Rebar spacing affects both structural performance and material planning. Bars placed too far apart may not satisfy design requirements, while overly tight spacing can increase cost and make concrete placement harder. This calculator estimates the bar count and final spacing for a rectangular slab after accounting for edge cover.
The rebar spacing formulas
For each direction, with the slab dimension
The
Worked example
The calculator’s defaults are a 20 ft × 12 ft slab, 2-inch edge cover, 12-inch maximum spacing.
Convert and subtract cover: usable length
Bars across the width (running parallel to length):
Bars across the length (running parallel to width):
Total:
What your results mean
Bars running parallel to length are spaced across the slab width. Bars running parallel to width are spaced across the slab length. The labels describe the direction each bar travels, not the direction in which the spacing is measured.
The actual spacing values show the layout produced by the required whole number of spaces. For the default example, the 12-inch maximum becomes 11.67 inches across the width and 11.80 inches across the length. Both values remain below the selected limit.
The total of 34 bars is a planning estimate for a single rectangular grid. It does not include additional reinforcement at openings, supports, edges, corners, construction joints, or concentrated loads. It also does not calculate bar length, lap splices, chairs, tie wire, or waste.
Practical checks before ordering
Verify the required bar diameter and grade separately from the spacing. Confirm that edge cover is appropriate for the exposure and placement conditions, and check whether the design calls for one grid or reinforcement at multiple elevations. For a structural slab, the engineer’s drawings and local code take priority over this estimate.
Total length and stock bars to order
Each bar runs the slab’s dimension minus the cover at both ends. For the default slab:
- 13 bars at
in (19.67 ft) = about 255.7 ft. - 21 bars at
in (11.67 ft) = 245.0 ft.
That’s about 501 feet of bar. Rebar is commonly sold in 20-foot lengths. Each 19.67-ft bar uses one stick, and two 11.67-ft bars won’t fit in one 20-ft stick, so this layout needs 34 sticks, with offcuts saved for chairs or ties.
For #4 bar, which weighs about 0.668 lb per foot, the grid weighs about
Bars longer than stock length
When a slab is longer than the bars you can buy, two bars must overlap at a lap splice. The required lap length depends on the bar size and the design, and is set by the engineer or local code. It’s often stated as a multiple of the bar diameter. Add the laps to your total length before ordering.
Common mistakes
- Subtracting cover from one side only. Cover applies at both edges, so take
off each dimension. - Measuring spacing edge to edge. Spacing is measured centre to centre of the bars.
- Leaving bars on the ground. Rebar must sit on chairs or blocks at the specified depth. Bar lying on the base does little once the concrete is poured.
- Forgetting laps and extra bars. Openings, corners and thickened edges often need extra reinforcement on top of the basic grid.