Rebar Calculator
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17 #4 (1/2") stock bars, 317 ft in total
How the rebar calculator works
Bars running crosswise = ⌊(slab length − 2 × edge clearance) ÷ spacing⌋ + 1
Total length = bars × run length + laps · Stock bars = total length × (1 + waste) ÷ stock length
When a run is longer than one stock bar, two bars overlap by the lap length and are tied together. The calculator adds that overlap to the total. Waste covers offcuts that are too short to reuse.
Worked example: a 20 × 12 ft slab
With #4 bar at 18 inches on center and 3 inches of edge clearance, you need 8 bars running the length (19.5 ft each) and 14 bars running across (11.5 ft each). That is 317 ft of bar, or 17 twenty-foot bars with 5% waste, weighing about 222 lb. There are 112 crossings to tie.
Rebar sizes and weights
| US bar | Diameter | lb per ft | lb per 20 ft bar |
|---|---|---|---|
| #3 | 0.375 in | 0.376 | 7.5 |
| #4 | 0.5 in | 0.668 | 13.4 |
| #5 | 0.625 in | 1.043 | 20.9 |
| #6 | 0.75 in | 1.502 | 30 |
| #7 | 0.875 in | 2.044 | 40.9 |
| #8 | 1 in | 2.670 | 53.4 |
| Metric / Canadian bar | Diameter | kg per m | kg per 6 m bar |
|---|---|---|---|
| 10M (Canada) | 11.3 mm | 0.785 | 4.71 |
| 15M (Canada) | 16 mm | 1.570 | 9.42 |
| 20M (Canada) | 19.5 mm | 2.355 | 14.13 |
| 8 mm bar | 8 mm | 0.395 | 2.37 |
| 10 mm bar | 10 mm | 0.617 | 3.7 |
| 12 mm bar | 12 mm | 0.889 | 5.33 |
| 16 mm bar | 16 mm | 1.580 | 9.48 |
| 20 mm bar | 20 mm | 2.469 | 14.81 |
US weights are the standard ASTM A615 unit weights. Metric bar weights follow d² ÷ 162 kg/m; Canadian M sizes use their CSA nominal weights.
Placing rebar in a slab
- Keep it off the ground. Support the grid on chairs or concrete blocks so it ends up near the middle of the slab thickness.
- Keep clear of the edges. Leave 2 to 3 inches (50–75 mm) between the bars and the forms; concrete cast against soil typically needs about 3 inches of cover.
- Tie the crossings. Wire ties at intersections stop the grid shifting when concrete is placed. Tying every second crossing is common for light slabs.
- Lap splices. A common rule of thumb is 40 bar diameters, but follow the lap length on your plans or in local code.
This calculator handles the quantities. Bar size and spacing for structural slabs, footings and walls must come from your plans or an engineer.
Frequently asked questions
How far apart should rebar be in a slab?
Residential slabs commonly use #3 or #4 bars at 12 to 24 inches on center in both directions. Structural slabs, footings and anything a building inspector will check must follow the spacing on your plans.
How much does rebar weigh?
Standard US bars weigh 0.376 lb/ft (#3), 0.668 lb/ft (#4), 1.043 lb/ft (#5) and 1.502 lb/ft (#6). Metric bars weigh about d² ÷ 162 kg per meter, so a 10 mm bar is about 0.617 kg/m and a 12 mm bar about 0.888 kg/m.
How much should rebar overlap?
A common rule of thumb is 40 bar diameters, which is 20 inches for #4 bar. Required lap lengths actually depend on bar size, concrete strength and position, so use the lap length on your plans or the one your local code requires.
How far should rebar be from the edge of a slab?
Keep bars about 2 to 3 inches (50–75 mm) in from the edges and support them on chairs or bricks so they sit near the middle of the slab thickness, not on the ground. Concrete cast directly against soil typically needs about 3 inches of cover.
Should I use rebar or wire mesh?
Welded wire mesh is common in thin, lightly loaded slabs such as patios and walkways. Rebar is stiffer and easier to keep at the right height, so it is the usual choice for driveways, thicker slabs and anything specified on plans.