Rebar Calculator

How Rebar Quantity Is Calculated

Rebar for a concrete slab is laid in a grid pattern with bars running in both directions (lengthwise and widthwise). The number of bars in each direction equals the slab dimension divided by the spacing, plus one. For a 20-foot-long, 20-foot-wide slab with 18-inch (1.5-foot) spacing: lengthwise bars = (20 x 12 / 18) + 1 = 14 bars, each 20 feet long; widthwise bars = (20 x 12 / 18) + 1 = 14 bars, each 20 feet long. Total bars = 28, total linear feet = 560 feet. This calculator performs these calculations automatically and computes the weight based on the selected bar size.

When ordering rebar, remember that standard stock length is 20 feet. For slabs longer or wider than 20 feet, bars must be spliced (overlapped). Each splice adds additional length that must be factored into the total. For a 30-foot slab with 18-inch spacing, each bar in the long direction requires one splice of 24 inches (for #4 rebar), so the total material per bar is 30 + 2 = 32 feet, requiring two pieces of 20-foot rebar cut to length. Always order 10% extra rebar to account for splices, waste, and field adjustments.

Rebar Sizes: Complete Reference Table (#3 through #8)

Rebar is designated by bar numbers that correspond to the diameter in eighths of an inch. A #4 bar is 4/8 or 1/2 inch in diameter. The bar number also determines the cross-sectional area and weight per foot, which are important for structural calculations and material ordering. Here is a complete reference for the most commonly used rebar sizes:

For most residential concrete work, #3 and #4 rebar handle the job. #3 bars are sufficient for 4-inch sidewalks and lightly loaded slabs. #4 bars are the workhorse for driveways, garage floors, basement slabs, and standard footings. #5 bars enter the picture for foundation walls, larger footings, and any application where an engineer specifies increased reinforcement. Sizes #6 through #8 are typically reserved for commercial construction, retaining walls, and structural engineering applications.

Rebar Spacing Requirements by Application

Rebar spacing determines how many bars go into a given area and directly affects the slab's load-carrying capacity. Closer spacing provides more reinforcement but costs more in material and labor. The spacing is always measured "on center" (OC), meaning from the center of one bar to the center of the next.

Lap Splice Length and Overlap Requirements

When rebar runs are longer than the standard 20-foot stock length, bars must be overlapped (spliced) to maintain structural continuity. The overlap length, called the lap splice, depends on the bar size, concrete strength, and the type of splice (tension or compression). For standard residential construction with 3,000-4,000 PSI concrete, the ACI 318 building code requires a tension lap splice of at least 40 bar diameters.

Splice locations should be staggered so that adjacent bars do not splice at the same point. ACI 318 requires that no more than 50% of bars be spliced at any cross-section for Class B splices (the most common type). In practice, this means offsetting splices by at least the splice length. Tie spliced bars together with 16-gauge tie wire at each end and in the middle of the overlap to keep them in contact during the concrete pour.

Proper Rebar Placement and Concrete Cover

Rebar must be positioned at the correct depth within the concrete to function properly. For slabs on grade (ground-supported), bars should sit at approximately the mid-depth or slightly below mid-depth of the slab. For a 4-inch slab, this means the rebar centers should be about 2 inches from the bottom surface. Use rebar chairs (also called bar supports or bolsters) to hold bars at the correct height -- never let rebar rest directly on the subgrade, as it will sink to the bottom during the pour and provide no reinforcement.

Concrete cover is the distance from the nearest rebar surface to the nearest concrete surface. ACI 318 specifies minimum cover requirements to protect rebar from moisture, chemicals, and fire. For concrete cast against and permanently exposed to earth (bottom of footings, slab on grade), the minimum cover is 3 inches. For concrete exposed to weather (exterior walls, columns, beams), the minimum is 1.5 to 2 inches. For interior concrete not exposed to weather, 0.75 inch minimum for #5 and smaller bars. In areas where deicing chemicals are used, increase cover to at least 2 inches to slow chloride-induced corrosion.

Rebar vs. Wire Mesh: When to Use Each

Welded wire mesh (WWM or WWF) is an alternative to rebar for light-duty concrete slabs. It consists of steel wires welded into a grid, typically 6x6 inches with W1.4 or W2.0 wire (6x6-W1.4xW1.4 is the most common residential specification). Wire mesh is faster to install than individual rebar pieces and costs less for small projects, making it popular for sidewalks, small patios, and non-structural slabs.

However, rebar is superior to wire mesh in several important ways. Rebar provides significantly more cross-sectional steel area, meaning greater tensile strength. A grid of #4 rebar at 12-inch spacing provides roughly 4 times the steel area of standard 6x6 wire mesh. Wire mesh is also difficult to keep at the proper depth during a pour -- it tends to sink to the bottom of the slab unless carefully supported on chairs, which largely negates its installation speed advantage. For driveways, garage floors, and any slab that will support vehicles or heavy loads, rebar is the better choice.

This calculator is for informational purposes only and does not constitute financial, tax, or legal advice. Always consult a qualified professional for decisions specific to your situation.