Calculate total weight, linear footage, and number of bars for your structural projects. Supports custom grid spacing and standard rebar sizes.
Last updated: August 2026
Concrete is strong under compression but weak under tension, which is why nearly every slab, footing, and foundation gets a steel skeleton underneath it. Get too little rebar in there, or space it too far apart, and the slab is prone to cracking under load; get the size or spacing wrong the other way and you're paying for steel you didn't need. The calculator above handles the math for both a grid layout (slabs and walls) and a linear layout (beams and footings), including the overlap you lose to lap splices.
In the US, a bar's number tells you its diameter in eighths of an inch — a #4 bar is 4/8", or 1/2". Here's how the common residential sizes stack up:
| Bar Size | Diameter | Weight (lb/ft) | Weight (kg/m) | Common Use |
|---|---|---|---|---|
| #3 | 3/8" (10mm) | 0.376 | 0.560 | Stirrups, sidewalks |
| #4 | 1/2" (13mm) | 0.668 | 0.994 | Patios, driveways, slabs |
| #5 | 5/8" (16mm) | 1.043 | 1.552 | Footings, small foundations |
| #6 | 3/4" (19mm) | 1.502 | 2.235 | Retaining walls, columns |
| #8 | 1" (25mm) | 2.670 | 3.973 | Foundations, structural beams |
The calculator above automates this, but it helps to know what it's doing. For a 10x10 ft slab on 12" spacing, you'd run 11 bars in each direction (counting both edges), for 22 bars at 10 ft each — 220 linear feet before overlap. Lap splices then eat into that: a 10% allowance is typical for residential work, though structural projects may call for more under your local code, so add that percentage on top before pricing steel.
Getting the total footage right is only half the job — where the steel actually sits inside the pour determines whether it does anything. Rebar is usually held mid-depth (or per engineered spec) using plastic or concrete chairs. Too shallow and the bar is exposed to moisture and eventually spalls the surface as it rusts; too deep and it stops contributing meaningful tensile strength. In coastal or high-moisture areas, epoxy-coated rebar is worth the upcharge to slow corrosion.
Most rebar sold today is Grade 60, meaning it yields — starts to permanently deform — at 60,000 PSI. Grade 40 is still available and cheaper, but Grade 60 is the default for anything beyond the lightest residential work. When you order steel based on the numbers above, confirm the grade with your supplier matches what your plans call for; it's not always the default stock.
Q: What does #4 rebar mean?
A: The number refers to the bar's diameter in eighths of an inch — a #4 bar is 4/8", or 1/2" (12.7 mm).
Q: How far apart should rebar be in a 4-inch slab?
A: Most residential patios and driveways use 12–18" on-center spacing. Always confirm against local building codes.
Q: How long is a standard rebar?
A: Typically 20, 40, or 60 ft lengths. 20 ft is standard for most DIY projects.
Q: Why is rebar ribbed instead of smooth?
A: The ribs create a mechanical bond with the concrete so the bar can't slip once it cures.
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