How this calculator works
The calculator fits as many bars as the grid spacing allows inside the edge clearance in each direction, multiplies each set by its bar length, then adds an allowance for laps and offcuts. Weight uses standard nominal weights per foot for each bar size.
Bars along length = floor( (Width×12 − 2×Clearance) ÷ Spacing ) + 1
Bars along width = floor( (Length×12 − 2×Clearance) ÷ Spacing ) + 1
Linear ft = bars × bar length (each direction), × (1 + Lap%/100)
Weight = Linear ft × lb/ft (#3 0.376 · #4 0.668 · #5 1.043 · #6 1.502)
Assumptions and tips
- Single mat, straight bars. Laps where bars join are covered by the allowance; your engineer or code may specify exact lap lengths.
- Bar size and spacing must come from your plans.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Slab length | 20 ft |
| Slab width | 20 ft |
| Grid spacing | 12 in |
| Edge clearance | 3 in |
| Bar size | #4 (1/2 in) |
| Stock bar length | 20 ft |
| Lap and waste allowance | 10 % |
| Result | Value |
|---|---|
| Total rebar | 858 linear ft |
| 20 ft bars to buy | 43 bars |
| Bars running lengthwise | 20 bars |
| Bars running widthwise | 20 bars |
| Approximate weight | 573 lb |
Step by step:
- Bars lengthwise = floor((240 − 6) ÷ 12) + 1 = 20, each 19.5 ft
- Bars widthwise = 20, each 19.5 ft
- Linear ft = 780; + 10% laps = 858 ft
- 20 ft bars = 858 ÷ 20 → 43
Rebar for square slabs (12 in grid, #4)
3 in edge clearance, 10% lap allowance, 20 ft bars.
| Slab | Total rebar | 20 ft bars | Approximate weight |
|---|---|---|---|
| 10 × 10 ft | 209 linear ft | 11 bars | 140 lb |
| 12 × 12 ft | 304 linear ft | 16 bars | 203 lb |
| 20 × 20 ft | 858 linear ft | 43 bars | 573 lb |
| 24 × 24 ft | 1,241 linear ft | 63 bars | 829 lb |
Common mistakes to avoid
- Laying rebar on the ground — it needs chairs so it sits within the slab.
- Too little edge cover, which lets moisture reach the steel.
Frequently asked questions
What does #4 rebar weigh?
About 0.668 lb per foot, so a 20 ft bar weighs roughly 13.4 lb.
Do I need rebar in a patio slab?
Many small patios use wire mesh, fibers or no reinforcement; driveways and structural slabs usually need rebar. Follow your plans and local code.