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.
Your rebar plan
- Slab
- 480 sq ft
- Bar size
- #4 (1/2 in)
- Spacing
- 18 in
Rebar materials estimate
$231 – $363
Est. as of 2026-09 — not a live quote
Shopping list
- Rebar #4 (1/2 in) 656 total linear ft, 438.2 lb. 33 stock bars (20 ft)
Email me this plan
Get the shopping list, cost range and next steps in your inbox, to have at the store or when you talk to a contractor. Free.
Need 33 rebar stock lengths?
Find a building-material supplier near you for rebar by size and quantity.
View shopping listPouring the slab too?
Use the Concrete Slab Calculator for bag count or ready-mix quantity alongside this rebar grid.
Assumptions
- Two-way grid: 14 bars one direction, 16 the other, at 18 in center-to-center spacing.
- 3 in clearance kept from each slab edge, per typical residential slab practice.
- Stock bars assume 20 ft lengths with no overlap/splice allowance — add extra for laps on larger pours.
- Prices are national-average DIY retail ranges as of 2026-09, not live quotes.
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: