Rebar Calculator
This free rebar calculator turns your slab dimensions, bar size and spacing into a two-way rebar grid — total bars, linear feet, weight and a materials cost range.
What this calculator does
This planner works out a standard two-way rebar grid for a rectangular slab: how many bars run each direction at your spacing, the total linear feet and weight, and how many stock-length bars to buy.
How the math works
- Bars per direction: (usable width or length ÷ spacing) + 1, after subtracting edge clearance from both sides.
- Total length: bars one way × the other dimension, plus bars the other way × that dimension.
- Weight: total length × the bar size's weight per foot.
Worked example
A 10 ft by 10 ft slab, #4 rebar, 18 in spacing, 3 in edge clearance: 7 bars each direction, 140 total linear ft, about 93.5 lb, or 7 stock 20 ft bars.
Frequently asked questions
How much rebar do I need for a slab?
A 10x10 ft slab with #4 rebar at 18 in spacing needs about 140 linear ft, or 7 stock 20 ft bars.
What size rebar is used for a driveway or patio slab?
#3 or #4 is typical for residential slabs; #5 or larger for structural slabs or heavier commercial loads.
How far apart should rebar be spaced?
12-24 in is typical, with 18 in a common default; heavier loads use tighter spacing with larger bars.
How much does rebar weigh per foot?
#3 ≈ 0.376 lb/ft, #4 ≈ 0.668 lb/ft, #5 ≈ 1.043 lb/ft, #6 ≈ 1.502 lb/ft.
Do I need rebar for a shed or patio slab?
Many light slabs use wire mesh instead; rebar is stronger and more common for driveways — check your local code.
Limitations
This tool estimates a simple rectangular two-way grid with no overlap/splice allowance — larger pours need extra length for laps. It is not a substitute for a structural engineer's design on load-bearing slabs.
Put this calculator on your site
Run a blog or business site for DIYers, homeowners or contractors? You can embed the Rebar Calculator as a free widget — responsive, no registration, and no ads or tracking added to your page.
Cite this calculator
Using this tool in a class, a paper or an article? Copy a ready-made reference: