This rebar calculator lays out a reinforcement grid for a slab or footing: bars each way at your spacing, total linear feet, approximate weight, and how many standard 20-foot sticks to buy.
Bars are inset 3″ from slab edges (concrete cover) — the grid math below assumes edge-to-edge for estimating; real layouts also need lap splices of ~24–30 bar diameters where sticks join, so add a couple of extra pieces to the truck.
Grid Arithmetic
Bars run both directions: across the width every spacing inches gives ⌊width ÷ spacing⌋ + 1 bars, each the slab’s length (the +1 is the far-edge bar), and symmetrically the other way. Multiply by the bar’s weight per foot for the tonnage estimate your truck cares about. The concrete itself is our Concrete Calculator‘s department. Part of our Construction Calculators collection.
Structural disclaimer in plain words: grid spacing and bar size for anything structural (driveways, footings, retaining anything) come from your local code or an engineer — this calculator estimates quantities for a given grid, it does not design the grid.
Worked Example (Verified)
A 20 × 20 ft slab, #4 bar at 12″ spacing:
- Bars each way: 240″ ÷ 12″ = 20 spaces + 1 = 21 bars
- Total: 21 × 20 ft + 21 × 20 ft = 840 linear feet
- Sticks: 840 ÷ 20 ft = 42 pieces; weight: 840 × 0.668 = ≈ 561 lb of steel
- At 18″ spacing the same slab drops to 2 × 14 bars = 560 lin ft — spacing is the budget lever
Field Notes
- Chairs and cover: bars belong in the slab’s middle-to-lower third, held on chairs/dobies with ~3″ of concrete below them against ground. Rebar lying in the dirt at the pour is decoration, not reinforcement.
- Laps: where bars join, overlap ~24–30× the bar diameter (for #4, roughly 12–15″ minimum, code often wants more) and tie. Continuous steel is the point of the exercise.
- Mesh vs rebar: welded wire mesh handles shrinkage cracking in light slabs; driveways, hot-tub pads and anything with real loads graduate to a rebar grid like this one.
- Cut with a grinder or bolt cutters, bend with a hickey — and cap exposed ends; rebar injuries are an ER staple.
Related Tools
- Concrete Calculator — the slab itself
- Concrete Block Calculator — walls on the footing
- Construction Calculators — the full collection
Frequently Asked Questions
How much rebar do I need for a 20×20 slab?
At 12″ grid spacing with #4 bar: 21 bars each way = 840 linear feet = 42 twenty-foot sticks, about 561 lb. At 18″ spacing it drops to 560 linear feet.
What spacing should a rebar grid be?
Common slab grids run 12–24 inches depending on load and slab thickness — 12–18″ for driveways and structural slabs, 18–24″ for light patios. Your code or engineer specifies; this calculator prices the grid you choose.
What does #4 rebar mean?
The number is the diameter in eighths of an inch: #4 = 4/8 = ½″ diameter, 0.668 lb per foot. #3 is ⅜″, #5 is ⅝″. Bigger number, fatter bar, more steel per foot.
How much do I overlap rebar at splices?
A common rule is 24–30 times the bar diameter (roughly 12–15″ for #4), with many codes and engineers specifying 40× or a fixed minimum — when in doubt, lap longer and tie well, and follow your local spec for structural work.
Rebar or wire mesh for a patio?
Mesh is acceptable for light-duty patios and walks; rebar grids win where loads, vehicle traffic or poor soils appear. Many builders use both philosophies by zone: mesh in the field, rebar at edges and thickened sections.
Is this rebar calculator free?
Yes — free, instant, no sign-up.