Rebar Calculator
Calculate rebar quantity for a slab or footing. Enter dimensions, bar spacing and stock length to get bars, linear feet and weight.
Rebar needed
47 bars
- Bars running the length
- 21 × 20 ft
- Bars running the width
- 21 × 20 ft
- Total linear feet
- 924 ft
- Estimated weight
- 617 lb
- Stock bars (20 ft)
- 47
- Estimated rebar cost
- $564.00
Assumes a grid on 12 in centers and a 10% allowance for cuts and lap splices. Increase the allowance for runs longer than a stock bar.
Rebar is sold in stock lengths and placed in a grid, so the count depends on spacing, slab dimensions and how many bars you can cut from each stick. This calculator builds the grid in both directions, adds a lap allowance, and converts the total into stock bars and weight.
Use it for slabs on grade, driveways, patios and footings. Bar size and spacing are structural decisions — this tool sizes the order, not the design. Follow the engineered plans or local code for the actual reinforcement.
How the rebar calculator works
Bars in one direction = ceiling(width ÷ spacing) + 1, and bars in the other direction = ceiling(length ÷ spacing) + 1. Multiply each count by its run length, add both directions, then apply the cut and lap allowance. Divide the total linear feet by the stock length and round up. Weight uses the nominal weight per foot for the selected bar size.
Worked example
A 20 × 20 ft slab with #4 bar at 12 in centers needs 21 bars each way: 840 linear feet. With a 10% allowance that is 924 ft, or 47 bars at 20 ft each. The total weighs about 617 lb.
How rebar spacing works
- Slabs on grade commonly use #3 or #4 bar at 12–18 inches on center. Driveways, structural slabs and expansive soils usually need more steel.
- The grid should sit in the middle third of the slab, supported on chairs or bar supports — not lying on the subgrade.
- Lap splices are typically 40 bar diameters. A #4 bar needs about 20 inches of overlap; a #5 needs about 25 inches.
- Order stock lengths that match the run. A 20 ft bar is easy to handle; 30 ft and 40 ft bars reduce laps but need more people and space.
Rebar size quick reference
| Bar size | Diameter | Weight |
|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft |
| #4 | 1/2 in | 0.668 lb/ft |
| #5 | 5/8 in | 1.043 lb/ft |
| #6 | 3/4 in | 1.502 lb/ft |
Common rebar mistakes
- Counting only the slab area instead of the total length of both bar directions.
- Forgetting lap splices when the run is longer than a stock bar.
- Letting rebar rest on dirt, which leaves it at the bottom of the slab where it does little for cracking.
- Switching bar size or spacing without an engineer because the estimate looked high.
Frequently asked questions
How much rebar do I need for a 10 × 10 slab?
At 12 inches on center with #4 bar, you need 11 bars each way, or 220 linear feet. With a 10% allowance that is 242 ft, which is 13 bars at 20 ft. The total weighs about 162 lb.
What size rebar should I use for a 4-inch slab?
A typical patio or shed slab uses #3 or #4 bar at 12–18 inches on center. The final choice depends on loads, soil and local code; ask the engineer or building department if the slab is structural.
How far apart should rebar be spaced?
Residential slabs on grade often use 12–18 inch spacing in both directions. Driveways and structural slabs may use closer spacing or larger bars. Follow the plans rather than a generic rule.
How much overlap do I need for rebar laps?
A common rule is 40 bar diameters. That is about 15 inches for #3, 20 inches for #4 and 25 inches for #5. Code requirements can be longer in tension zones.
Can I use wire mesh instead of rebar?
Welded wire mesh is lighter and faster for crack control in slabs on grade, but it is not a substitute for structural reinforcement. Mesh also needs to be pulled up into the slab, or it ends up on the bottom.
Does rebar need to be tied together?
Yes. Tie wire keeps the grid in place while concrete is placed. Use bar chairs or supports to hold the grid at the correct depth, and check it again during the pour.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology