Rebar calculator
This lays out a grid and totals the steel — counts and lengths only. It does not choose the bar size, the spacing or the cover for any load; those are set by an engineer or your local requirements, and a wrong guess there is not a wasted stick of bar but a failed slab. Order to a design you trust.
Enter the slab size, the spacing you were told to use and the edge clearance, and the tool lays out a two-way grid: how many bars run each way, the total length of bar to order, and how many intersections you will tie.
Ejemplo resuelto
A 20 ft × 12 ft slab, bars at 12 inches on centre, 3 inches clear of each edge. The usable field is 19.5 ft × 11.5 ft. Across the 11.5 ft width at 1 ft spacing that is 11 spaces, so 12 bars running the length at 19.5 ft each. Along the 19.5 ft length, 19 spaces means 20 bars running the width at 11.5 ft each. That is 234 + 230 = 464 ft of bar; add 10% for laps and you order about 510 ft (26 sticks at 20 ft), with 240 intersections to tie.
The same in metric: a 6 m × 3.6 m slab, bars at 300 mm on centre, 75 mm clear of each edge. The usable field is 5.85 m × 3.45 m. Across the 3.45 m at 0.3 m that is 11 whole spaces, so 12 bars running the length at 5.85 m each. Along the 5.85 m, 19 spaces means 20 bars running the width at 3.45 m each. That is 70.2 + 69 = 139.2 m of bar; add 10% and you order about 153 m — 26 bars of 6 m stock — with 240 ties. The bar size (10M, 15M, #4) is a name from your drawings; it does not change the count.
En la obra
A shop slab on 18 in centres, ordered by the stick. 24 ft × 30 ft, 3 in clear, so the field is 23.5 × 29.5 ft. Across the 23.5 ft (282 in) at 18 in: 15 full spaces, 16 bars running the 29.5 ft length. Across the 29.5 ft (354 in): 19 spaces, 20 bars running 23.5 ft. Steel: 16 × 29.5 + 20 × 23.5 = 472 + 470 = 942 ft, plus 10% is 1,036 ft. Bar comes in 20 ft sticks: 1,036 ÷ 20 = 51.8, so 52 sticks, and 16 × 20 = 320 ties.
Where the lap factor really comes from. Those 29.5 ft bars are longer than a 20 ft stick, so every one is two pieces with a lap in it. If your drawings call for a 24 in lap (600 mm), each long bar is 29.5 + 2 = 31.5 ft of stock, and the sixteen of them are 504 ft instead of 472. That is a 7% uplift on that set alone before an offcut hits the ground — which is why 10% is a starting point, not a ceiling. The lap length is the engineer's number; type it into your waste.
Two mats. A thicker slab with top and bottom steel is the same grid twice, plus chairs to hold the top mat up. Run it once and double the bar and the ties; the chairs are counted off the intersections of the top mat and are a separate line on the order.
What went wrong?
Quick referencefor a slab grid
- Bars in a run
- floor(usable ÷ spacing) + 1
- Usable field
- slab dimension − 2 × edge clearance
- Ties
- bars one way × bars the other way
- Steel per 100 ft² of slab (10 × 10, no cover)
- at 12 in: 220 ft · at 18 in: 140 ft · at 24 in: 120 ft
- Steel per 9 m² of slab (3 × 3 m, no cover)
- at 300 mm: 66 m · at 450 mm: 42 m · at 600 mm: 36 m
- Largo comercial
- commonly 20 ft sticks — a bar longer than the stick is two pieces and a lap
- Metric stock
- commonly 6 m (also 12 m) lengths — set yours in the tool
- Nominal weight per foot
- #3 = 0.376 lb · #4 = 0.668 lb · #5 = 1.043 lb (mill figures, for the truck)
- Nominal mass per metre
- 10M = 0.785 kg · 15M = 1.570 kg · 20M = 2.355 kg (mill figures, for the truck)
Waste & overagewhat to add, and why
- Rebar is ordered 10–15% over the linear count for laps and the cut-offs at odd lengths — the waste/lap field carries both. Your drawing sets the lap as a number of bar diameters; whatever that number is, the lap is a real length of steel per splice, not a percentage, so count the splices if the slab is long.
- Buy in stock lengths (20 ft is common at the yard) and let the count round up to whole bars — a 17 ft bar is a 20 ft bar with a cut.
- Ties run in the thousands; buy a full bag or roll rather than the exact count.
Mistakes to catchbefore you order
- Laying the grid out on the full slab dimension and forgetting the edge clearance both sides — the usable field is the slab minus twice the clearance.
- Typing the bar spacing in inches into a field set to feet, or 12 in as 12 ft — one bar across the slab is the tell.
- Reading the bar count as the count of one direction; the grid needs bars both ways and the tool gives both, so add them.
What an inspector looks forthe building inspector or the engineer, at the pre-pour review
On this part of the job
- Bar size, spacing and the number of bars in each direction, measured against the reinforcement schedule rather than counted by eye.
- Cover — the clear distance from steel to form, to soil and to the top of the pour. Chairs, dobies and the right bar supports are the whole point, and they get counted.
- Laps at splices and the way bars turn a corner. The length of a lap comes from the drawing or the code; do not guess it and do not take it from a calculator.
- Ties: enough that the mat cannot shift when a boom and a vibrator land on it.
- Clean steel — heavy loose rust, mud, form oil or ice on the bar will be called out.
Have this ready
- The reinforcement schedule and any bending details.
- Mill certificates or the delivery ticket for the bar, if the job asked for them.
- A bar marker and a tape so spacing can be checked in two places without moving anything.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La fórmula
bars = floor(usable ÷ spacing) + 1 · length = Σ (bars × run) × (1 + waste)
Trim the clearance off each side to get the usable field, divide by the spacing and add one for the edge bar. Each set's total length is its bar count times how far it runs; the two sets add together, and the lap factor tops it up. Intersections are simply one count times the other. Spacing and clearance go in as inches (÷ 12 for feet) or millimetres (÷ 1,000 for metres) so they share the slab's unit.
What this tool assumes
A single rectangular slab and one spacing both ways. Spacing, clearance and the lap factor are all yours to set — nothing about the bar is fixed, because everything that matters structurally is a decision you bring to the page, not one it makes.
Preguntas
- How do I work out how much rebar a slab needs?
- Count the bars each way and add up their lengths. Along the width, a bar every so-many inches gives you a set running the length; along the length, another set runs the width. This tool takes your slab size, your spacing and your edge clearance and returns both bar counts, the total linear footage and the number of grid intersections to tie.
- Does this tell me what size rebar to use?
- No, and that is deliberate. Bar size and spacing for a structural slab come from an engineered design or your local requirements, never from a quantity tool. This page assumes you already know the spacing you were told to use and simply lays out the grid and totals the steel to order.
- Why the +1 in the bar count?
- Because a grid has a bar at both edges as well as in the gaps. If a run fits five spaces it holds six bars — the fencepost rule. The tool divides the usable width by the spacing, drops any part-space, and adds one, so the outer bars at each edge are always counted.
- What is the edge clearance for?
- Steel is kept back from the edge of the pour so concrete fully surrounds it — the cover. The clearance you enter is trimmed off each side before the grid is laid out, so the bars stop short of the forms and the counts reflect real, buildable positions rather than bars running out to the very edge.
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