How the grid is counted
When a run is shorter than a stock bar, the calculator cuts as many runs as fit from each bar. When a run is longer, it splices bars with your lap length and counts every piece. Offcuts aren’t carried from one run to another, so the count leans toward having a bar or two spare — which you’ll want for chairs and corners anyway.
Worked example: 20 × 10 ft slab, #4 at 12 in, 3 in clearance
- Usable area: 19 ft 6 in × 9 ft 6 in.
- Lengthwise: 10 bars × 19 ft 6 in — one per 20 ft bar, so 10 bars.
- Widthwise: 20 bars × 9 ft 6 in — two per 20 ft bar, so 10 bars.
- Total: 20 bars, 385 linear ft, 257 lb of #4.
Bar sizes
| Bar | Diameter | Area | Weight |
|---|---|---|---|
| #3 | 0.375 in (⅜) | 0.11 in² | 0.376 lb/ft |
| #4 | 0.500 in (½) | 0.20 in² | 0.668 lb/ft |
| #5 | 0.625 in (⅝) | 0.31 in² | 1.043 lb/ft |
| #6 | 0.750 in (¾) | 0.44 in² | 1.502 lb/ft |
The bar number is its diameter in eighths of an inch: #4 is 4/8 = ½ in.
Lap splices: the rule of thumb and what the codes say
“Forty bar diameters” is the lap most people quote. It’s right for Grade 40 bar and short for Grade 60:
| Bar | IRC wall table, Grade 40 | IRC wall table, Grade 60 | ACI 318 tension lap, Grade 60* |
|---|---|---|---|
| #4 | 20 in | 30 in | 29 in at 3,000 psi · 25 in at 4,000 psi |
| #5 | 25 in | 38 in | 36 in · 31 in |
| #6 | 30 in | 45 in | 43 in · 37 in |
*Class B laps for bottom bars with normal cover and spacing, as tabulated by Dayton Superior from ACI 318. Top bars need about 1.3× these laps, tight cover or spacing about 1.5×, and both together about 2×. IRC values are Table R608.5.4(1).
Plans override all of this
Bar size, spacing, cover and laps are structural decisions. If you have drawings or an engineer, use their numbers in the calculator.
Does a slab need steel at all?
Not every residential slab does. The Ohio Ready Mixed Concrete Association’s guidelines for driveways, patios and walks say wire mesh “is not necessary in residential slabs”. Others reinforce as a matter of course, and your plans may require it. Where steel is used, it only helps if it ends up inside the slab — supported on chairs, not lying on the ground.
Questions people ask
How much rebar for a 10 × 10 slab?
At 12 in on center with 3 in clear of the edges: 10 bars each way, each 9 ft 6 in long — 190 ft in all. Two runs come out of each 20 ft bar, so you buy 10 bars. In #4 that’s 127 lb of steel; in #3, 71 lb.
How much does a 20 ft bar weigh?
#3: 7.5 lb. #4: 13.4 lb. #5: 20.9 lb. #6: 30.0 lb. (ASTM A615 nominal weights of 0.376, 0.668, 1.043 and 1.502 lb/ft.)
Is 40 bar diameters enough for a lap?
For Grade 40 bar it matches the IRC’s concrete-wall table (20 in for #4, 25 in for #5). For Grade 60 the same table asks for about 60 diameters — 30 in for #4 — and ACI 318 tension laps for Grade 60 bottom bars in normal conditions work out around 50–58 diameters. Use the lap on your plans when you have one.
What’s the difference between Grade 40 and Grade 60?
The grade is the steel’s minimum yield strength in thousands of psi — 40,000 or 60,000. Grade 60 is stronger but needs longer laps. A lot of rebar sold at home centers is Grade 40 — check the tag.
Sources for the numbers on this page
- Caltrans — Bridge Design Details, ch. 13 — ASTM A615 bar sizes, areas and weights · checked October 1, 2026
- Element ICF — TB-4 Lap splices (IRC Table R608.5.4(1) values for Grade 40 and Grade 60) · checked October 1, 2026
- Dayton Superior — Rebar splicing handbook — tabulated ACI 318 lap lengths · checked October 1, 2026
- Ohio Ready Mixed Concrete Association — Guidelines for durable driveways, patios and walks · checked October 1, 2026
Corrections and updates are logged on the sources page.