The rebar calculator above works out how many reinforcing bars a slab or footing needs at a given grid spacing, how much total linear length that is once lap splices are added, how many tie-wire connections it involves, what the steel weighs, and how many stock bars you have to buy to get there. It handles imperial bar designation numbers and metric nominal diameters.
Arb Digital publishes this in a free construction estimating set. It is an estimating tool and nothing more. Bar size, spacing, cover and lap length are structural design decisions governed by the structural drawings, by the adopted building code and by a licensed engineer. This tool counts what you tell it to count.
What This Rebar Calculator Does
It computes the reinforced dimension in each direction by subtracting cover from both edges, counts bars at your spacing using the standard on-centre count, works out how many stock lengths each run takes and adds a lap for every joint. It sums the linear length in both directions, converts that to weight from the nominal bar diameter, counts intersections for tie wire, applies a waste factor and rounds the purchase up to whole bars.
It is a quantity tool for reinforcing steel specifically, which is what separates it from our material weight calculator — that tool weighs any stock profile in any material, while this one counts a grid, adds the laps and returns a bar schedule. If you need the concrete that goes around the steel, the concrete calculator handles the pour, and the cubic yard calculator converts the volume into ordering units.
How to Use It
- Enter the slab or footing dimensions. Use the concrete dimensions. Cover is subtracted from both edges automatically, so do not deduct it yourself.
- Choose the bar size. Imperial bar numbers are approximately the diameter in eighths of an inch. Metric mode takes the nominal diameter in millimetres directly.
- Set the spacing in each direction. Both directions can differ. A one-way slab commonly has closer spacing for the main steel than for the distribution steel.
- Enter cover and lap length from the drawings. Lap length is expressed in bar diameters, which is how structural drawings normally specify it, and the tool converts that into a real length.
- Set the stock length and waste factor. Stock length is what the supplier sells. Five percent covers ordinary cutting and offcuts on a rectangular slab.
The Formula / How It's Calculated
Counting comes first. The reinforced dimension is overall dimension − 2 × cover. Bars in one direction are floor(reinforced dimension ÷ spacing) + 1 — the plus one is the closing bar at the far edge, and it is the single most commonly dropped term in the whole calculation. Each bar runs the reinforced dimension of the other axis.
Laps come next. If a run is longer than a stock bar, it needs ceil(run ÷ stock) − 1 splices, and each splice adds lap in bar diameters × bar diameter to the length. On a long slab this is not a rounding error: a 60 foot run in 20 foot stock needs two laps per line, and at 40 bar diameters on a #5 bar that is an extra 4.2 feet in every single line.
Weight follows from the bar's nominal cross-section: π × (diameter ÷ 2)² × density. In imperial that produces 0.668 pounds per foot for a #4 and 1.044 for a #5, which reproduces the tabulated values in ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement to within a fraction of a percent. That standard also defines the four minimum yield strength grades — 40, 60, 80 and 100 in inch-pound units — and the deformation and marking requirements that let you identify a bar on site. The Concrete Reinforcing Steel Institute's reinforced concrete terminology defines the bar number as approximately the bar diameter in eighths of an inch, and defines a lap splice as the overlapping of two bars side by side, in contact or non-contact.
Ties are simply bars in one direction × bars in the other × ties per intersection, which gives the number of crossings in the mat.
What This Tool Does Not Decide, and Why
Bar size, spacing, cover, lap length, grade and whether the slab needs reinforcement at all are structural design outputs. They come from an engineer's analysis of loads, span, soil conditions, exposure and the adopted code, and they appear on the structural drawings.
In most of the United States the governing documents are the International Building Code or the International Residential Code as adopted by your state or municipality, published by the International Code Council, which in turn reference ACI 318 for structural concrete design. Adoption varies: two neighbouring jurisdictions can be on different code editions with different requirements, and local amendments are common. Cover in particular is not a single number — it changes with exposure, with whether the concrete is cast against earth, and with bar size.
The practical instruction is simple. Read the values off your drawings and type them in. If you do not have drawings for a structural element, the answer is not to guess a spacing from a calculator; it is to get the element designed. This tool estimates quantities so you can order material and check an invoice, and a licensed professional governs the design.
Laps Are Where the Steel Order Grows
Lap splices are the most under-estimated item in reinforcing steel. They are invisible on a plan — the drawing shows a continuous line of steel — but every joint between two bars adds real length, and long slabs have a lot of joints.
Work through it. A 40 foot run in 20 foot stock needs one splice. At 40 bar diameters on a #5 bar, the lap is 40 × 0.625 = 25 inches, just over two feet. That is a five percent addition to that line. Now take a 100 foot run: four splices, over eight feet of extra steel per line, and if you have forty lines that is 330 extra feet of bar. Order without counting laps and you will be one full bundle short.
Two related points. Lap length in bar diameters means bigger bars get longer laps in absolute terms, so upsizing bar diameter increases the lap allowance more than proportionally. And splice location is usually restricted by the drawings — you generally cannot splice wherever a bar happens to run out, because splices must be staggered and kept away from points of maximum stress. That constraint can force you to cut a bar short and lose the offcut, which is what the waste factor is for.
Cover, Chairs and the Steel That Ends Up in the Wrong Place
Reinforcement only works where it is placed. Steel lying on the ground at the bottom of a slab that needs it near the top does close to nothing, and steel with too little cover corrodes. Cover is a durability requirement as much as a structural one — the concrete around the bar is what protects it from moisture and chloride.
This is why the bar count is only half of an order. The other half is the support system: bar chairs, bolsters and spacers that hold the mat at the correct height while the concrete is placed and while people walk on it. Those are not in this calculator's output because their spacing depends on bar stiffness and site practice, but they are a real line item and forgetting them is the usual reason a mat ends up on the subgrade.
Ties matter for the same reason. Tie wire does not add structural capacity — a tied intersection is not a structural connection — but it keeps the mat at the spacing you calculated while it is being walked on and poured around. The tie count here lets you estimate wire; the standard rolls are long enough that the figure mostly matters for deciding how many rolls to open.
Reading the Placed Length Against the Purchased Length
The tool separates them deliberately. The placed length is the honest total of steel in the finished element, and it is the figure to use for a weight take-off or to check what an engineer specified. The purchased figure is whole stock bars, after laps and waste, and it is what appears on the delivery note.
The gap between them tells you something useful. If it is large, your stock length is a poor fit for your slab dimensions. A 20 foot slab with 3 inch cover needs 19.5 foot bars, which come out of 20 foot stock with almost nothing wasted. Change the slab to 21 feet and each bar needs a splice or a longer stock length, and the order jumps. Where you have any freedom over dimensions, checking two or three stock lengths before ordering is worth the two minutes.
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Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Dropping the closing bar — a grid at a fixed spacing always needs one more bar than the division suggests, in both directions.
- Ignoring lap splices — every joint adds real length, and on long runs the total is a bundle of steel rather than a rounding error.
- Measuring to the slab edge instead of to the cover line — bars stop short of the edge, and using the full dimension overstates both count and length.
- Leaving the mat on the subgrade — without chairs and bolsters the steel ends up at the bottom, where it does not do the job it was designed for.
- Guessing spacing without drawings — bar size, spacing, cover and lap length are engineering outputs, not estimating inputs you are free to choose.
Related Free Tools From Arb Digital
Pair this with the concrete calculator for the pour itself, the cubic yard calculator for ordering volume, the material weight calculator for any other steel section in the job, the gravel calculator for the sub-base under the slab and the square footage calculator for an irregular footprint. The free online tools hub lists every calculator we publish.
Frequently Asked Questions
With 3 inch cover and #4 bars at 12 inches on centre both ways, the reinforced dimension is 19.5 feet, giving 20 bars in each direction, each 19.5 feet long. That is 780 linear feet of steel weighing about 521 pounds, and with 5 percent waste it comes to 41 twenty-foot bars.
About 0.668 pounds per foot, which the tool derives from the nominal half-inch diameter and the density of steel. That figure matches the tabulated value in ASTM A615 to within a fraction of a percent, and a #5 comes out at 1.044 pounds per foot the same way.
It is set by the structural drawings and the adopted code, and it is normally expressed as a number of bar diameters. Enter that number in the lap field and the tool converts it into a real length for your bar size. Splice locations are usually restricted too, so read the drawings rather than splicing wherever a bar runs out.
CRSI defines the bar number as approximately the bar diameter in eighths of an inch, so a #4 is about half an inch and a #8 is about one inch. Metric bars are designated by their nominal diameter in millimetres instead.
Because a grid at a fixed spacing needs one more member than the division gives. Twenty feet at one foot spacing is twenty gaps and twenty-one positions, and after cover is deducted the count lands where the calculator shows it. Dropping that closing bar is the most common counting error in reinforcement take-offs.
The tool multiplies the bars in one direction by the bars in the other to get the number of intersections, then applies your ties-per-intersection figure. Enter 0.5 if you are tying alternate crossings. Tie wire holds the mat in position during the pour; it is not a structural connection.
Yes. Switch the units selector and enter dimensions in metres with bar diameter, spacing and cover in millimetres. Bar mass is derived from the nominal diameter and the density of steel, and results come back in metres and kilograms.
No, and it should not. Bar size, spacing, cover, grade and lap length are structural design decisions that depend on loads, spans, soils, exposure and the code adopted in your jurisdiction. Take them from the structural drawings prepared by a licensed engineer.
This tool produces material estimates only. Reinforcement design, bar size, spacing, cover and lap length are governed by the structural drawings and the building code adopted in your jurisdiction, and must be determined by a licensed engineer.
