Rebar Calculator
Enter your slab or footing size, bar size, and spacing to estimate rebar sticks — and the weight the supplier charges by.
Runs entirely in your browser — your numbers never leave this page.
How it works
Rebar is ordered in 20-foot sticks and priced by weight. For a slab grid:
Bars each way = (Width ÷ Spacing) + 1
Weight = Total feet × lb/ft (#4 = 0.668, #5 = 1.043)
- Spacing: 18 in on-center is standard for residential slabs; 12 in for driveways and heavy loads.
- Chairs: rebar must sit in the middle of the slab on chairs/dobies — steel lying on soil rusts and adds zero strength.
- Laps: splice bars with 30× diameter overlap (#4 ≈ 15 in) and tie with wire. The 10% waste covers laps and cuts.
- Footings: typically 2–3 continuous #4/#5 bars top and bottom, with vertical dowels tying the wall above.
- For structural slabs, retaining walls, or anything over 4 ft — get an engineer's bar schedule, not a calculator.
Frequently asked questions
How much rebar do I need for a slab?
Grid math: a 24×20 ft slab at 18 in spacing needs 15 bars one way × 17 the other ≈ 700 ft + 10% ≈ 770 ft → 39 sticks of #4 (~514 lb).
What size rebar for a concrete slab?
#4 (1/2 in) for residential slabs and footings; #5 (5/8 in) for heavy loads, thick slabs, and commercial work.
How far apart should rebar be?
18 inches on-center is typical for residential slabs; 12 inches for driveways, heavy loads, and structural work.
How much does rebar weigh?
#4 weighs 0.668 lb/ft, #5 weighs 1.043 lb/ft. A 20-ft stick of #4 is ~13.4 lb — suppliers price by the ton.
How long should rebar laps be?
30× the bar diameter minimum: ~15 inches for #4, ~19 inches for #5. Tie every splice with rebar wire.
Do I need chairs under rebar?
Yes — non-negotiable. Rebar belongs in the middle third of the slab. Steel sitting on dirt rusts and contributes nothing.
Can I use wire mesh instead of rebar?
For light-duty slabs (sheds, walkways), 6×6 WWM works. For driveways and structural slabs, rebar's strength wins — mesh ends up at the bottom anyway without chairs.
How do I cut rebar?
Angle grinder with a metal cutoff wheel, or bolt cutters up to #4. Cut outside — the sparks and dust are brutal indoors.
Will rebar rust inside concrete?
Concrete's alkalinity protects it — that is the whole system. Problems start only with cracks + water + salt exposure (coastal/road salt).
How much does rebar cost?
Roughly $0.50–1.00 per foot for #4 depending on region and quantity (~$400–800/ton). Buy full bundles from a steel supplier, not per-stick at big-box stores.