On-center spacing and edge clearance
Rebar (reinforcing bar) is typically placed in a two-way grid inside a concrete slab, with bars running in both directions at a set on-center (o.c.) spacing — the distance measured from the center of one bar to the center of the next, not the clear gap between bar edges. Edge clearance is the gap left between the outermost bars and the slab edge, commonly required to maintain concrete cover over the steel.
A simple rectangular grid works out how many bars fit at the specified spacing within the usable slab area — the slab dimensions minus edge clearance on both sides — then sums the lengths of all bars running in both directions.
The formula
Usable slab dimensions equal the slab length and width minus twice the edge clearance (clearance is subtracted from both sides of each dimension). The number of bars running lengthwise equals the usable width divided by the spacing, rounded down, plus 1 — the extra bar accounts for the one placed at each end of the range. The same logic gives the bar count running widthwise from the usable length. Total length sums each direction's bar count multiplied by its usable run length.
- Usable length/width (m) = Slab dimension − 2 × Edge clearance
- Bars per direction = floor(Usable dimension ÷ Spacing) + 1
- Total length (m) = (Bars lengthwise × Usable width) + (Bars widthwise × Usable length)
Worked example: a 6 m × 6 m slab
For a 6 m × 6 m slab with 30 cm (0.3 m) bar spacing and 8 cm (0.08 m) edge clearance: usable dimension in each direction = 6 − 2(0.08) = 5.84 m. Bars per direction = floor(5.84 ÷ 0.3) + 1 = 19 + 1 = 20 bars. That gives 40 bars total (20 running lengthwise, 20 running widthwise), each 5.84 m long, for a total combined length of 40 × 5.84 = 233.6 m of rebar.
| Quantity | Value |
|---|---|
| Usable dimension (each direction) | 5.84 m |
| Bars per direction | 20 |
| Total bars | 40 |
| Total rebar length | 233.6 m |
What this estimate doesn't cover
This is a material take-off estimate, not a structural design. Bar size (diameter), spacing and required concrete cover for a given slab are structural decisions made by a qualified engineer based on the applicable building code and the slab's actual loading — this method only estimates quantity for a spacing that has already been specified elsewhere.
It also doesn't add lap splices — the extra length needed where two bars overlap and are tied together across a slab larger than standard bar stock length (commonly 6 m or 12 m). Any slab dimension exceeding stock length needs a lap allowance added on top of this base grid calculation.
Preguntas frecuentes
How do I calculate how much rebar I need for a slab?
Work out the usable slab area (slab dimensions minus edge clearance on both sides), then divide by the on-center bar spacing in each direction to get the bar count, and multiply by the usable run length to get total length.
What does 'on-center' spacing mean for rebar?
On-center (o.c.) spacing is measured from the center of one bar to the center of the next parallel bar, rather than the clear gap between bar edges.
How many bars does a 6x6 metre slab need at 30 cm spacing?
With 8 cm edge clearance, 20 bars run in each direction (40 bars total), each 5.84 m long, for 233.6 m of total rebar length.
Does this rebar estimate include lap splices?
No — it estimates the base grid length only. Where a slab dimension exceeds standard bar stock length (commonly 6 m or 12 m), extra length must be added for the overlap where two bars are spliced together.
Who decides the correct rebar spacing for a slab?
Bar size, spacing and required concrete cover are structural design decisions determined by a qualified engineer based on the applicable building code and the slab's loading.
Referencias
- American Concrete Institute (ACI 318) — Building Code Requirements for Structural Concrete: reinforcement spacing and cover provisions.
- Concrete Reinforcing Steel Institute (CRSI) — reinforcing bar detailing and placement reference.
- International Residential Code (IRC) / International Building Code (IBC) — general slab reinforcement provisions.