Rebar Calculator
Enter your slab size and the spacing your plan calls for, and get bars each direction, total linear feet, a lap-splice allowance from your stock length, and the steel weight from the ASTM nominal table. This tool counts a layout you already have — whether a slab needs reinforcement, and of what size and spacing, is your plan's or engineer's call.
Slab and grid (from your plans)
Example: a 20 × 20 ft slab at 12 in o.c. with 3 in edge clearance → 20 bars each way, 780 lf.
Structural boundary: whether this slab needs steel, of what size, at what spacing, with what lap and cover — all of that is design, from your plans, engineer, or code official. This tool arithmetic-checks a layout that already exists.
Span, spacing, splice — in that order
The worked slab: 20 ft minus 3 in of clearance each side leaves a 19.5 ft clear span. At 12 in on-center that is floor(19.5) + 1 = 20 bars each direction, every bar 19.5 ft long: 40 bars, 780 linear feet. At #4's ASTM nominal 0.668 lb/ft the grid weighs 521.04 lb. Give the tool your stock length and lap requirement and it adds the splice overlap before the weight. Computed at build time by the same engine wired to the form.
What is sourced, what is yours
The one sourced table on this page is the ASTM A615 nominal weight per foot for #3–#8 bar, verified against a distributor's published chart and locked by automated tests — a property of the standardized sizes, not a design recommendation. Spacing, clearance, stock length, and lap are all user-entered because they are all plan-governed; the 3 in clearance and 12/16/18 in spacings appear only as labeled conventions and placeholders.
The concrete around the steel
The slab this grid sits in is the concrete slab calculator's job — volume, waste, and quarter-yard ready-mix ordering. Bagged-mix comparisons live in the concrete bag calculator, and footings under the slab edge in the concrete footing calculator.
Frequently asked questions
How is the number of bars computed?
floor(clear span ÷ spacing) + 1 in each direction: one bar at each end of the span plus one per full spacing interval. The clear span is the slab dimension minus the edge clearance on both sides — a 20 ft slab with 3 in clearance has a 19.5 ft clear span, so 12 in on-center gives floor(19.5) + 1 = 20 bars each way.
What spacing and bar size should I use?
Whatever your plans, engineer, or local code say — that is a design decision this tool refuses to make. 12, 16, and 18 inches on-center appear as placeholders only because they are common plan callouts; the field takes any spacing. The same goes for choosing rebar versus wire mesh versus fiber.
Where do the weight figures come from?
The ASTM A615 nominal weights, verified against a steel distributor’s published size chart and a CRSI-derived reference: #3 = 0.376, #4 = 0.668, #5 = 1.043 lb per foot (through #8 = 2.670). These are properties of the standardized bar sizes, identical across producers; the tool multiplies your total linear feet, including splice overlap, by the figure for the size you pick.
How is the splice allowance computed?
From your stock length and your plan’s lap requirement — both user-entered. Each bar longer than a stock piece needs ceil(bar ÷ stock) pieces and one splice per joint; every splice adds the overlap you specify. Lap length is design-governed (it varies with bar size, concrete strength, and code), so the tool never invents one.
Why the 3 inch edge clearance default?
It is a labeled convention, not a code citation: keeping steel back from slab edges maintains concrete cover, and 3 inches is the commonly used figure for slab-on-grade edge setback. Your plans win — the field accepts any value including zero.
Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.