Concrete Rebar Calculator

1. Enter the length and width of the area to reinforce, in feet.
2. Set the bar spacing on centre and the edge cover in inches, from your plans or code.
3. Add a lap and waste allowance for splices and offcuts, and pick the bar size.
4. Read the grid: bars each way, total linear feet, 20 foot sticks, weight and the number of tie intersections.

Here is what each result line means:

A concrete rebar calculator turns the size of a slab or footing and a chosen grid spacing into the quantity of reinforcing bar to buy. It counts the bars in each direction, totals their length and converts that to weight and to standard 20 foot sticks.

Reinforcing bar, or rebar, is the steel grid cast into concrete to carry tension the concrete cannot. This tool sizes the quantity for a grid you define; it does not decide whether a slab needs rebar or at what spacing, which come from a code or an engineer. To size the concrete around the grid use the concrete weight calculator for the placed weight, or the concrete mix calculator for the mix.

Rebar in flatwork is usually a grid: bars running one way crossed by bars running the other way. In each direction the number of bars is the clear span, from the first bar to the last, divided by the spacing, rounded down, plus one for the starting bar.

1. Convert the length and width to inches and subtract twice the edge cover to get the clear span each way.
2. Divide each clear span by the spacing, round down and add one, for the bar count in each direction.
3. Multiply each count by the length of those bars and add the two totals for the base linear feet.
4. Add the lap allowance, multiply by the bar unit weight for the total weight, and divide by 20 for the stick count.

To size the binder in the surrounding concrete, the cement calculator sizes the cement.

Suppose you are reinforcing a 10 foot by 10 foot slab with a 12 inch grid, 3 inch edge cover, number 4 bar and a 10 percent lap allowance.

Calculation: the clear span is 120 – 6 = 114 inches each way. Bars = 114 / 12 = 9.5, rounded down to 9, plus 1 = 10 bars each way. That is 10 x 10 ft + 10 x 10 ft = 200 ft, and with 10 percent lap, 220 ft. At 0.668 pounds a foot that is about 147 lb of number 4 bar, roughly 11 twenty-foot sticks and 100 tie intersections.

The lesson: the lap allowance and the edge cover both matter; drop them and you under-order the splices and miscount the bars.

Spacing

Tighter spacing means more bars. Halving the spacing roughly doubles the bar count and weight, so the grid you specify drives the whole order.

Bar Size

A heavier bar weighs far more per foot: number 5 bar is over half again the weight of number 4, and number 6 is more than double. The length is the same, but the tonnage is not.

Lap Splices

Bars longer than a stock length are overlapped and tied, and each lap adds bar that is not in the finished grid. A lap allowance of 10 to 15 percent covers typical splicing and offcuts.

Edge Cover

Starting the grid in from the edge shortens the clear span and can drop a bar from the count. Use the cover your code calls for so the bar count matches what you will actually place.

Light slabs are often reinforced with welded wire mesh instead of tied rebar. The two are sized differently and suit different jobs.

For the mesh side, the wire mesh calculator in the related tools sizes sheets by area instead of bars by the foot.

Ordering Steel for a Slab or Footing

Turn the grid on your plans into sticks and weight before you buy, so you order the right quantity and enough for the lap splices.

Pricing a Reinforcement Job

The weight is what a steel yard prices against, so the total pounds let you compare quotes and budget the reinforcement separately from the concrete.

Planning the Tie-up

The tie intersection count estimates how much tie wire and how many clips you need, and hints at how long the grid will take to tie.

1. Forgetting the Plus One

A grid has one more bar than the span divided by spacing, because of the starting bar. Dropping the plus one under-counts every row. The tool adds it for you.

2. Ignoring Lap Splices

Bars longer than a stick are overlapped, so the bar in the ground is more than the slab dimensions suggest. A lap allowance stops you running short on a long run.

3. Using the Wrong Bar Size Weight

Ordering by the wrong size throws the tonnage off, since weight per foot changes sharply between sizes. Pick the size on your plans; the tool uses the ASTM nominal weight.

4. Skipping the Edge Cover

Rebar is held in from the edge for cover, which shortens the clear span. Leaving cover out slightly over-counts the bars.

5. Assuming a Slab Needs This Grid

Whether a slab needs rebar, at what size and spacing, is a code and engineering question. This tool counts a grid you give it; it does not design one.

The counting and the ASTM nominal weights are exact; the usefulness depends on the grid, cover and lap allowance you enter.

Count the bars each way as the clear span divided by the spacing, rounded down, plus one, then multiply by the bar lengths and add a lap allowance. A 10 by 10 foot slab at 12 inch spacing needs about 10 bars each way, roughly 200 feet before laps.

At ASTM A615 nominal weights, number 3 bar is 0.376 pounds a foot, number 4 is 0.668, number 5 is 1.043 and number 6 is 1.502. The bar number is the diameter in eighths of an inch, so number 4 is 4/8 or half an inch.

Common slab grids are 12 to 18 inches on centre, but the right spacing depends on the slab, the load and the code. This tool counts the grid at whatever spacing you enter; it does not set the spacing, which comes from your plans or an engineer.

Divide the total linear feet, including laps, by the stock length, usually 20 feet, and round up. The 10 by 10 foot example at 220 feet with laps works out to about 11 twenty-foot sticks. The tool shows the stick count for your grid.

A grid has one more bar than the span divided by the spacing, because you need a bar at the start as well as after each spacing step. Counting without the plus one under-counts every row, so the tool adds it automatically.

Bars longer than a stock length are overlapped and tied, and that overlap is extra bar not shown by the slab size. A 10 to 15 percent lap and waste allowance covers typical splices and offcuts so you do not run short.

It estimates the tie intersections where the grid crosses, which guides tie wire and clips, but it does not quantify chairs, dowels, hooks or bends. Add those from your drawings.

Yes. Enter the footing length and width as the area and the grid spacing, and it counts the bars the same way. For a narrow footing the cross bars may reduce to one, which the tool handles.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• American Concrete Institute (ACI; reinforced concrete and the ACI 318 design standard).
• CRSI reinforcing bar basics (Concrete Reinforcing Steel Institute; ASTM A615 bar sizes and properties. Live in a browser; CRSI blocks automated checks.)
• American Cement Association (American Cement Association; reinforced concrete basics).