Rebar Calculator
Bar count includes a 10% allowance for lap splices on runs longer than a single 20-foot bar.
How much rebar you need comes down to laying a grid across your slab and counting the bars, splices, and ties. Enter your slab length, width, and grid spacing below for total linear footage, the number of 20-foot bars, and how many wire ties the grid takes.
Standard Slab Rebar Grids
| Slab Footprint | Total Grid Intersections | Total Linear Feet | Required 20ft Bars |
|---|---|---|---|
| 10 ft x 10 ft Area | 64 Ties | 152 Linear ft | 9 Full Bars |
| 20 ft x 20 ft Area | 225 Ties | 585 Linear ft | 33 Full Bars |
| 30 ft x 30 ft Area | 484 Ties | 1,276 Linear ft | 71 Full Bars |
| 40 ft x 40 ft Area | 900 Ties | 2,370 Linear ft | 131 Full Bars |
How the rebar grid is calculated
Working out how much steel a slab needs is more than square footage. You set the edge clearance, pull the grid in from those edges, then lay out grid lines at your chosen spacing and add up the length. The result is a linear-footage total, a bar count, and a tie count — the three numbers you actually order and lay out from.
Edge clearance
Rebar should not sit flush against the forms or rest on the soil. Water reaches exposed steel, it rusts, and the rust expands and cracks the surrounding concrete, a failure called spalling. The calculator holds the grid in from the edges so the steel stays covered.
Lap splices
Rebar comes in fixed lengths, typically 20-foot bars, so any run longer than that needs a second bar overlapped with the first. That overlap, the lap splice, runs at least 40 times the bar diameter so the joint carries the same load as a continuous bar. The extra length adds up on wide slabs, so the total includes an allowance for it.
Slabs, walls, footings, and mats
The same grid math drives most residential rebar takeoffs, but where the steel goes changes which numbers matter. Set your dimensions above for the layout you're pouring — the calculator treats every one as a grid or mat and returns the linear feet, bar count, and ties.
Slab and footing grids
A slab uses a flat mat of bar spaced evenly both directions, which is exactly what the inputs above describe. A footing is the narrow strip of that same idea: two, three, or four continuous bars running its length with cross-ties or stirrups at intervals rather than a full grid. To size a footing here, enter its length and set the width to the bar spacing you want between the longitudinal bars — the linear-footage total still holds, since it's the running length of steel that you order.
Walls
A reinforced concrete or block wall carries a grid of vertical and horizontal bar much like a slab stood on end, so the grid spacing and linear-footage logic transfer directly. Enter the wall height and length as your two dimensions. For a concrete block wall, the vertical bars drop into grouted cores rather than being tied into a mat — our concrete block calculator handles the block count and core fill that go with it.
Grid and mat layout
"Grid," "mat," and "mesh" all describe the same thing this calculator builds: bars crossing at a set spacing, tied at each intersection. The tie count above is your intersection count, one twisted wire tie per cross, which is what tells you how much tie wire and labor the mat takes. Prefabricated welded wire mesh is the factory version of the same mat and is estimated by sheet area instead of by bar.
Rebar weight, tonnage, and cost
Suppliers price and ship rebar by weight, so once you have the linear-footage total above, multiply it by the weight per foot for your bar size to get pounds, then divide by 2,000 for tons. Bar sizes are numbered in eighths of an inch — a #4 bar is 4/8, or ½ inch, in diameter.
| Bar size | Diameter | Area (sq in) | Weight (lb/ft) |
|---|---|---|---|
| #3 | 3/8" | 0.11 | 0.376 |
| #4 | 1/2" | 0.20 | 0.668 |
| #5 | 5/8" | 0.31 | 1.043 |
| #6 | 3/4" | 0.44 | 1.502 |
| #7 | 7/8" | 0.60 | 2.044 |
| #8 | 1" | 0.79 | 2.670 |
For a worked example, a 20-by-20-foot slab on a 16-inch grid takes about 585 linear feet of bar. In #4, that's roughly 390 pounds, just under a fifth of a ton; in #5, closer to 610 pounds. Multiply the pounds by your local price per pound, or the bar count by the per-bar price, for a material cost — steel is quoted both ways, so having the linear feet, the bar count, and the weight all in hand lets you match whichever a yard uses.
Common mistakes and edge cases
- Forgetting the splice overlap. A slab wider than one 20-foot bar needs spliced bar, and each 40-diameter overlap is steel you buy but don't span with. The 10% allowance above covers typical flatwork splicing; a long structural run with many joints needs more.
- Skipping chairs and supports. Rebar has to sit at mid-depth, not on the ground. Bar chairs or dobie blocks hold the mat up during the pour, and they're a separate line item the linear-footage total doesn't include — count roughly one support per few feet of bar in each direction.
- Buying the wrong bar size. Bar count and linear feet stay the same across sizes, but weight and price don't. Confirm whether your plan calls for #3, #4, or #5 before you convert to tons or dollars.
- Irregular footprints. This calculator lays a rectangular grid. For an L-shape, curve, or slab with a cutout, split it into rectangles, run each, and add the linear feet together — the same approach that works for irregular areas on the gravel calculator.
- Treating it as engineered design. These figures size a standard residential mat for takeoff and ordering. Bar size, spacing, and splice class on a structural slab, grade beam, or retaining wall are an engineer's call under ACI 318, not a default.
About these figures: bar diameters, cross-sectional areas, and weights per foot follow the ASTM A615 rebar dimensions verified against the Inch Calculator reference. Minimum cover and the 40-diameter lap-splice rule of thumb follow the ACI 318 provisions cited below. Confirm spacing, bar size, and splice requirements against your stamped plans or local code before ordering steel for anything structural.
Frequently asked questions
What is the standard spacing for rebar in a slab?
Most residential slabs like driveways and patios use 12 to 18 inches between bars, with 16 inches a common default. Tighter spacing carries more load, so the right interval depends on what the slab has to support. Check your local code or engineer for anything beyond a standard flatwork pour.
Why does rebar need edge clearance?
Concrete code calls for about 3 inches of cover between the soil and the steel on ground-cast slabs. If a bar sits against the dirt or pokes out of the pour, moisture reaches it and it rusts, and rusting steel expands and cracks the concrete around it. Keeping the grid pulled in from the edges protects the steel and the slab.
What are the intersections and why do they matter?
Intersections are the points where the horizontal and vertical bars cross in the grid. Each one gets a twisted wire tie to hold the mat together while you pour, so the tie count tells you how much wire and labor the grid takes.
Why does rebar have to overlap?
Rebar comes in fixed lengths, usually 20-foot bars, so any slab wider than that needs two bars joined end to end. The overlap, called a lap splice, runs about 40 times the bar diameter so the joint carries the same tension as a single bar. That overlap adds length you have to buy, which is why it factors into the total.
What does the 10% waste allowance cover?
The calculator adds 10 percent to the total linear footage to cover lap splices on runs longer than one bar, plus bends at the edges and the odd offcut. It is a working buffer so you are not a few feet short on pour day; trim it if your slab fits within single bar lengths.
Does concrete thickness change how much rebar I need?
No. A standard residential slab uses a single layer of rebar held at mid-depth on chairs, so the steel total depends on the length and width of the slab, not how thick it is. Thicker structural slabs may call for a second mat, but that is an engineered detail rather than a flatwork default.
How much rebar do I need for a concrete slab?
For a standard residential slab, lay a grid of #3 or #4 bar at 16-inch spacing and total the linear feet, which is what this calculator does from your dimensions. As a quick check, a 16-inch grid uses roughly 1.5 linear feet of rebar for every square foot of slab, so a 400-square-foot pad (about 20 by 20 feet) needs close to 585 feet, or around 33 twenty-foot bars once the splice allowance is added. Enter your exact length and width above for the real count.
How long should a rebar lap splice be?
For standard residential slabs and footings, a tension lap splice runs about 40 times the bar diameter, roughly 20 inches for #4 bar or 25 inches for #5. That overlap lets two bars share load as though they were one continuous bar. Development length is the related figure for a single bar anchoring into concrete, and a lap splice is essentially two development lengths overlapped, which is why splices come out longer than people expect. For anything structural beyond flatwork, follow the splice class your engineer or ACI 318 specifies.
How much does rebar weigh per foot?
Rebar weight depends on bar size: #3 weighs 0.376 lb/ft, #4 weighs 0.668 lb/ft, #5 weighs 1.043 lb/ft, and #6 weighs 1.502 lb/ft. Multiply your total linear footage by the figure for your bar size to get total pounds, then divide by 2,000 for tons when a supplier quotes steel by weight. The full size chart, including cross-sectional areas, is in the weight and tonnage section above.
Related calculators
Sources
- ACI 318 — Building Code Requirements for Structural ConcreteAmerican Concrete Institute (ACI)Source for reinforcement provisions: minimum concrete cover and tension lap-splice lengths expressed as a multiple of bar diameter.
- Tension Development and Lap Splice Lengths of Reinforcing Bars Under ACI 318Simpson Strong-Tie Engineering Technical Note (ETN-D-1-15)Engineering reference on how development and lap-splice lengths are derived from ACI 318, supporting the splice-length estimate.
- Rebar — Standard Sizes and 20-foot Stock LengthsHarris Supply SolutionsReference for standard reinforcing bar sizes and the common 20-foot stock length used to estimate the number of bars.