Calculate rebar bar count, spacing, linear footage, and total weight for slabs, footings, and walls.
| Bar Size | Diameter (in) | Area (in²) | Weight (lb/ft) |
|---|---|---|---|
| #3 | 0.375 | 0.11 | 0.376 |
| #4 | 0.500 | 0.20 | 0.668 |
| #5 | 0.625 | 0.31 | 1.043 |
| #6 | 0.750 | 0.44 | 1.502 |
| #7 | 0.875 | 0.60 | 2.044 |
| #8 | 1.000 | 0.79 | 2.670 |
| #9 | 1.128 | 1.00 | 3.400 |
| #10 | 1.270 | 1.27 | 4.303 |
| #11 | 1.410 | 1.56 | 5.313 |
| #14 | 1.693 | 2.25 | 7.650 |
| #18 | 2.257 | 4.00 | 13.600 |
Select your project type (Slab/Footing, Wall, or Circular Slab) using the tabs above. Enter your dimensions in feet, choose your rebar spacing in inches, select the bar size from the dropdown, and enter the standard bar length you'll be purchasing (typically 20 or 40 ft). Hit Calculate Rebar to instantly get bar counts, total linear footage, number of full bars to buy, and total weight.
The diagram updates to reflect your slab dimensions and spacing, giving you a visual preview of the rebar grid layout.
Ordering the right amount of rebar before a pour is one of those things that sounds simple but causes real headaches on job sites every day. Over-order and you're hauling scrap; under-order and you're stopping a concrete pour mid-job. A typical 20×20 ft residential driveway slab with #4 bar at 12-inch spacing requires about 82 bars — that's roughly 1,640 linear feet and over 1,000 lbs of steel. Getting that number right before you call the supplier saves time and money.
This rebar calculator is useful for contractors, homeowners doing DIY foundations, structural engineers doing quick estimates, and anyone preparing a material takeoff. It handles the most common scenarios: flat slabs (driveways, garage floors, patios), concrete footings, retaining walls, and circular pads for columns or round tanks. The lap splice allowance is especially important — a 18-inch overlap on every joint adds up to 5–10% more material than the bare dimension calculation suggests.
For a 30×8 ft retaining wall with #4 bar at 12-inch spacing in both directions, you'd need roughly 76 bars and about 460 lbs of steel — numbers you can quote to a supplier in under a minute using this tool.
Bar Count (one direction): bars = floor(dimension / spacing) + 1
Linear Footage per direction: linear_ft = bars × perpendicular_dimension
With lap splices: Each bar run longer than the standard bar length requires one or more splices. The extra footage per splice equals the overlap distance (e.g., 18 inches = 1.5 ft).
Total Weight: weight (lb) = total_linear_ft × lb_per_ft where lb/ft comes from the ASTM A615 standard weight table for each bar designation.
Bars to Purchase: ceil(total_linear_ft / standard_bar_length) — always rounds up so you never come up short.
For circular slabs, the calculator computes the chord lengths of each parallel rebar pass across the circle using the formula: chord = 2 × sqrt(r² − d²) where r is the radius and d is the distance from center to that bar's row.
#4 rebar (1/2-inch diameter) at 12-inch spacing is the most common choice for residential driveways and garage floors up to 4 inches thick. For heavier loads like RV pads or commercial driveways, step up to #4 at 8–10 inches or #5 at 12 inches. Always check your local building department's requirements before pouring.
Rebar spacing is the center-to-center distance between parallel bars in one direction of the grid. A 12-inch spacing means each bar is 12 inches from the next, measured from the center of one bar to the center of the adjacent bar. Closer spacing increases strength but also increases material cost and installation time.
Rebar is most commonly sold in 20-foot and 40-foot lengths at suppliers and big-box stores. Custom lengths are available through steel distributors. The 20-ft length is the standard for most residential jobs since it fits in a pickup truck bed with a small overhang.
#4 rebar weighs 0.668 lb/ft, so a 20-ft bar weighs about 13.4 lbs. #5 weighs 1.043 lb/ft (20.9 lbs per 20-ft bar). Use the bar size reference chart on this page for a full table. Weights are based on the ASTM A615 standard for deformed steel reinforcing bars.