Rebar Material Calculator

How the bar count works: one bar per grid line

Rebar in a slab forms a grid, and the count starts from one simple idea: you need one bar for every grid line in each direction. If your slab is 16 feet wide and your bars sit 18 inches apart, then 192 inches of width divided by 18 inches per space gives you 10.67 spaces. Round up to 11 spaces, add the closing bar at the far edge, and you need 12 bars running that direction.

The same arithmetic runs the other way, and then the two counts multiply out into total length:

• Bars across the width: 16 ft x 12 = 192 in, 192 / 18 = 10.67, so 11 spaces + 1 = 12 bars
• Bars along the length: 20 ft x 12 = 240 in, 240 / 18 = 13.33, so 14 spaces + 1 = 15 bars
• Total: 12 x 20 = 240 ft plus 15 x 16 = 240 ft, which is 480 ft of bar

Always round the space count up, never down, and never forget the closing bar. Every calculator for rebar uses this same grid rule, including the one at the top of this page, so you can check any layout in a few seconds.

Worked example: a 20 by 16 foot patio slab

Say you are pouring a 20 foot by 16 foot patio, 4 inches thick, with No. 4 bar on an 18 inch grid. That is one of the most common residential layouts, and it shows every step of the estimate.

• Length direction: 12 bars, each 20 ft long, for 12 x 20 = 240 ft
• Width direction: 15 bars, each 16 ft long, for 15 x 16 = 240 ft
• Total bar length: 240 + 240 = 480 ft
• Sticks: 480 / 20 = 24 sticks of 20 ft bar
• With 10 percent waste: 480 x 1.1 = 528 ft, and 528 / 20 = 26.4, so order 27 sticks

The weight comes from the pounds-per-foot figure for No. 4 bar. At 0.668 pounds per foot, the slab needs 480 x 0.668 = 320.64 pounds of steel. With the waste allowance it is 528 x 0.668 = 352.70 pounds. Those two numbers, feet and pounds, are what a steel supplier quotes against, so write both down before you call.

The figure below shows the same grid-count arithmetic laid out line by line.

Bar sizes, diameters and weight per foot

Rebar sizes are numbered in eighths of an inch, so a No. 4 bar is 4/8 of an inch across, which is half an inch. The bar number also drives the lap splice length, the chair spacing and, most importantly for your order, the weight. The weights below follow the standard bar tables published under ASTM A615, the specification that covers the deformed bars sold at every lumber yard.

You can sanity-check any bar by its weight: a No. 4 stick is 20 x 0.668 = 13.36 pounds. When a supplier quotes by the ton, divide your total pounds by 2,000 to compare quotes fairly. The Concrete Reinforcing Steel Institute publishes the full table for larger bar numbers, but the three above cover almost every residential slab.

Lap splices: why long runs need extra length

Bars come in 20 foot sticks, so any run longer than 20 feet needs a splice. The standard overlap is 40 bar diameters, which the Concrete Reinforcing Steel Institute treats as the common residential splice and which your local code may exceed. The two bars sit side by side inside the concrete, wired together, so tension passes from one bar into the concrete and back into the next.

• No. 3 bar: 40 x 0.375 = 15 inches of overlap
• No. 4 bar: 40 x 0.500 = 20 inches of overlap
• No. 5 bar: 40 x 0.625 = 25 inches of overlap

Each splice adds one lap to the run, so a 30 foot No. 4 run is a 20 ft stick plus a 10 ft cut, with one 20 inch overlap, giving 31.67 feet of bar per finished run. On the 20 foot patio runs above, no splice is needed, which is why the worked example stays clean at 480 feet. If your slab is longer than 20 feet in either direction, add one lap for every joint and let the calculator at the top of this page do the rounding for you.

Weight, cost and what to budget

Feet and pounds both matter when you buy. Retail yards price by the 20 foot stick, typically 5 to 8 dollars for No. 4 depending on region and steel prices. Steel suppliers quote by weight, and the pounds figure is also what you need to plan delivery, because a pallet of rebar gets heavy fast.

For the patio example, a typical order looks like the breakdown below: 27 sticks at 6 dollars each is 162 dollars, plus ties and chairs. Note that two of the slabs in the table run past 20 feet in one direction, so their real orders include the lap feet discussed above. The calculator adds laps automatically.

The figure below breaks that typical order down by line item.

Chairs, edge distance and the rest of the mat

Rebar does no good lying on the dirt. Concrete protects steel from rust only when the bar is fully surrounded, so the mat has to sit in the middle of the pour. Three small details handle that:

• Chairs: plastic or concrete supports that hold the bar about 2 inches off the subbase, spaced every 2 to 3 feet
• Edge distance: keep the outer bars 2 to 3 inches in from the formed edge on all four sides
• Wire ties: tie every intersection, or at minimum every other one, so the grid survives foot traffic during the pour

Chairs run a few dollars per hundred, and a five gallon bucket of tie wire lasts most homeowners years. The steel in a 20 by 16 foot slab weighs 320.64 pounds, which is more than one person should carry in a single trip, so plan on moving sticks to the pour site in batches before you start.

Common mistakes when ordering rebar

Most rebar estimates go wrong in the same few ways, and all of them are easy to catch before you load the truck.

One more habit worth stealing from contractors: write the final cut list on the slab plan before ordering. Knowing that the width direction needs 15 bars at 16 feet each tells you instantly that every 20 foot stick leaves a 4 foot offcut, and offcuts are exactly what you use for the little hooks and dowels every pour seems to need.