Reinforcing a slab means laying a grid of steel bars at a set spacing, then tying the crossings so that the grid stays put while the concrete is placed. The shopping list for that grid is a count of bars, a number of stock sticks and a weight, and this rebar calculator produces all three from the slab dimensions, the bar size and the spacing.
How to use the rebar calculator
- Enter the slab length and width.
- Pick the bar size and the spacing, measured center to center, used in both directions.
- Set the edge cover — how far the outer bars sit from the slab edge. Rebar must stay inside the concrete to be protected from rust.
- Set the lap length in bar diameters for places where one stick is not long enough.
Rebar grid formula
Bar length = slab dimension − 2 × cover
Pieces per bar = 1 if the bar fits a 20 ft stick; otherwise ⌈ (length − lap) ÷ (20 − lap) ⌉
Weight = total length × weight per foot
Weight per foot comes from the bar’s cross-section and the density of steel; see the Rebar Weight Calculator for the table. Lap length equals the lap in bar diameters multiplied by the diameter; for #4 bar at 40 diameters it is 20 inches.
Worked example: a 20 × 20 ft slab with #4 bars at 16 in
- Bars each way: (240 − 6) ÷ 16 = 14.6 → 14, plus 1 = 15 bars in each direction, 30 in total.
- Each bar is 20 − 0.5 = 19.5 ft, so it fits a 20 ft stick with no splice.
- Total length: 30 × 19.5 = 585 ft, so 30 sticks.
- Weight: 585 × 0.668 = 391 lb.
How spacing changes the order (20 × 20 ft, #4 bar)
| Spacing | Bars | Total length (ft) | 20 ft sticks | Weight (lb) |
|---|---|---|---|---|
| 12 in | 40 | 780 | 40 | 521 |
| 16 in | 30 | 585 | 30 | 391 |
| 18 in | 28 | 546 | 28 | 365 |
| 24 in | 20 | 390 | 20 | 261 |
Placement tips
- Support the grid. Chairs or dobies hold the steel at the specified height; bars dragged up with a hook during the pour are not reliable.
- Keep cover. Typical cover to the bottom or sides is 3 inches where the concrete is cast against soil, with less in formed work. Your plans give the requirement.
- Tie the crossings at intervals; every crossing is not always needed.
- Don’t forget bends. Footings and walls need hooks and corner bars not counted here.
Welded wire mesh is an alternative in light slabs; compare it with the Concrete Reinforcing Mesh Calculator. The pour itself comes from the Concrete Calculator, and the Concrete Footing Calculator covers the foundation.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 16 inches on center with #4 bar, 3 inch edge cover and 40-diameter laps, the grid has 30 bars, needs 30 twenty-foot sticks and weighs about 391 lb. Your slab thickness and design may call for something different.
How far apart should rebar be in a slab?
Typical residential patios and driveways use #3 or #4 bar at 12 to 24 inches on center in both directions. The right spacing depends on loads, soil and the thickness, so follow your plans or local code.
How are the bar counts calculated?
Bars start at the edge cover and are placed at the spacing across the slab: count = ⌊(span − 2 × cover) ÷ spacing⌋ + 1 in each direction.
Why add lap splices?
Where a bar must be longer than a stock stick, two bars overlap to carry the load across the joint. Each splice uses extra steel equal to the lap length.
Does the rebar need to sit off the ground?
Yes. Rebar should be supported on chairs or dobies so that it sits in the middle or upper third of the slab, never lying on the soil. Chairs and tie wire are not counted here.