ft
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Standard rebar is sold in 20 ft lengths
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Total Rebar Bars Needed
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🔩 Note: Results include 10% waste factor for lap splices and cuts. Rebar is typically sold in 20-foot lengths. Always verify requirements with your engineer or local building code.
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How to Calculate Steel Rebar for Concrete
The number of rebar bars needed depends on the concrete area dimensions, bar spacing, and whether you're laying bars in one direction or a grid. Our calculator uses a standard grid layout (both directions) and adds 10% waste for lap splices and cutting.
Rebar Calculation Formula
Bars needed = ⌈Length / Spacing⌉ + 1 (per direction)
Bars L-direction = ⌈Length / Spacing⌉ + 1 (these span the Width)
Bars W-direction = ⌈Width / Spacing⌉ + 1 (these span the Length)
Total linear ft = (BarsL × Width + BarsW × Length) × 1.10
Total bars = Total linear ft ÷ Bar length (rounded up)
Example: 20×15 ft, 12" spacing, 20 ft bars:
21 bars × 15 ft + 16 bars × 20 ft = 635 ft × 1.10 = 699 ft → 35 bars, 467 lbs
Bars W-direction = ⌈Width / Spacing⌉ + 1 (these span the Length)
Total linear ft = (BarsL × Width + BarsW × Length) × 1.10
Total bars = Total linear ft ÷ Bar length (rounded up)
Example: 20×15 ft, 12" spacing, 20 ft bars:
21 bars × 15 ft + 16 bars × 20 ft = 635 ft × 1.10 = 699 ft → 35 bars, 467 lbs
Rebar Weight Per Linear Foot
- #3 (3/8"): 0.376 lb/ft — light residential walkways
- #4 (1/2"): 0.668 lb/ft — standard residential slabs and driveways
- #5 (5/8"): 1.043 lb/ft — structural slabs, retaining walls
- #6 (3/4"): 1.502 lb/ft — heavy structural elements
- #8 (1"): 2.670 lb/ft — heavy beams, bridge decks
💡 Buying Tip: Rebar is sold in standard 20-foot lengths at most hardware stores and steel suppliers. Buy in quantity for significant savings — bulk orders from steel distributors can be 20–40% cheaper per foot than retail hardware stores.
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Frequently Asked Questions
How many rebar bars do I need?
Divide your concrete element length by the rebar spacing and add 1 to get the number of bars in one direction. Multiply by the element width to get linear footage. Repeat for the other direction if using a grid layout. Always add 10% for overlaps and waste.
How much does a 20-foot rebar weigh?
A 20-foot length of #4 rebar (1/2 inch, the most common size) weighs 20 × 0.668 = 13.36 lbs. #3 rebar at 20 ft = 7.5 lbs; #5 rebar at 20 ft = 20.86 lbs. Rebar is sold in 20-foot standard lengths.
What spacing should I use for rebar in concrete?
12-inch spacing on center in both directions is the most common standard for residential concrete slabs. Use 6–8 inch spacing for structural applications. Use 18-inch spacing for light-duty patios and walkways. Always check local building codes and engineer specifications.
How much does rebar cost?
#3 rebar typically runs $0.25–$0.45/ft, #4 runs $0.35–$0.60/ft, and #5 runs $0.55–$0.85/ft at retail. Bulk purchases from steel suppliers are significantly cheaper. Get quotes from local suppliers for current pricing as steel prices fluctuate.