About the Rebar Calculator
Rebar for a slab runs in a grid in both directions, so the total length needed is more than just the slab's perimeter, it's every row and column of bar laid across the full area.
How It Works
The calculator figures out how many rows of rebar fit across the width and how many across the length at your chosen grid spacing, multiplies each by the perpendicular dimension, and adds both totals together before converting to standard 20 ft bars.
Examples
A garage slab
A 20×15 ft slab at 18 inch grid spacing needs about 496 ft of total rebar length, or 25 bars of standard 20 ft stock.
Tighter spacing for a structural slab
The same slab at 12 inch spacing instead of 18 needs noticeably more bar, since more rows fit across both dimensions.
Advantages
- Calculates a full perpendicular grid, not just a simple perimeter estimate
- Converts total length directly into standard 20 ft bar counts
- Shows the number of tie-wire intersection points for labor planning
Common Mistakes
- Not adding extra length for splice overlaps between bars
- Using perimeter alone instead of a full grid calculation
- Ignoring local code or engineer specifications for grid spacing on structural slabs
Edge Cases to Watch For
- This assumes a simple perpendicular grid with no overlap splices; real installations typically add 12-18 times the bar diameter in overlap at every splice joint, which adds meaningfully to the total length needed.
- Rebar needs support chairs to hold it at the correct height within the slab, a separate material not included in this length estimate.
- Tighter spacing near slab edges or around openings is common in structural applications and isn't captured by a single uniform grid spacing.
Common Use Cases
- Estimating rebar for a driveway, patio, or garage slab
- Budgeting a DIY reinforced concrete project
- Comparing material needs between different grid spacings