Rebar Calculator: Calculate Reinforcement for Concrete Slabs
Use our free rebar calculator to determine the quantity, spacing, and weight of rebar needed for concrete slabs, footings, and walls. Built for contractors, estimators, and DIY concrete pours, it converts your slab dimensions and chosen bar spacing into a bar count, total linear footage, and total steel weight so you can order the right amount the first time.
How to use this calculator.
- Enter slab dimensions. Input the length and width of your concrete slab in feet.
- Set rebar spacing. Enter the center-to-center spacing between bars, in inches (default 18").
- Select bar size. Choose the rebar size, #3 through #11 (default #4, the standard for residential slabs).
- Review your results. The calculator shows the lengthwise and crosswise bar counts, total linear feet of rebar, and total weight.
- Order with a waste allowance. Add 5-10% to the total length for laps, splices, and cut-offs before placing your order.
How do you calculate rebar quantities and weight?
The calculator first works out how many bars run in each direction based on spacing, then adds up their combined length, then converts that length to weight based on bar size.
Lengthwise bars = (Slab Width × 12 ÷ Spacing) + 1
Crosswise bars = (Slab Length × 12 ÷ Spacing) + 1
Total length (ft) = (Lengthwise bars × Slab Length) + (Crosswise bars × Slab Width)
Total weight (lbs) = Total Length × (Bar Size² ÷ 24) × 3.4
Where:
- Slab Length / Slab Width: The outer dimensions of the slab, in feet.
- Spacing: The center-to-center distance between parallel bars, in inches. Multiplying width or length by 12 converts feet to inches so the units match.
- Bar Size (#): The rebar's nominal size number, such as #4 for a 1/2-inch diameter bar. Larger numbers mean thicker, heavier bars.
- The "+1": Accounts for the bar placed at the starting edge of the slab, in addition to the evenly spaced bars that follow.
Example calculation.
Let's calculate rebar for a garage floor slab.
Scenario: You're reinforcing a 24 ft long, 12 ft wide slab with #4 rebar at 12-inch spacing (typical for a garage floor that carries vehicle loads).
- Lengthwise bars: (12 ft × 12 ÷ 12) + 1 = (144 ÷ 12) + 1 = 12 + 1 = 13 bars
- Crosswise bars: (24 ft × 12 ÷ 12) + 1 = (288 ÷ 12) + 1 = 24 + 1 = 25 bars
- Total length: (13 × 24) + (25 × 12) = 312 + 300 = 612 ft
- Total weight: 612 × (4² ÷ 24) × 3.4 = 612 × 0.667 × 3.4 = 1,387 lbs
Result: This slab needs 38 bars of #4 rebar totaling 612 linear feet and approximately1,387 lbs. Add 5-10% for waste, which brings the order to roughly 640-675 linear feet.
Tips from the field.
- Add 5-10% extra length for laps. Splices between bar lengths need 12-24 inches of overlap depending on bar size, and this consumes material beyond the raw slab dimensions.
- Support bars at mid-depth. Use rebar chairs or dobies to hold the steel at the center of the slab thickness. Rebar resting on the ground or dirt does nothing to reinforce the concrete above it.
- Keep rebar 2-3 inches from edges. Steel too close to the surface or edge of a slab is prone to rust staining and spalling as moisture reaches it.
- Match bar size to the job. #3 bar suits light-duty patios and walkways, #4 is standard for driveways and garage floors, and #5 or larger is reserved for heavy commercial or structural slabs.
- Tighten spacing under vehicle loads. Use 12-16 inch spacing for driveways and garage floors; 18 inches is adequate for standard patios and walkways that only see foot traffic.
Disclaimer: This calculator provides estimates for planning purposes only. Actual rebar requirements vary based on slab design, load requirements, local building codes, and site conditions. For structural designs or load-bearing applications, consult a licensed civil or structural engineer. PavementCalculator.com assumes no liability for material overages or shortages.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How do I calculate rebar for a concrete slab?
For lengthwise bars: divide slab width (in inches) by the spacing, then add 1. For crosswise bars: divide slab length (in inches) by the spacing, then add 1. Total length = (number of lengthwise bars × slab length) + (number of crosswise bars × slab width). For a 30×20 ft slab with 18-inch spacing, you need about 20 lengthwise and 27 crosswise bars.
What size rebar should I use for a concrete slab?
#3 (3/8 inch) rebar is used for light-duty patios and walkways. #4 (1/2 inch) is standard for residential driveways, garage floors, and slabs. #5 (5/8 inch) is used for heavy-duty commercial floors and structural slabs. Spacing is typically 12-18 inches on center for slabs and 6-12 inches for footings.
How much does rebar weigh?
Rebar weight varies by size: #3 bar = 0.376 lb/ft, #4 bar = 0.668 lb/ft, #5 bar = 1.043 lb/ft, #6 bar = 1.502 lb/ft. Calculate total weight: total linear feet × weight per foot. For 500 feet of #4 rebar: 500 × 0.668 = 334 lbs. Our calculator computes this automatically.
How far apart should rebar be placed?
For residential slabs: 18 inches on center is standard. For driveways and garage floors: 12-16 inches. For footings: 6-12 inches depending on load. Place rebar in the middle of the slab thickness, supported by chairs or dobies. Rebar should be at least 2 inches from any slab edge to prevent rust.
How much rebar do I need for a footing?
Footings typically use 2-4 continuous horizontal bars (#4 or #5) running the full length, with vertical dowels every 4-6 feet. For a 150-linear-foot footing with 2 bars: 150 × 2 = 300 linear feet of horizontal rebar. Add vertical dowels: 150 ÷ 5 ft spacing ≈ 30 dowels at 4 ft each = 120 ft. Total: 420 linear feet.
How much does rebar cost?
Rebar costs $0.50 to $1.50 per linear foot depending on size. #3 bar: ~$0.50/ft, #4 bar: ~$0.75/ft, #5 bar: ~$1.00/ft. For a 30×20 ft slab needing about 900 linear feet of #4 rebar: 900 × $0.75 = $675. Bulk pricing, pre-cut lengths, and epoxy-coated rebar cost more. Always add 5-10% for overlaps and waste.