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:\n(20 × 12) / 12 + 1 = 21 bars\n\nLength per bar: 20 ft\nTotal length: 21 + 21 = 42 bars × 20 ft = 840 ft\n\nWeight: 840 × 0.668 lb/ft = 561 lbs\n\nStandard 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\nLength each: 100 ft\nTotal: 400 ft\n\nWeight: 400 × 1.043 = 417 lbs\n\nStandard bars: 400 / 20 = 20 bars\n(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:\n6 bars × 12 ft = 72 ft\n\nTotal 8 columns:\n72 × 8 = 576 ft\n\nWeight: 576 × 0.668 = 385 lbs\n\nBars 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.