# Free online rebar calculator

> A rebar calculator counts the reinforcing bars needed for a concrete slab, giving you total linear footage, weight, and material cost before you order.

Calculate the number of rebar bars, total linear footage, weight, and estimated cost for concrete slabs — with bar size and spacing options.

**Interactive version:** https://onlinecalculator.me/construction/rebar/
**Category:** Construction

## How to use

1. Enter the slab length and width in feet.
2. Choose bar spacing in inches (12 in is standard residential).
3. Select the bar size (#4 is most common for slabs).
4. Set a waste factor (10% is typical).
5. Read bar counts, linear footage, weight, and cost.

## Formulas

```
barsAlongLength = ceil(slabWidth × 12 / spacing) + 1
barsAlongWidth  = ceil(slabLength × 12 / spacing) + 1

linFtLength     = barsAlongLength × slabLength  [+ lap allowance if length > 20 ft]
linFtWidth      = barsAlongWidth  × slabWidth   [+ lap allowance if width > 20 ft]

totalLinearFt   = (linFtLength + linFtWidth) × (1 + waste% / 100)
totalWeightLbs  = totalLinearFt × weightPerFt
totalTons       = totalWeightLbs / 2000
estimatedCost   = totalWeightLbs × $0.75
```

Bar weights (CRSI):

| Bar | Diameter | lb/ft |
|-----|----------|-------|
| #3  | 3/8 in   | 0.376 |
| #4  | 1/2 in   | 0.668 |
| #5  | 5/8 in   | 1.043 |
| #6  | 3/4 in   | 1.502 |

## Worked example

20 ft × 20 ft slab, 12-in spacing, #4 bar, 18-in lap, 10% waste:

```
barsAlongLength = ceil(20 × 12 / 12) + 1 = 20 + 1 = 21
barsAlongWidth  = ceil(20 × 12 / 12) + 1 = 21

linFtLength     = 21 × 20 = 420 ft  (slab = 20 ft, no lap needed)
linFtWidth      = 21 × 20 = 420 ft

totalLinearFt   = (420 + 420) × 1.10 = 840 × 1.10 = 924 lin ft

totalWeightLbs  = 924 × 0.668 = 617.23 lbs
totalTons       = 617.23 / 2000 = 0.31 tons
estimatedCost   = 617.23 × $0.75 ≈ $463
```

## Notes

- This calculator provides a material quantity estimate for ordering purposes. Rebar placement details (cover depth, support chairs, tie patterns) must follow your structural drawings or engineer's specifications.
- Always verify spacing and bar size with your local building official for any structural slab.

## Frequently asked questions

### How far apart should rebar be spaced in a concrete slab?

12-inch on-center spacing is standard for residential slabs (driveways, patios, floors). Structural slabs and footings often use 8-inch or 6-inch spacing as specified by a structural engineer.

### What bar size should I use?

#4 bar (1/2-inch diameter) is the most common choice for residential slabs. #3 bar (3/8 in) is used for sidewalks and light-duty flatwork. #5 and #6 bars are specified for heavier structural applications.

### What is a lap splice and why does the calculator add footage for it?

When a slab is longer than a standard bar length (20 ft), bars must be overlapped (spliced) to maintain continuity. IRC and ACI 318 require a minimum lap of 40 bar diameters — about 18 in for #4 bar. The calculator adds this footage automatically for slabs longer than 20 ft.

### How heavy is rebar?

Weight per linear foot by bar size (CRSI standard): #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft. These values are used directly in the weight calculation.

### How is the estimated cost calculated?

The calculator uses $0.75 per pound as a rough market reference. Actual prices vary by region, supplier, and steel market conditions. Get a current quote from your local supplier.

### How do I share my rebar estimate?

Click Share to copy the page link, or Share with my numbers to encode your inputs in the URL.

## Sources

- [Rebar — bar sizes, spacing, and unit weight](https://en.wikipedia.org/wiki/Rebar) — Wikipedia
- [Reinforced concrete — role of steel reinforcement](https://en.wikipedia.org/wiki/Reinforced_concrete) — Wikipedia

## Related calculators

- [Concrete calculator](https://onlinecalculator.me/construction/concrete/) — Calculate bags or yards of concrete to go with your rebar.
- [Asphalt calculator](https://onlinecalculator.me/construction/asphalt/) — Compare asphalt as an alternative to concrete for driveways.
- [Square footage calculator](https://onlinecalculator.me/construction/square-footage/) — Measure your slab area before ordering materials.
- [Framing calculator](https://onlinecalculator.me/construction/framing/) — Estimate lumber for forms and slab-edge framing.
- [Gravel calculator](https://onlinecalculator.me/construction/gravel/) — Calculate gravel for the base under your slab.

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