Rebar Calculator

Pick the bar size, say how you want to count the bars, enter the length of each bar, and the calculator returns the number of stock bars to buy, the total length, the weight and a cost. It is built for the quantity side of reinforcing steel. You tell it what the drawings call for, and it turns that into an order.

There are two ways to count. In the first you type the number of bars, which suits footings, beams and columns where the detail says 'three bars continuous' or 'four vertical bars'. In the second you give a run and a spacing, which suits walls and anything with bars at a pitch such as 16 inches on centre. The spacing mode divides the run by the spacing, rounds down and adds one for the end bar, so a 24 ft wall at 16 inches has 19 bars.

The 'sets' box multiplies the whole job. Use it for six identical columns, two faces of a wall, or two layers of steel. It keeps the entry simple: describe one member, then say how many.

The calculator does not choose a bar size or a spacing. Those come from the structural drawings, an engineer or the local code. Think of this page as the take-off sheet that sits beside the drawings.

In the United States a rebar number is its diameter in eighths of an inch. A #4 bar is four eighths, or half an inch. A #8 is a full inch. The bar size table lists the nominal diameter, area and weight for #3 to #11, using the values in the ASTM A615 standard for deformed carbon-steel bars. Metric names are rounded diameters in millimetres, so #4 is No. 13.

Weight per foot grows quickly with size because it follows the area. A #3 bar weighs 0.376 lb per foot and a #8 bar weighs 2.670, about 7.1 times as much for 2.67 times the diameter. That is why changing one bar size on a big job moves the total weight and the cost.

The 'nominal' diameter is the diameter of a plain round bar with the same weight per foot as the deformed bar. The ribs make the outside slightly bigger, which matters when you check cover and clearances but not when you order. A 20 ft bar of #4 weighs about 13.4 lb, and a 20 ft bar of #5 weighs about 20.9 lb.

Most small jobs reduce to a count multiplied by a length. For a continuous footing, the count is the number of longitudinal bars on the detail and the length is the footing length. The footing table shows three #5 bars for footings from 20 to 150 ft. A 20 ft footing needs no laps at all, while a 60 ft footing needs 3 laps in every bar.

For a wall, use the spacing mode. The wall table shows how the number of vertical bars on a 24 ft wall changes with spacing: 13 bars at 24 inches, 19 at 16 inches and 37 at 8 inches. Set sets to 2 when there is a bar curtain on each face. Horizontal bars are a second job: enter the wall height as the run, the horizontal spacing, and the wall length as the bar length.

For columns, enter the vertical bars per column as the count, the column height plus any dowel or lap as the bar length, and the number of columns as sets. The column table shows what happens to the order for four #6 bars in columns 12 ft tall. Ties, stirrups, hooks and bent shapes are cut and bent to a bar schedule, so take those lengths from the schedule and add them separately.

Rebar is sold straight, usually in 20 ft or 40 ft lengths in the US and 6 m or 12 m elsewhere. When a bar must be longer than one stock bar, two bars overlap, and the overlap is the lap splice. Reinforcement can also be anchored by embedding it deeply, and one reference gives 40 to 60 bar diameters for that kind of embedment, which is why 40 diameters is the default lap here. It is a planning figure only. The lap on your job comes from the drawings, and it changes with bar size, steel grade and concrete strength.

The lap table shows the lap lengths for 30, 40 and 50 diameters. A #4 lap at 40 diameters is 20 inches, and a #8 lap is 40 inches. Each lap uses up that much bar, so a footing with many splices needs more steel than its plain length suggests.

The calculator builds long bars from whole stock bars. One lap covers 2 x stock - lap, two laps cover 3 x stock - 2 x lap, and so on. It never reuses an offcut, so the answer leans a little high on purpose. Shorter bars are cut from stock, and the box 'pieces cut from one stock bar' tells you how many fit. Cutting 6 ft pieces from a 20 ft bar gives 3 pieces and 2 ft of scrap. Mechanical couplers are another way to join bars, and they replace the lap, so set the lap to 0 if you use them.

Weight is total length times the nominal weight per foot. Example 1 has 198.75 ft of #5 steel, which is 207.3 lb. The calculator shows weight without waste and with waste, in both pounds and kilograms, so you can check it against a delivery ticket or a freight limit. Short tons are pounds divided by 2,000, and the stock bar table gives the number of bars in a ton for every size.

Waste covers cutting, trimming and the occasional damaged bar. The default of 5 percent is a typical allowance. Raise it for complex cutting and lower it when a supplier cuts to a schedule. The waste table shows how the allowance changes the bars to buy for Example 1.

Cost is the number of stock bars times a price you enter. The default of $12.00 per bar is a placeholder so that the result shows on first load. Rebar prices move with the steel market, bar size and region, so replace it with a current quote. If your quote is per pound or per hundredweight, multiply by the order weight shown, or divide the quote by the stock bar weight in the stock bar table.

Switch the units to metric and the page asks for metres and centimetres, uses the kilograms per metre column of the table and offers 6 m and 12 m stock bars. Bars appear with their soft-metric names, so #5 is No. 16 and #8 is No. 25. These are rounded versions of the same US bars. True metric bars, with sizes like 10, 12, 16 and 20 mm, are a different series with different diameters and weights, so choose the size your supplier actually lists.

To estimate steel per square metre of a slab or wall, count the metres of bar in one square metre. With bars every s centimetres in both directions, that is 2 × 100 / s metres of bar. The square metre table gives the weight per square metre for No. 13 bars. At 20 cm spacing it is 9.94 kg per square metre, so 100 m2 holds roughly 994 kg before edge setback and laps. This is a shortcut for budgeting. For an exact slab order use the real length and width and count the edge bars.

If your plans give steel in kilograms per cubic metre of concrete, that is a design quantity. Multiply it by the concrete volume to cross-check, and treat any gap against the bar-by-bar count as a reason to look at the drawings.

This calculator estimates a quantity. It is not a structural design tool and it does not replace the drawings, an engineer or the local building code. The bar size, spacing, cover, lap length, bends, hooks and the steel grade all come from the structural design. Have an engineer or inspector check reinforcement for foundations, walls, columns, beams and slabs before concrete goes in.

The common mistakes are easy to avoid. Using a bar count from the wrong detail, forgetting a second curtain of steel in a wall, ignoring the laps on long runs and forgetting that corner and step bars are separate pieces will all leave the order short. Counting each end of a run twice is the usual way to end up with one bar too many. Rounding the order to whole stock bars is right, but keep an eye on offcuts, because a good supplier or a good crew can reuse them.

Reinforcement also has to be held at the right height and covered by the right thickness of concrete. Bar chairs or supports, and ties, keep it in place while the pour goes in, and the cover figure is on the drawings. Cover for concrete cast against the ground is 3 inches in the ACI 318-19 code table, which is one reason footing steel is not placed hard against the earth.

Reinforcing steel is only part of a concrete or masonry job. For the base under a slab or footing, the gravel calculator works out the stone volume and tons. If your wall is built from concrete masonry units with steel in the cores, the CMU block calculator counts the blocks, and the stone calculator covers facing or fill. For timber formwork and bracing, the board foot calculator turns lumber sizes into board feet.

Keep the drawings, the bar schedule and your supplier's quote beside this page, and treat the numbers here as the first pass of a take-off that you confirm before you order.