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Rebar Calculator — How Much Rebar Do You Need?
Calculate rebar weight and count for concrete slabs. Enter slab dimensions and spacing to get total rebar length, weight, and pieces needed.
About This Calculator
Proper rebar reinforcement is what separates a concrete slab that lasts decades from one that cracks within the first few years. Whether you're pouring a residential driveway, a patio, or a foundation, getting the right amount of reinforcing steel ordered and placed correctly is critical for structural integrity. Rebar comes in 20-foot sticks and is sold by weight, so you need to know both the total linear feet and pounds to order from your steel supplier. Our rebar calculator takes your slab dimensions and bar spacing to compute total bar count, linear footage, and weight.
The Formula Behind This Calculator
Pieces = Floor(Length / Spacing) + 1 Total length (ft) = Pieces * Member length Weight (lbs) = Total length * Weight per ft
Understanding the math helps you verify results and make better decisions for your project.
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How to Use
- 1Enter the slab length and width in feet.
- 2Set the rebar spacing in inches (18" O.C. is standard for 4" slabs).
- 3Choose bar size (#4 is most common for residential slabs).
- 4Set the edge distance from slab boundary.
- 5Click Calculate for bar count, total length, and weight.
When to Use
- →Ordering rebar from a steel supplier for a concrete slab, driveway, or foundation pour
- →Estimating rebar costs when budgeting a concrete project with reinforced steel
- →Planning rebar placement layout before a pour to know how many sticks to buy and cut
Tips
- ✓Always use rebar chairs or dobies to keep rebar centered in the slab — rebar on the ground provides zero reinforcement
- ✓Tie intersections with wire ties — don't weld residential rebar as it can weaken the steel
- ✓Overlap splices by at least 30 times the bar diameter (15 inches for #4 bar) at every joint
FAQ
What size rebar should I use for a concrete slab?
#3 (3/8") for light-duty patios and walkways. #4 (1/2") for standard residential slabs and driveways. #5 (5/8") for heavy loads like RV pads.
How far apart should rebar be spaced?
12-18 inches on center is typical for residential slabs. Driveways carrying heavy loads should use 12" spacing. Larger spacing is acceptable for non-structural pads.
Does rebar need to overlap at joints?
Yes. Lap splices should be at least 30 times the bar diameter. For #4 bar that's about 15 inches of overlap at every joint.
Should rebar sit on chairs or the ground?
Always use rebar chairs or dobies to keep it in the middle of the slab. Rebar on the ground provides no reinforcement — it needs to be embedded in the concrete.
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