Rebar Weight Calculator
Calculate the total weight of rebar by bar size and length. Covers bar sizes #3 through #11 with weight per foot and metric equivalents.
Rebar Spacing Calculator
Calculate the number of rebar bars needed for a slab or wall. Enter span, spacing, and cover to get bar count for both directions.
ft
ft
in
in
Bars (length dir)⧉
12
Each 20 ft long
Bars (width dir)⧉
20
Each 12 ft long
Total Bars⧉
32
Total Linear Feet⧉
480
24 pcs of 20' bars
Planning notes, formulas, and examples
About the Rebar Spacing Calculator
Reinforcing steel (rebar) gives concrete its tensile strength, preventing cracks from becoming structural failures. The spacing of rebar determines how well the reinforcement distributes loads across the concrete section. Too wide a spacing leaves unreinforced zones; too tight wastes material and makes concrete placement difficult.
This calculator computes the number of rebar bars needed in each direction for a rectangular slab, wall, or footing. Enter the span dimensions, the on-center spacing, and the concrete cover distance. The tool calculates bars in both directions (for a grid) and gives you total bar counts and total linear feet of rebar.
Whether you're laying out a #4 bar grid at 12 inches on center for a driveway slab or #5 bars at 18-inch spacing for a foundation wall, this calculator ensures you order the right number of bars and don't run short during placement.
When This Page Helps
Rebar is sold by the piece (typically 20-foot lengths), and running short during placement means a delay while you source more bars. Over-ordering wastes money on heavy material that's expensive to return. This calculator gives you exact counts for both directions of a reinforcement grid.
How to Use the Inputs
- Enter the span length in feet (the longer dimension).
- Enter the span width in feet (the shorter dimension).
- Enter the on-center spacing of bars in inches (e.g., 12, 16, 18, or 24).
- Enter the concrete cover distance in inches (typically 1.5–3 inches).
- Review the bar count in each direction and total linear feet.
- Use the total to order 20-foot bars with an allowance for lap splices.
Formula used
Bars in length direction = floor((Width − 2 × Cover) / Spacing) + 1
Bars in width direction = floor((Length − 2 × Cover) / Spacing) + 1
Total bars = Bars(L) + Bars(W)
Total LF = Bars(L) × Length + Bars(W) × WidthExample Calculation
Result: 29 bars total
A 20×12 ft slab with #4 rebar at 12" on center and 2" cover: bars running the 20 ft direction = (12 − 4) / 1 + 1 = 9 bars. Bars running the 12 ft direction = (20 − 4) / 1 + 1 = 17 bars. Total = 26 bars needing 20 ft × 9 + 12 ft × 17 = 384 linear feet.
Tips & Best Practices
- Standard slab reinforcement is #4 bars at 12–18 inches on center in both directions.
- Concrete cover protects the rebar from corrosion; minimum cover is 1.5 inches for slabs on grade, 3 inches for footings.
- Use rebar chairs or bolsters to hold bars at the correct height within the form.
- Add 10–15% extra rebar length for lap splices (overlaps where bars meet end to end).
- Mark your spacing on the forms with a lumber crayon to ensure even placement.
- Tie bar intersections with wire ties — you don't need to tie every intersection, just enough to hold the grid in place.
Understanding Rebar Grids
A rebar grid consists of bars running in two perpendicular directions, tied together at intersections. The grid provides multi-directional tensile strength to resist cracking from loads, settlement, and thermal changes. Grid spacing is specified by the structural engineer based on load requirements.
Common Rebar Spacing Standards
Residential slabs: #4 at 18" OC both ways. Driveways: #4 at 12" OC both ways. Foundation walls: #4 or #5 at 12" OC vertically, #4 at 24" OC horizontally. Always verify with your structural plans or local building code.
Ordering and Cutting Rebar
Order 20-foot bars and cut to length on site using a rebar cutter or reciprocating saw with a metal blade. Account for lap splice length when calculating total material. A typical lap splice adds 24–40 bar diameters of extra length per splice.
Sources & Methodology
Last updated:
Frequently Asked Questions
On-center (OC) spacing measures from the center of one bar to the center of the next bar. 12-inch OC means bars are 12 inches apart, center to center. This is different from clear spacing, which measures the gap between bar edges.
#3 (3/8") or #4 (1/2") rebar is standard for residential slabs. #4 is most common for driveways and garage floors. #5 or larger is used for structural slabs, commercial applications, and heavily loaded areas.
Cover is the distance from the nearest concrete surface to the nearest rebar. It protects the steel from moisture and corrosion. ACI 318 requires 1.5" cover for slabs on grade, 2" for formed surfaces exposed to weather, and 3" for concrete cast against soil.
For circular slabs, lay bars in a grid pattern across the full diameter in both directions. The number of bars in each direction is: (diameter − 2 × cover) / spacing + 1. Bars will vary in length — outer bars are shorter.
Yes, for slabs subjected to loads, rebar should run in both directions (a grid). One-way reinforcement is only used in specific structural elements like beams and one-way slabs where the load path is clearly directional.
Rebar is typically sold in 20-foot lengths (sometimes 30 or 40 feet for large projects). Where bars must be longer than available stock, lap splices are used to connect bars end-to-end.
More in this topic
Rebar Lap Length Calculator
Calculate required rebar lap splice length by bar size and concrete strength. Includes tension and compression multipliers per ACI 318.
Concrete Slab Volume Calculator
Calculate concrete volume needed for a slab in cubic yards. Enter length, width, and thickness to get accurate material estimates for your project.