Linear feet
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Reinforcement
Rebar calculator
Lay out a two-way rebar grid for a slab and get the bar count, total linear feet, and weight by bar size — the numbers you actually order and tie.
How this is calculated
Lay out a grid at your chosen spacing (commonly 12–18 inches on center), set back from each edge by the clearance (about 3 inches). This calculator counts the bars both directions, totals the linear feet, and weighs it by bar size so you can order by the stick.
bars per direction = ⌊(span − 2×clearance) ÷ spacing⌋ + 1 · weight = total linear ft × lb/ft for the bar size
Bar unit weights: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb/ft. See the methodology for sources. Need the concrete too? Use the slab calculator.
Worked example
For a 20 × 20 ft slab reinforced with #4 bar at 12 inches on-center and a 3-inch edge clearance, the grid runs 20 bars each direction (40 total). With a 10% waste/lap allowance that's about 858 linear feet of rebar. A #4 bar weighs 0.668 lb/ft, so:
858 ft × 0.668 lb/ft ≈ 573 lb of #4 rebar — roughly 43 twenty-foot sticks. Order to the next full stick and allow extra for lap splices (typically 40× the bar diameter) on long runs.
Rebar for a 20 × 20 slab by spacing (#4 bar)
| Spacing | Total bars | Linear feet | Weight (#4) |
|---|---|---|---|
| 12 in o.c. | 40 | 858 ft | 573 lb |
| 16 in o.c. | 30 | 644 ft | 430 lb |
| 18 in o.c. | 28 | 601 ft | 401 lb |
| 24 in o.c. | 20 | 429 ft | 287 lb |
Bar size drives weight even when the layout is identical. That same 858 ft of grid weighs about 323 lb in #3, 573 lb in #4, 895 lb in #5, or 1,289 lb in #6 — so specifying a heavier bar than the design calls for adds cost fast. #4 at 12–18 inches is typical for residential slabs-on-grade; follow your engineer or local code for structural work.
FAQ
How much rebar do I need for a slab?
Lay out a grid at your chosen spacing (commonly 12–18 inches on center), set back from each edge by the clearance (about 3 inches). This calculator counts the bars both directions, totals the linear feet, and weighs it by bar size so you can order by the stick.
What size rebar for a concrete slab?
#3 (3/8") and #4 (1/2") are typical for residential slabs and patios; #5 is common for driveways and structural slabs. Heavier bars and tighter spacing add strength — and weight. The right spec is a structural decision; confirm with your engineer or local code.
How do I account for rebar overlap?
Where bars are spliced end to end, they overlap (lap) by roughly 40 times the bar diameter — about 20 inches for #4. Add a waste margin here to cover laps and offcuts; for long runs, plan splice locations explicitly.