Rebar Calculator

Estimate rebar count and total length for a slab grid.

Total rebar length

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Bars lengthwise

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Bars widthwise

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Total bars

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Grid spacing

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A simplified grid estimate with no laps or edge trim. Order extra for overlaps, and follow the structural drawings.

Laying out a reinforcing grid

Slabs are reinforced with a grid of steel bars running both ways. The number of bars in each direction comes from the span divided by the spacing, plus one for the first bar at the edge.

bars = ⌊span ÷ spacing⌋ + 1　·　total length = bars × bar length, both ways

Bars running the length of the slab are spaced across its width, and vice versa. Adding both gives the total grid length — a starting figure before laps, offcuts and edge cover are accounted for.

Worked example

A 5 × 4 m slab with bars at 200 mm spacing both ways needs about 21 bars running lengthwise and 26 running widthwise — roughly 209 m of rebar in total before laps.

Ordering steel

This grid estimate gets you a materials ballpark, but reinforcement is an engineered detail. Bar size, spacing, cover and lap lengths all come from the design, and getting them wrong undermines the slab. Use the figure to budget, not to build.

On-site notes

Add for laps. Overlaps at joins add length — often around 40 bar diameters.
Mind the cover. Bars sit in from edges and off the base on chairs.
Follow the drawings. Spacing and size are an engineer’s call.

Frequently asked questions

How is the bar count worked out?

For each direction, the span is divided by the spacing and one bar is added for the starting edge. The two directions form the reinforcing grid or mesh.

Does this include laps and overlaps?

No. It is the bare grid length. Real jobs add overlap at bar joins (often around 40 times the bar diameter), so order extra to allow for laps and offcuts.

What spacing should I use?

It depends on the design — slabs commonly use 150–300 mm (6–12 in). Spacing, bar size and cover come from a structural engineer’s drawings, not a rule of thumb.

What about edge cover?

Bars are set in from the slab edge for protection. This estimate runs bars full length for simplicity; trim each by twice the cover if you need a tighter figure.