Rebar Calculator — How Much Rebar for a Concrete Slab?
Enter your slab size and rebar spacing to get the number of bars in both directions, total length and how many 20-ft bars to buy. No sign-up, no data stored.
Rebar needed
How rebar quantity is estimated
- Bars along length = floor((width - 2 x edge) ÷ spacing) + 1.
- Bars across width = floor((length - 2 x edge) ÷ spacing) + 1.
- Total length (ft) = bars-along x length + bars-across x width.
- 20-ft bars = ceil(total length ÷ 20), then add 10% for splice overlaps.
The +1 accounts for the first bar at the edge; spacing is measured center-to-center. Bars shorter than a full stick need overlap splices - add roughly 10% to the total for #4 rebar laps.
Slab size → rebar reference
16-in spacing, 3-in edge clearance, including ~10% splice allowance.
| Slab | Lengthwise | Across | Total ft | 20-ft bars |
|---|---|---|---|---|
| 4x8 pad | 3 | 4 | 44 ft | 3 |
| 10x10 patio | 8 | 8 | 160 ft | 9 |
| 10x20 driveway | 8 | 12 | 280 ft | 16 |
| 20x20 patio | 12 | 12 | 480 ft | 27 |
| 24x40 shop | 16 | 26 | 1,280 ft | 71 |
Last updated: October 2026 · Verify spacing, bar size and cover with local code or an engineer - this is an estimate for planning, not a structural design.
Rebar FAQ
Structural slabs and anything load-bearing (footings, driveways, patios with vehicles) should be reinforced. Small pads like a 4x4 post base can skip it. Wire mesh is weaker than rebar but better than nothing.
12-in spacing on center is typical for driveways and heavy loads; 16-18 in works for most patios and house slabs; 24 in suits light-duty paths. The closer the spacing, the stronger the mat - and the more bars you buy.
Keep rebar 3 in from the edges and 2-3 in from the top surface (concrete cover). The cover protects the steel from moisture and keeps it from rusting and spalling the concrete.
Overlap bars by at least 30 bar diameters (about 15 in for #4 rebar) at splices, and stagger the splices so they do not all line up in one row. Tie the overlap with tie wire where bars cross.
#3 (3/8 in) for light slabs and patios, #4 (1/2 in) for driveways and footings, #5 (5/8 in) for heavy structural work. Your engineer or local building code should dictate size - spacing alone is not enough.
Related calculators & guides
How Much Rebar Do I Need?
Spacing rules, bar sizes and splice math for reinforced slabs.
Read the guide →