How much rebar for a slab? The grid math
A slab grid is bars both ways at an on-center spacing. The count in each direction comes from the clear span between edge clearances, divided by the spacing, plus one:
bars one way = floor((width − 2 × edge) ÷ spacing) + 1, each (length − 2 × edge) long
bars other way = floor((length − 2 × edge) ÷ spacing) + 1, each (width − 2 × edge) long
weight = total linear ft × lb/ft for the bar size
A 24×12 ft driveway slab at 16″ OC with 3″ edges takes 10 long bars and 18 short bars — about 446 linear feet, or 298 lb of #4 bar, cut from twenty-three 20-ft sticks. How far apart? 12–24″ OC is common practice for residential slabs and driveways, but spacing is a structural decision: follow your project plans and local requirements, and involve an engineer where required.
Footing rebar calculator
Footings use continuous horizontal bars running the footing’s length — the footing tab multiplies your total footing length by the number of continuous bars your plans call for (two is a common residential arrangement) and returns linear feet, weight and sticks. Where bars lap at stick ends and corners, the lap length comes from your plans; the waste field gives you room to cover it.
Rebar size & weight chart
Imperial bar numbers encode the diameter in eighths of an inch: a #4 bar is 4/8 = 1/2″. Everything else follows from geometry — area = πd²/4, and weight per foot = area × steel’s 490 lb/ft³. The chart below is computed from that bar-designation geometry (the metric name is just the diameter in millimetres, rounded).
| Bar | Diameter (in) | Diameter (mm) | Metric name | Area (in²) | Weight (lb/ft) | kg/m |
|---|---|---|---|---|---|---|
| #3 | 0.375 | 9.5 | #10 | 0.110 | 0.376 | 0.559 |
| #4 | 0.500 | 12.7 | #13 | 0.196 | 0.668 | 0.994 |
| #5 | 0.625 | 15.9 | #16 | 0.307 | 1.044 | 1.553 |
| #6 | 0.750 | 19.1 | #19 | 0.442 | 1.503 | 2.237 |
| #7 | 0.875 | 22.2 | #22 | 0.601 | 2.046 | 3.045 |
| #8 | 1.000 | 25.4 | #25 | 0.785 | 2.672 | 3.977 |
| #9 | 1.125 | 28.6 | #29 | 0.994 | 3.382 | 5.033 |
| #10 | 1.250 | 31.8 | #32 | 1.227 | 4.176 | 6.214 |
| #11 | 1.375 | 34.9 | #36 | 1.485 | 5.053 | 7.520 |
Computed from bar designation geometry (d = n/8″; weight = πd²/4 × 490 lb/ft³). #9–#11 actual mill diameters differ fractionally from the n/8 rule (under 1%).
Rebar weight per foot — quick lookups
The two sizes most DIY slabs use: #3 weighs 0.376 lb/ft (a 20-ft stick ≈ 7.5 lb) and #4 weighs 0.668 lb/ft (a 20-ft stick ≈ 13.4 lb). Weight matters for hauling — 500 linear feet of #4 is 334 lb, which is a pickup-bed load, not a trunk load. The weight-only tab handles any size and length directly.
Wire mesh calculator (remesh)
Welded wire mesh — remesh — covers a slab by area instead of by bar count. The mesh tab divides your slab area by the sheet or roll area and adds an overlap allowance, since adjacent sheets must overlap at least one full square. 5×10 ft sheets are the common flat format and rolls run 5 ft wide by 50–150 ft; both are editable, so enter whatever your supplier stocks. Whether a slab calls for mesh or bar — or fiber, or nothing — is a plans question, not a calculator question.