How this rebar calculator works
Reinforcing a slab usually means laying a grid of rebar — bars running one way crossed by bars running the other way — set in from the edges by a margin of concrete cover. To count the bars in one direction, take the slab dimension across which they are spread, subtract twice the edge clearance to get the clear span, divide by the spacing on-center and round down to count the gaps, then add one bar for the starting edge. Doing this for both directions gives the count running each way.
The length of each bar is the slab's other dimension minus twice the clearance, because every bar also stops short of the far edges. Multiply the bar count by the bar length in each direction and add the two together for the total linear length of rebar to buy. Finally, wherever a lengthwise bar crosses a widthwise bar there is a grid intersection; that count — the two bar counts multiplied together — is the maximum number of tie-wire ties you would use.
Reference note: results are a planning estimate for a single mat in a rectangular slab, simplified. Overlap laps and splices (typically 40 bar diameters), dowels, edge bars, double mats, bar chairs and waste all add material, and bar size and grade are set by the engineered design. Always check your local building code and span tables before buying.
Common rebar spacing, cover and sizes
| Item | Common value | Notes |
|---|---|---|
| Grid spacing | 12 in OC | Typical for driveways and footings; 16–18 in for lighter slabs. |
| Edge clearance / cover | 3 in | Concrete cover to edges and bottom of a slab on grade. |
| Bar count each way | floor(clear span ÷ spacing) + 1 | Clear span = slab dimension − 2 × clearance. |
| Common bar sizes | #3 (3/8 in), #4 (1/2 in) | #4 for vehicle loads; #5+ for heavier structural slabs. |
| Lap splice | ≈ 40 bar diameters | Add extra length where bars overlap and are tied. |
Frequently asked questions
- How much rebar do I need for a slab?
- For a rectangular slab with a single mat, the number of bars in each direction is the clear span (the slab dimension minus twice the edge clearance) ÷ spacing on-center, rounded down, plus one. Add the two directions for the total bar count, and multiply each count by its bar length for total linear feet. For a 20 ft × 12 ft slab at 12 in spacing with 3 in clearance that is about 32 bars and roughly 464 linear feet. Enter your slab size, spacing and clearance above and the calculator returns the totals.
- What spacing should rebar be?
- Residential slabs are most commonly tied on a grid at 12, 16 or 18 inches on-center, with 12 inches typical for driveways and footings and wider spacing for lighter slabs. The correct spacing depends on slab thickness, loads and local code, so confirm against the engineered design. Closer spacing uses more rebar but adds crack control and load capacity.
- How do I calculate a rebar grid?
- Take each slab dimension, subtract twice the edge clearance to get the clear span, divide by the spacing on-center, round down and add one bar for the starting edge. Do this for both length and width to get the count running each way. The bar length in one direction is the slab's other dimension minus twice the clearance. Multiply count by bar length and sum both directions for the total linear length.
- What is rebar clearance or cover?
- Clearance — often called concrete cover — is the distance from the edge or face of the concrete to the nearest rebar. It protects the steel from moisture and corrosion and keeps it off the slab edges. A common rule for slabs on grade is about 3 inches of cover, and the bars are inset from each edge by that clearance, which shortens both the bar lengths and the number of bars that fit.
- How many tie wires do I need?
- Tie wires go where the bars cross, so the maximum number of ties equals the grid intersections — the bar count one way × the bar count the other way. In practice many builders tie every perimeter crossing and every second or third crossing in the field, so use the intersection count as an upper bound and buy a roll of tie wire accordingly. This calculator reports the intersection count.
- What size rebar should I use for a slab?
- Common residential slabs use #3 (3/8 in) or #4 (1/2 in) rebar, with #4 typical for driveways, footings and slabs carrying vehicle loads and #3 for lighter slabs. Heavier structural slabs use #5 or larger. Bar size affects strength but not the bar count from this tool, which works from spacing and slab size; check your engineered drawings or local code for the required size and grade.