Rebar Calculator
Free Rebar Calculator for construction. Enter project dimensions to estimate materials, costs, and requirements.
Formula
Bars = (Dimension / Spacing) + 1
Calculate number of bars based on spacing, multiply by length to get total linear feet, then multiply by weight per foot.
Worked Examples
Example 1: Slab Rebar Grid
Problem:20×20 ft slab, #4 rebar at 12" spacing both ways.
Solution:Bars in each direction: (20 × 12) / 12 + 1 = 21 bars Length per bar: 20 ft Total length: 21 + 21 = 42 bars × 20 ft = 840 ft Weight: 840 × 0.668 lb/ft = 561 lbs Standard 20-ft bars: 42
Result:42 bars × 20 ft (#4) = 561 lbs
Example 2: Foundation Rebar
Problem:Footing requires 4 continuous #5 bars, 100 ft long each.
Solution:Bars: 4 Length each: 100 ft Total: 400 ft Weight: 400 × 1.043 = 417 lbs Standard bars: 400 / 20 = 20 bars (with laps)
Result:20 #5 bars, 417 lbs total
Example 3: Column Vertical Rebar
Problem:8 columns need 6 vertical #4 bars each, 10 ft tall, plus 2 ft into footing.
Solution:Per column: 6 bars × 12 ft = 72 ft Total 8 columns: 72 × 8 = 576 ft Weight: 576 × 0.668 = 385 lbs Bars needed: 576 / 20 = 29
Result:29 bars, 385 lbs
Frequently Asked Questions
What size rebar do I need?
#3 and #4: Common for residential slabs, driveways, sidewalks. #5 and #6: Structural applications, retaining walls, foundations. #7 and #8: Heavy structural, commercial. Most DIY concrete uses #3 or #4 rebar. Check building code or engineer specs.
How much does rebar weigh?
#3: 0.38 lb/ft. #4: 0.67 lb/ft. #5: 1.04 lb/ft. #6: 1.50 lb/ft. Weight matters for handling and structural calculations. A 20-ft #4 bar weighs ~13 lbs. Truck delivery often required for large quantities.
What spacing for rebar in concrete slab?
Typical residential: 12-18 inch grid. Heavy loads: 8-12 inch grid. Placed in bottom third of slab (usually on chairs). Some codes require rebar, others allow wire mesh. Engineer may specify based on soil conditions and loads.
How do I overlap (lap) rebar?
Minimum lap length typically 20-40× bar diameter. #4 bar: 20-40 inches. #5 bar: 25-50 inches. Longer laps for tension areas. Overlap at least 6 inches and tie with wire. Stagger splices to avoid creating weak planes.
Can I use wire mesh instead of rebar?
Wire mesh (WWF - welded wire fabric) is alternative for slabs. Common: 6×6-W1.4×W1.4 (6-inch squares, W1.4 wire). Easier to place than rebar grid. Rebar is stronger and required for structural applications. Both work for residential slabs.
What is rebar cover?
Cover is concrete thickness between rebar and surface. Minimum cover protects rebar from corrosion and provides fire resistance. Typical: 1.5-2 inches for formed work, 3 inches for concrete cast against earth. Use plastic chairs to maintain proper cover.
How do I cut rebar?
Hacksaw: Slow but works for a few bars. Angle grinder with cutoff wheel: Fast, creates sparks. Rebar cutter (manual or electric): Cleanest cuts, best for many bars. Bolt cutters: Work for #3 and small #4. Always wear safety glasses.
What is epoxy-coated rebar?
Rebar with epoxy coating for corrosion protection. Required for structures exposed to de-icing salts (parking structures, bridges) or marine environments. Costs 50-70% more than regular rebar. Handle carefully to avoid coating damage. Not needed for most residential work.
Do I need to tie all rebar intersections?
Code requires tying intersections to prevent movement during concrete placement. Use 16-gauge tie wire twisted tight with pliers or tie-wire tool. Don't need to tie every single intersection - alternate pattern acceptable for large grids. All perimeter intersections should be tied.
Can I reuse old rebar?
Only if free from heavy rust, damage, and coatings. Surface rust is okay (even helpful for bonding). Bent rebar can't be straightened safely - bending stresses weaken steel. Most demolition rebar is recycled as scrap, not reused structurally.