Rebar calculator for a slab grid
Enter the slab size, the grid spacing your plans call for and the bar size to count the sticks of rebar to buy. The calculator counts steel; it does not choose a spacing.
Enter your measurements
More options: Extra
- Bars in the grid
- 21 bars × 19.5 ft and 21 bars × 19.5 ft
- Total length of bar (incl. laps)
- 819 ft
- Weight of the sticks
- 561 lb
- Crossings to tie (every one)
- 441 ties
- Chairs at about one per 4 sq ft (check your plans)
- 96 chairs
Bars run inside the edge distance: 19.5 ft × 19.5 ft
Across 19.5 ft at 12 in: 21 bars; along 19.5 ft: 21 bars
819 ft of bar → 42 bars of 20 ft · 0.668 lb/ft = 561 lb
- Bars must sit inside the concrete, not on the ground: use chairs or dobies at the height your plans show.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much rebar for a 20×20 slab | 28 bars of 20 ft #4 on an 18 in grid (42 on a 12 in grid) |
| How much rebar for a 30×40 slab | About 91 bars of 20 ft #4 on an 18 in grid |
Your slab at other grid spacings
| Grid spacing (from your plans) | Rebar sticks | Total length of bar (incl. laps) | Weight of the sticks | Crossings to tie (every one) |
|---|---|---|---|---|
| 12 in | 42 bars | 819 ft | 561 lb | 441 ties |
| 16 in | 32 bars | 624 ft | 428 lb | 256 ties |
| 18 in | 28 bars | 546 ft | 374 lb | 196 ties |
| 24 in | 22 bars | 429 ft | 294 lb | 121 ties |
A rebar grid is two sets of parallel bars: one running the length of the slab, spaced across its width, and one running the width, spaced along its length. Count each set, work out how many stock bars that takes, and you have the order. The spacing, bar size, edge cover and lap length are structural decisions that belong to your plans or engineer — this calculator does the counting.
How the count works
bars running the length = (width − 2 × edge) ÷ spacing, rounded up, + 1 bars running the width = (length − 2 × edge) ÷ spacing, rounded up, + 1 runs shorter than a stock bar: cut as many as fit from each stick longer runs: splice bars with the lap your plans give ties = bars one way × bars the other way
| Bar | Diameter | lb per ft | A 20 ft stick weighs |
|---|---|---|---|
| #3 | ⅜ in (9.5 mm) | 0.376 | 7.5 lb |
| #4 | ½ in (12.7 mm) | 0.668 | 13.4 lb |
| #5 | ⅝ in (15.9 mm) | 1.043 | 20.9 lb |
| #6 | ¾ in (19.1 mm) | 1.502 | 30.0 lb |
Worked examples
- How much rebar for a 20×20 slab — at an example 18 in and 12 in grid.
- How much rebar for a 30×40 slab — where runs are longer than a stick and splices come in.
Practical notes
- Chairs. Rebar lying on the ground ends up under the slab, where it does nothing. Support the grid on chairs or concrete blocks (dobies) at the height your plans show; the chair count here is a rough planning figure.
- Ties. Tying every crossing is the simple rule for a grid that will be walked on during the pour. Some crews tie every other crossing; follow your plans or inspector.
- Cutting. Bars can be cut with a manual cutter, a grinder with an abrasive or metal-cutting wheel, or a reciprocating saw. Wear eye protection.
- Mesh instead? Many light slabs use welded wire mesh. The wire mesh calculator counts sheets and rolls.
What you’ll need
- Rebar tie wire and twisterBudget price
Ties bars together where they cross so the grid stays put while you pour.
See options on Amazon (opens in a new tab) - Rebar chairsBudget price
Hold rebar or mesh off the ground so it ends up inside the concrete, not under it. Match the chair height to where your plans place the steel.
See options on Amazon (opens in a new tab) - Rebar cutter and benderMid-range price
Cuts and bends #3–#5 bar by hand. For a few cuts, a grinder or reciprocating saw with the right blade also works.
See options on Amazon (opens in a new tab) - Safety glassesBudget price
Splashes of wet concrete and chips from cutting block both go for the eyes.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
What rebar spacing should I use for a slab?
That is a structural decision for your plans, engineer or building department — it depends on the slab’s use, thickness and soil. Enter the spacing they give you; the calculator doesn’t recommend one.
How many ties do I need for a rebar grid?
Tying every crossing, the number of bars one way times the number the other way. A 20 × 20 slab on a 12 in grid with 3 in edges has 21 × 21 = 441 crossings.
How long is a stick of rebar?
Stores and suppliers commonly stock 10 ft and 20 ft bars, and steel suppliers list longer stock such as 30, 40 and 60 ft. Ask what your supplier carries before you plan splices.
Related guides
Sources
- Caltrans Bridge Design Details 13.1 — Table 13.1.1, ASTM standard reinforcing bar data (opens in a new tab) — Bar sizes #3–#18 with nominal diameter, area and weight per foot (ASTM A615).
- Continental Steel — Straight rebar chart (opens in a new tab) — Soft-metric bar designations (#3 = #10, #4 = #13 …) and kg per metre.
- Continental Steel — #4 reinforcing bar product page (opens in a new tab) — 0.668 lb/ft; stock lengths of 20, 30, 40 and 60 ft.