๐ฉ Rebar Calculator PRO
Professional steel reinforcement calculator โ slabs, columns, beams, footings. Bar schedules, lap splices, wire ties, and cost per ACI 318.
Slab Rebar Grid โ updates as you type
Two-way bar layout at the set spacing. Steel weight follows bar count times length.
1 Layout Type
2 Dimensions
2 Column Dimensions
2 Beam Dimensions
3 Rebar Settings
4 Cost & Details
Labour Cost
Results
Bar Counts
Weights & Schedule
Costs
Bar Bending Schedule
| Type | Qty | Length | Shape |
|---|---|---|---|
Material Breakdown by Size
โ ๏ธ Pre-Export Checklist
- Dimensions verified from structural drawings
- Rebar size and spacing per engineer's specification
- Concrete cover maintained per exposure condition
- Lap splices specified (Class A or B per ACI 318)
- Chairs/bolsters for proper rebar elevation
- Temperature and shrinkage steel included where required
- 10% extra for waste, bends, and field adjustments
Rebar Reference & Professional Guide
Show bar size table, formulas, and worked examples โผ
Rebar Size Reference Table
| Size | Diameter | Area (inยฒ) | Weight (lb/ft) | Typical Use |
|---|---|---|---|---|
| #3 | 3/8" | 0.11 | 0.376 | Light slabs, driveways, stirrups |
| #4 | 1/2" | 0.20 | 0.668 | Residential slabs, footings |
| #5 | 5/8" | 0.31 | 1.043 | Structural slabs, walls |
| #6 | 3/4" | 0.44 | 1.502 | Heavy foundations |
| #7 | 7/8" | 0.60 | 2.044 | Commercial footings, columns |
| #8 | 1" | 0.79 | 2.670 | Heavy commercial, bridge decks |
| #9 | 1-1/8" | 1.00 | 3.400 | Columns, transfer beams |
| #10 | 1-1/4" | 1.27 | 4.303 | High-rise columns, piers |
| #11 | 1-3/8" | 1.56 | 5.313 | Bridge piers, foundations |
ACI 318 Formulas
Bars (one direction)= (Dimension perpendicular to bars รท Spacing) + 1
Total Linear Feet= (Bars in A ร Length A) + (Bars in B ร Length B)
Weight= Total Linear Feet ร Weight per Foot (lb/ft)
Lap Splice (Class A)= 40 ร bar diameter (min overlap)
Lap Splice (Class B) = 48 ร bar diameter (required when >50% spliced)
Temp & Shrinkage Steel (fy=60,000)= 0.0018 ร b ร h (ACI 318 ยง24.4.3.2)
Column Tie Spacing= min(16ร long. bar dia, 48ร tie dia, column dim)
Wire Tie Length = Perimeter + 2 ร hook allowance (approx. 6" per tie for #3 ties)
Worked Example โ Grid Slab
20 ft ร 12 ft slab, #4 rebar at 12" spacing, no overlay:
- Bars along length: (12 รท 1) + 1 = 13 bars (each 20 ft long)
- Bars along width: (20 รท 1) + 1 = 21 bars (each 12 ft long)
- Total bars: 13 + 21 = 34 bars
- Linear feet: (13 ร 20) + (21 ร 12) = 260 + 252 = 512 ft
- Weight: 512 ร 0.668 = 342 lbs
- Ties: 13 ร 21 = 273 ties (2ร wire per tie = 546 wire ties)
- Lap splices (48ร dia for #4 = 24"): estimated 4 extra bars per direction
Concrete Cover Requirements
- 1.5" โ Interior (cast against ground not included)
- 2" โ Exterior (exposed to weather or in contact with ground)
- 3" โ Buried (concrete cast against and permanently exposed to earth)
Common Mistakes to Avoid
- Using wrong weight per foot โ #4 and #5 look similar but differ 56%
- Forgetting overlay doubles all quantities
- Not accounting for lap splices (add 40-48ร diameter per splice)
- Ignoring concrete cover โ rebar must be elevated on chairs
- Omitting temperature and shrinkage steel in slabs on grade
Tips
- #4 is the most common for residential slabs (4" thick)
- Use 12" spacing for standard slabs, 6" for heavy loads
- Always order 10% extra for lap splices, waste, and bends
- Wire ties: 2 ties per intersection, 6" wire per tie minimum
- Chairs every 2-3 ft in both directions to maintain cover
Related Calculators
Frequently used together on the same project.
Client & Quote (prints on PDF & Excel)
Adjustments (applies to exports)
Contingency is added as a separate line on the exported cost summary. Waste % is printed as an advisory reminder.
Project notes (prints on PDF & Excel)
Result compare (A/B)
| Item | Pinned | Current | Change |
|---|---|---|---|
Saved rates auto-apply from your Price Library. Advanced details stay in your browser โ nothing is uploaded except inside your own PDF/Excel exports.
Reinforcing steel is the skeleton of every concrete structure, and getting the quantity wrong - too few bars or too many laps - compromises structural integrity and wastes money. Our rebar calculator computes exact bar counts, total weight, lap splice lengths, and material costs from your slab, footing, wall, or column dimensions. Enter the concrete area, bar spacing, and rebar size, and get back the number of bars, total linear footage, weight in pounds and tons, and the number of lap splices required.
Rebar is sized by number, where the number represents the diameter in eighths of an inch. A #3 bar is 3/8 inch in diameter (0.375 inches, weighing 0.376 pounds per foot). A #4 bar is 1/2 inch (0.500 inches, 0.668 pounds per foot). A #5 bar is 5/8 inch (0.625 inches, 1.043 pounds per foot). The most common residential sizes are #3 for light slabs and walls, #4 for driveways and foundations, and #5 for structural footings and commercial slabs. Metric equivalents are #10 (10mm), #12 (12mm), #16 (16mm), and #20 (20mm).
Bar spacing is measured center-to-center and typically ranges from 6 inches to 18 inches on center, depending on the structural engineer's design. A typical residential driveway slab uses #4 rebar at 12 inches on center in both directions, creating a grid pattern. A 20-foot by 20-foot driveway requires 21 bars in each direction (including the end bars), totaling 42 bars at 20 feet each - 840 linear feet of #4 rebar, weighing approximately 561 pounds (0.28 tons).
Lap splices occur when one bar length ends and another begins. Standard lap splice length for #4 bar in residential footings is 24 bar diameters (24 x 0.5 inches = 12 inches minimum overlap), but structural engineers typically specify 30 to 40 bar diameters for seismic zones. A 20-foot bar with a 24-inch lap splice becomes an effective 18-foot span. The calculator factors in lap splice length automatically when you enter the total bar length required, so you know exactly how many 20-foot, 40-foot, or 60-foot bars to order.
๐ How to Use the Rebar Calculator
Determine Concrete Dimensions
Measure the length and width of the concrete element (slab, footing, wall, or column). For slabs, measure the plan dimensions. For footings, measure the trench length and width. For walls, measure the wall height and length. For columns, measure the cross-section dimensions and height.
Select Rebar Size
Choose the rebar size (#3, #4, #5, #6, #7, or #8) based on the structural engineer's specifications. If no engineer is involved, use #4 for residential slabs and driveways, #5 for footings and foundations, and #3 for light walls and partition reinforcement.
Enter Bar Spacing
Enter the center-to-center bar spacing in inches. Common spacings are 6, 8, 10, 12, 14, 16, or 18 inches on center. Closer spacing provides more reinforcement but increases material cost. Structural drawings specify the spacing for each direction.
Set Lap Splice Length
Enter the required lap splice length in inches. For residential construction, 24 bar diameters is the minimum (12 inches for #4 bar). For seismic zones, increase to 30-40 bar diameters. The calculator uses this to adjust total bar count.
Choose Bar Length
Select standard bar lengths available at your supplier: 20, 40, or 60 feet. Longer bars reduce lap splices but are harder to transport and place. Most residential projects use 20-foot bars.
Review Material Quantities
The calculator outputs the number of bars, total linear footage, total weight in pounds and tons, number of lap splices, and estimated cost. Order 5 to 10 percent extra for waste, cutoffs, and corner reinforcement.
๐ Rebar Formulas & Worked Examples
Rebar calculations are based on simple geometric relationships, but precision matters because structural engineers specify exact quantities and spacings. Here are the key formulas:
Number of Bars: Divide the concrete dimension perpendicular to the bars by the bar spacing and add one for the end bar. Bars = (Dimension / Spacing) + 1. For a 20-foot-wide slab with bars at 12 inches on center: 20 feet = 240 inches. 240 / 12 = 20, plus 1 = 21 bars. This formula applies to each direction independently.
Total Linear Footage: Multiply the number of bars by the bar length. Total Length = Bars x Bar Length. For 21 bars at 20 feet each: 21 x 20 = 420 linear feet per direction. For a two-way grid: 420 x 2 = 840 linear feet total. Subtract the concrete cover (typically 3 inches on each side) if bars do not extend to the slab edge.
Rebar Weight: Multiply total linear footage by the weight per foot for the bar size. Total Weight = Total Length x Weight Per Foot. For 840 linear feet of #4 rebar at 0.668 pounds per foot: 840 x 0.668 = 561 pounds, or 0.28 tons. Rebar is sold by weight (tons or pounds), so convert linear footage to weight for ordering.
Lap Splice Count: Divide the total bar length required by the usable length per bar (bar length minus lap splice). Splices = Total Length Required / (Bar Length - Lap Length). For 840 feet required with 20-foot bars and 2-foot laps: each bar provides 18 feet of effective length. 840 / 18 = 46.7, which rounds to 47 bars (with 46 lap splices). The calculator adjusts the bar count to account for this.
Weight Per Bar: Multiply the bar length by the weight per foot. Weight Per Bar = Bar Length x Weight Per Foot. For a 20-foot #4 bar: 20 x 0.668 = 13.36 pounds per bar. For 47 bars: 47 x 13.36 = 628 pounds. This differs from the simple calculation above because lap splices add extra bar length that must be ordered.
โ ๏ธ Common Rebar Calculation Mistakes
Forgetting to add one bar for the end
โ The formula Bars = (Dimension / Spacing) + 1 includes the end bar. Forgetting the +1 results in one fewer bar than required, leaving the last bay without proper reinforcement.
Not accounting for lap splices
โ Each lap splice reduces the effective length of each bar by the splice length. For 20-foot bars with 2-foot laps, each bar provides only 18 feet of reinforcement. Failing to account for this means ordering 10 to 15 percent fewer bars than needed.
Ignoring concrete cover
โ Rebar must be positioned at least 3 inches from the concrete surface (2 inches for interior slabs) to prevent corrosion and ensure proper bond. This reduces the effective bar length by 6 inches total (3 inches at each end).
Using weight instead of length for ordering
โ Rebar is ordered by weight (tons), but calculations start with length (linear feet). Forgetting to convert between the two units results in ordering the wrong quantity. Always verify the weight-per-foot for the specific bar size.
Not adding waste factor
โ Always add 5 to 10 percent extra for waste, cutoffs at edges, and corner reinforcement. A 20-foot slab with 21 bars per direction needs material for at least 23 bars per direction.
Ignoring temperature and shrinkage reinforcement
โ Slabs require minimum temperature reinforcement (typically #3 bars at 18 inches on center) perpendicular to the main reinforcement. This is in addition to the structural reinforcement and must be counted separately.
Wrong bar size for the application
โ Using #3 bar where #4 is specified reduces the cross-sectional area by 44 percent (0.11 vs 0.20 square inches per bar), which may compromise structural capacity. Always verify the bar size against the structural drawings.
๐ก Pro Tips for Rebar Installation
Place rebar on 2-inch plastic chairs or concrete blocks to maintain proper cover. Bar chairs prevent the rebar from touching the ground or formwork, ensuring the minimum 3-inch cover required for exterior exposure and 2 inches for interior slabs.
Tie rebar intersections with wire ties using a automatic rebar tying gun or manual pliers. Tie every intersection in one direction and every other intersection in the perpendicular direction for a stable grid that does not shift during concrete placement.
Cut rebar with a chop saw with an abrasive blade, a portable band saw, or bolt cutters for #4 and smaller. Do not use a torch on epoxy-coated rebar - the heat destroys the coating and voids the corrosion protection.
Stagger lap splices in adjacent rows so that no two splices line up within 3 feet. Staggered splices distribute the load transfer and prevent a weak plane across the slab. Alternate splice locations by at least 24 inches in each row.
Verify bar placement before pouring concrete. Walk the grid and check that bars are on chairs, ties are secure, and spacing matches the structural drawings. Correcting bar placement after a pour is expensive and structurally compromised.
Order rebar in standard lengths (20, 40, or 60 feet) to minimize waste. For a 25-foot span, use a 20-foot bar plus a 10-foot bar with a 5-foot overlap rather than ordering a custom 25-foot bar that may not be available at your supplier.
Mark bar locations on the formwork before placement. Use a tape measure and marker to mark spacing on the forms, then align each bar with the marks. This eliminates guesswork and ensures consistent spacing across the entire slab.
โ Rebar Calculator - Frequently Asked Questions
For a 20x20-foot slab with #4 rebar at 12 inches on center in both directions, you need 21 bars per direction (42 bars total). Each bar is 20 feet long (minus 6 inches for cover at each end), totaling 840 linear feet. At 0.668 pounds per foot, the total weight is approximately 561 pounds, or 0.28 tons.
The minimum lap splice length for #4 rebar is 24 bar diameters (12 inches) for non-seismic applications. In seismic zones, structural engineers typically specify 30 to 40 bar diameters (15 to 20 inches). The splice length also depends on concrete strength and bar location - consult the ACI 318 code for specific requirements.
Divide the total weight in pounds by 2,000 to get tons. For 561 pounds: 561 / 2,000 = 0.28 tons. Rebar is sold by weight at steel service centers, with minimum order quantities that vary by supplier. Most suppliers deliver in full-ton increments for commercial orders.
Yes. For columns, enter the column dimensions and the number of vertical bars specified by the structural engineer. The calculator computes the total vertical bar length and stirrup count based on the spacing. For beams, enter the beam width, depth, and bar count for top and bottom reinforcement.
Black rebar is standard carbon steel and is used in interior applications or where corrosion is not a concern. Epoxy-coated rebar has a green or gray epoxy coating that protects against corrosion in exposed environments (bridges, parking garages, marine structures). Epoxy-coated rebar costs approximately 30 to 50 percent more than black rebar.
Order 5 to 10 percent extra for waste, cutoffs, and corner reinforcement. For a straightforward rectangular slab, 5 percent is sufficient. For slabs with many penetrations (pipes, conduits, anchor bolts), increase to 10 percent. For complex geometries with extensive cutting, 15 percent may be necessary.
๐๏ธ Real-World Rebar Projects & Applications
Residential Driveway Slab: A standard two-car driveway measuring 20 feet wide by 24 feet long requires #4 rebar at 12 inches on center in both directions. The transverse bars (24 feet long) need 21 bars at 12 inches spacing, and the longitudinal bars (20 feet long) also need 21 bars. Total: 42 bars, each 20 feet long (cut from 20-foot stock). Total linear footage: 840 feet. Weight: 561 pounds (0.28 tons). At 2024 rebar pricing of approximately $0.80 to $1.20 per pound, material cost ranges from $450 to $675. Add 60 number-3 dowels at 18 inches on center for slab-to-footing connection at approximately $0.40 per pound. Total material cost: $500 to $750.
Residential Footing: A continuous footing for a 1,500-square-foot home with a perimeter of 160 feet typically requires two #5 bars (top and bottom) running continuously along the footing length. With 20-foot bars and 24-inch lap splices, each 20-foot bar provides 18 feet of effective length. Total bars needed: 160 / 18 = 8.9, rounding to 9 bars per line, 18 bars total for top and bottom. Weight per #5 bar: 20 x 1.043 = 20.86 pounds. Total weight: 18 x 20.86 = 375 pounds (0.19 tons). Stirrups (#3 bar at 12 inches on center) add approximately 160 stirrups at 1.5 feet each = 240 linear feet of #3 bar at 0.376 pounds per foot = 90 pounds. Total footing rebar: 465 pounds (0.23 tons).
Commercial Foundation Wall: A 100-foot-long by 10-foot-tall foundation wall with #5 vertical bars at 12 inches on center and #4 horizontal bars at 12 inches on center requires 101 vertical bars at 10 feet each (1,010 linear feet) and 9 horizontal bars at 100 feet each (900 linear feet). Total linear footage: 1,910 feet. Weight: vertical #5 at 1.043 pounds per foot = 1,053 pounds; horizontal #4 at 0.668 pounds per foot = 601 pounds. Total: 1,654 pounds (0.83 tons). Material cost at $1.00 per pound: approximately $1,650. Add corner reinforcement, dowels, and accessories for a total of approximately $2,000 to $2,500 in rebar for this wall.
Our rebar calculator provides accurate weight, bar count, lap splice, and material cost estimates for concrete slabs, footings, walls, and columns. Enter your concrete dimensions, rebar size, and bar spacing to receive exact bar counts, total linear footage, weight in pounds and tons, and lap splice requirements. The calculator supports all standard rebar sizes (#3 through #8) and accounts for concrete cover, lap splices, and waste factors. Pair it with our concrete calculator for complete material estimates and our lumber calculator for formwork quantities.
โ Method, Sources & Review
Maintained by theBuildingCalc methods deskยท Last reviewedSeptember 23, 2026. This calculator implements the standards and references below; every figure can be rechecked against the formulas and worked example on this page.
Results are estimates, not engineering advice. Verify against local codes and licensed professionals for structural, electrical, and gas work. See ourTerms. Spotted an error? Report itโ corrections ship with the next review.