar. These laps consume extra steel, which the waste allowance covers.
How deep should rebar sit in concrete?
It should sit within the slab at the depth where tension occurs — often the middle or lower third — held up on supports called chairs, never lying on the ground. It also needs adequate concrete cover from the edges and surface to protect the steel from corrosion. Placement is set by the structural design.