Rebar spacing & sub-base basics (labeled typicals, not engineering)
Under and inside every good slab are two things you cannot see: a compacted gravel base and a grid of reinforcement. This guide shows how to figure quantities for both from simple geometry — strictly as labeled planning typicals, not a structural design.
First, an honest boundary
Everything here is a quantity calculation — how many linear feet of rebar, how many cubic yards of gravel — using common planning conventions. It is not a structural or geotechnical design. How much reinforcement a load-bearing slab actually needs, at what spacing, for your soil and code, is a job for a licensed engineer. Read the numbers below as "how to buy the material once the spacing is chosen," not "what spacing to choose."
Rebar: linear feet from a grid
Rebar in a slab is laid as a grid, and the linear footage is just geometry. For a slab L long and W wide with bars at a given spacing:
Bars along L = floor(W ÷ spacing) + 1, each L long
Bars along W = floor(L ÷ spacing) + 1, each W long
Total linear ft = (bars_L × L) + (bars_W × W), × (1 + overlap%)
The "+1" is the fencepost rule — a grid across a 10 ft span at 2 ft spacing has 6 bars, not 5. The overlap covers the lap where bars are tied end to end. The rebar calculator does all of this.
Worked example: a 12 ft × 10 ft slab at 16-inch (1.333 ft) spacing. Bars along L = floor(10 ÷ 1.333) + 1 = 7 + 1 = 8 bars × 12 ft = 96 ft. Bars along W = floor(12 ÷ 1.333) + 1 = 9 + 1 = 10 bars × 10 ft = 100 ft. Total = 196 linear feet; with 10% overlap, 196 × 1.10 ≈ 216 linear feet of rebar.
Typical spacing — labeled
Residential flatwork commonly uses #3–#4 bar (or welded wire mesh) at 12–18 inches on center. Tighter spacing means more steel and better crack control; wider spacing means less. Again, these are labeled planning typicals — the driveway thickness & rebar reference lists them, but the choice for a load-bearing slab is an engineering decision.
Sub-base: gravel by volume
The gravel base under a slab drains water and spreads load. You buy it by volume (and often by weight for delivery), and the formula is the slab volume formula with a compaction bump:
Cubic yards = area × (depth in ÷ 12) ÷ 27 × (1 + compaction%)
Tons = cubic yards × 1.4
The compaction factor accounts for the gravel settling when tamped — you need to buy a bit more loose material than the compacted volume. The 1.4 tons-per-yard figure is a labeled density typical for crushed gravel. Use the gravel / sub-base calculator (or gravel base for driveway).
Worked example: 300 sq ft at 4 inches deep: 300 × (4 ÷ 12) ÷ 27 = 100 ÷ 27 = 3.70 cubic yards; with 10% compaction, 3.70 × 1.10 = 4.07 cubic yards ≈ 4.07 × 1.4 = 5.7 tons.
Typical sub-base depth — labeled
A common planning range is 4–6 inches of compacted gravel under residential flatwork, deeper for driveways and heavy loads or poor soils. Like everything on this page, treat depth as a labeled typical to plan the buy, and defer the load-bearing decision to your engineer and local code.
Why both matter to cost
Reinforcement and base are the line items people are tempted to skimp on because they vanish under the finished slab — and they are exactly the ones that, done poorly, cause the cracks and settling that cost far more to fix later. In a driveway or slab budget (see driveway cost and slab cost) they are worth their place.
Mesh or bar?
Residential flatwork is reinforced two common ways. Welded wire mesh (a grid of light wires, in sheets or rolls) is inexpensive and quick to place, and it is plenty for many patios, walks and light slabs where the job is crack control rather than load. Rebar (deformed steel bar, sized #3 or #4 for most flatwork) is stiffer and is the usual choice for driveways, thicker slabs and anywhere the concrete carries real weight. The two are not interchangeable dollar-for-dollar: mesh costs less in material and labor but does less; bar costs more but ties a slab together better. For quantity, mesh is figured by area (sheets to cover the slab, plus overlap), while bar is the grid-geometry linear-foot calculation above. Which one a given slab should use is, again, a decision for a load-bearing design, not a rule of thumb — the rebar calculator tells you how much bar to buy once the spacing is set.
Where the steel sits matters as much as how much
A detail that quietly decides whether reinforcement does anything: its position in the slab. Steel laid flat on the subgrade and then buried by the pour ends up at the very bottom, where it does little — a classic field mistake. Rebar and mesh belong in roughly the middle-to-upper third of the slab thickness, held up on small supports called chairs (or lifted as the concrete is placed), so the steel is where the slab flexes and cracks. Bars are also lapped where they meet — overlapped by a set number of bar diameters and tied — which is what the overlap percentage in the quantity formula accounts for. You can buy exactly the right linear footage and still get a weak slab if the steel sits on the ground, so when you estimate the material, remember the chairs and ties that put it in the right place.
The bottom line
Rebar linear feet come from grid geometry (bars = floor(span ÷ spacing) + 1, times bar length, plus overlap) — about 216 feet for a 12×10 slab at 16 inches. Gravel comes from area × depth ÷ 27 with a compaction bump, and about 1.4 tons per yard. All figures here are labeled planning typicals for buying material, not a structural design; a licensed engineer sizes load-bearing concrete and reinforcement.