Rebar Calculator

What is a rebar calculator?

A rebar calculator works out the steel reinforcement grid for a concrete slab. Concrete is strong in compression but weak in tension, so a slab is reinforced with a mat of steel bars laid at right angles to each other. This calculator takes the slab’s dimensions, the spacing of that grid, the cover you leave at the edges, and the bar size, and returns the three numbers you actually need to order and place the steel: how many bars run each way, the total linear length of bar, and the total weight of steel.

Suppliers sell rebar by weight or by the length of bar, while the drawing specifies a grid — so the total-length and total-weight figures are what turn a spacing on paper into a delivery you can price.

How does the calculator work?

Enter the slab length and slab width, the rebar spacing (the centre-to-centre distance of the grid, commonly 12 in or 30 cm), and the edge cover (the distance from the slab edge in to the first bar, commonly 3 in or 7.5 cm). Then pick the rebar size.

The calculator reduces each side by twice the cover to get the reinforced span, divides that span by the spacing, rounds down, and adds one — the “fence post” count, because a grid with $n$ gaps needs $n + 1$ bars. It does this once for each direction, multiplies each bar count by the length of the bars in that direction, and adds the two totals. Finally it multiplies the total length by the unit mass of the chosen bar to get the weight of steel.

Every measurement has its own unit selector, so you can enter the slab in metres and the spacing in inches if that is how your drawing reads — the calculator converts everything internally. The total length reads in metres, feet, yards, and more; the total weight reads in kilograms, pounds, tonnes, or US tons.

Formulas

Take the slab length $L$, the slab width $W$, the grid spacing $s$, and the edge cover $c$. Reducing a side by the cover on both edges gives the reinforced span, so the number of bars spaced out along the length is:

$$n_L = \left\lfloor \frac{L - 2c}{s} \right\rfloor + 1$$

and the number spaced out along the width is:

$$n_W = \left\lfloor \frac{W - 2c}{s} \right\rfloor + 1$$

The floor function rounds down to a whole number of gaps, and the $+ 1$ adds the bar that closes the last gap. Note the crossing: the $n_L$ bars counted along the length each run widthwise, so each one is $W - 2c$ long, and the $n_W$ bars each run lengthwise at $L - 2c$ long. The total linear length of bar $T$ is therefore:

$$T = n_L \times (W - 2c) + n_W \times (L - 2c)$$

With a unit mass $m$ for the chosen bar size, the total weight of steel $M$ is:

$$M = T \times m$$

Rebar sizes and unit mass

The size dropdown carries the four bar sizes most often used in slabs, with the nominal masses published in ASTM A615/A615M, the US standard for carbon-steel reinforcing bars. The standard prints a soft-metric mass in kg/m and an inch-pound weight in lb/ft, which is why the two columns differ slightly in the last digit rather than converting exactly into one another.

The bar number is the diameter in eighths of an inch — a #4 bar is 4/8 in, or 1/2 in, across. The calculator uses the kg/m column internally and converts to whatever weight unit you select. Results are an estimate based on this published table; a structural drawing always overrules it.

Worked examples

Example 1: a 6 m × 4 m slab with a 300 mm grid

A slab is 6 m long and 4 m wide, reinforced with #4 bar on a 0.3 m grid, with 0.075 m of cover at the edges. The reinforced spans are $6 - 2 \times 0.075 = 5.85$ m and $4 - 2 \times 0.075 = 3.85$ m.

$$n_L = \left\lfloor \frac{6 - 2 \times 0.075}{0.3} \right\rfloor + 1 = \lfloor 19.5 \rfloor + 1 = 20$$ $$n_W = \left\lfloor \frac{4 - 2 \times 0.075}{0.3} \right\rfloor + 1 = \lfloor 12.83 \rfloor + 1 = 13$$

The 20 bars counted along the length each run widthwise at 3.85 m, and the 13 bars counted along the width each run lengthwise at 5.85 m:

$$T = 20 \times 3.85 + 13 \times 5.85 = 77 + 76.05 = 153.05 \, \text{m}$$ $$M = 153.05 \times 0.996 = 152.44 \, \text{kg}$$

So the slab takes 20 bars one way and 13 the other — 153.05 m of #4 bar, weighing about 152.44 kg (336.07 lb).

Example 2: a 20 ft × 10 ft slab with a 12 in grid

A slab is 20 ft long and 10 ft wide, reinforced with #4 bar on a 12 in (1 ft) grid, with 3 in of cover. The reinforced spans are 19.5 ft and 9.5 ft.

$$n_L = \left\lfloor \frac{19.5}{1} \right\rfloor + 1 = 20$$ $$n_W = \left\lfloor \frac{9.5}{1} \right\rfloor + 1 = 10$$ $$T = 20 \times 9.5 + 10 \times 19.5 = 190 + 195 = 385 \, \text{ft}$$

That is 385 ft of #4 bar, which weighs about 257.67 lb (116.88 kg). Notice how much the grid costs you in steel: halving the spacing to 6 in would roughly double both figures.

Practical notes

• Spacing is centre to centre. The grid dimension on a drawing is measured bar centre to bar centre, not between the faces of adjacent bars. Enter it that way here.
• Cover at the edge is not cover under the slab. This calculator’s cover is the horizontal setback from the slab edge to the first bar. The concrete cover beneath the mat — the depth of steel in the pour — is a separate structural requirement and does not change the bar count.
• Order in stock lengths and allow for laps. Rebar is sold in fixed stock lengths (commonly 20 ft or 6 m and 12 m). A bar longer than stock has to be spliced, and a lap splice typically overlaps by 40 or more bar diameters — that overlap is extra steel this calculator does not add, so pad the order for it on larger slabs.
• Add waste for cutting and bends. Cutting bars to fit and bending them around edges leaves offcuts. A small allowance on top of the calculated length keeps you from running short.
• Cross-check the concrete too. Once the steel is settled, use the concrete calculator to work out the pour volume and bag count for the same slab.