Rebar Calculator
A rebar calculator estimates the total quantity, length, and weight of reinforcing steel bars needed for concrete slabs, walls, footings, and other structural elements. Rebar provides the tensile reinforcement that concrete lacks on its own, preventing cracking and catastrophic failure under load. Getting the right quantity before ordering saves money and avoids costly project delays. This tool calculates the number of bars in each direction based on your slab or wall dimensions and the specified spacing, sums the total linear footage, applies a waste allowance for laps and cutting, and calculates the total weight based on the selected rebar size. Use this estimate to order materials and budget for reinforcing steel. Always follow the structural engineer's specified layout drawing.
Rebar quantity formula
Bars in length direction = floor(Width (in) / Spacing) + 1
Bars in width direction = floor(Length (in) / Spacing) + 1
Total LF = (Bars_L x Length) + (Bars_W x Width)
Total LF with waste = Total LF x (1 + Waste% / 100)
Total Weight = Total LF x lb/ft for selected bar size
This assumes a two-way mat (bars running in both directions). For one-way reinforcement, remove one direction's bars. Add lap splices (typically 1.3 x development length) for long runs that require multiple bars.
Standard rebar weights (ASTM A615)
- #3 (3/8 inch diameter): 0.376 lb/ft
- #4 (1/2 inch diameter): 0.668 lb/ft
- #5 (5/8 inch diameter): 1.043 lb/ft
- #6 (3/4 inch diameter): 1.502 lb/ft
- #7 (7/8 inch diameter): 2.044 lb/ft
- #8 (1 inch diameter): 2.670 lb/ft
Frequently asked questions
What is rebar and why is it used?
Rebar (reinforcing bar) is steel bar embedded in concrete to add tensile strength. Concrete is strong in compression but weak in tension. Rebar carries the tensile forces in beams, slabs, columns, and walls, preventing cracking and structural failure.
How do I calculate how much rebar I need for a slab?
Divide the slab length by the rebar spacing to get the number of bars in one direction, then repeat for the other direction. Multiply the number of bars by the slab dimension in the perpendicular direction to get total linear feet. Double the result for a two-way mat, then add 10% for waste and laps.
What rebar size should I use for a residential slab?
Most residential slabs on grade use #3 (3/8 inch) or #4 (1/2 inch) rebar at 18-inch centers. Structural slabs, walls, and footings typically require #4 or larger at closer spacing. Your engineer of record specifies the required size and spacing.
What does rebar spacing mean?
Rebar spacing is the center-to-center distance between parallel bars in the same direction. Common spacings are 12 inches, 16 inches, and 18 inches. Closer spacing provides more reinforcement and increases concrete cracking resistance.
How much does rebar weigh per foot?
Rebar weight per linear foot by size: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft. These are standard ASTM A615 deformed bar weights.
Official sources
- ASTM International: ASTM A615 - Deformed and Plain Steel Bars for Concrete Reinforcement.
- American Concrete Institute: ACI 318 Building Code Requirements for Structural Concrete.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.