Rebar Calculator
The rebar grid for a concrete slab: bars each way, linear feet, sticks to buy and weight.
Rebar sticks to buy
40
sticks
20 bars one way and 20 the other, 780 linear ft in total.
Grid
- Bars running the length
- 20
- Bars running the width
- 20
- Total length of bar
- 780 ft
- Weight
- 521 lb
Tying
- Intersections to tie
- 400
Cost
- Rebar
- –
How much rebar do I need for a slab?
Take off the edge clearance on both sides, then divide each dimension by the bar spacing and add one to get the number of bars running the other way. Multiply by their lengths for total feet, and weigh it with the bar's weight per foot (0.668 lb for #4). A 20 × 20 ft slab with #4 bars on a 12 in grid needs 20 + 20 bars, 780 ft in total: 40 sticks of 20 ft, about 521 lb.
Worked example: a 20 × 20 ft slab, #4 at 12 in
- Bar length: keep bars 3 in from each edge: 20 ft − 6 in = 19.5 ft.
- Bars each way: 19.5 ÷ 1 ft + 1 = 20 bars each direction.
- Total length: 40 bars × 19.5 ft = 780 ft.
- Sticks: each 19.5 ft bar comes from one 20 ft stick: 40 sticks.
- Weight and ties: 780 × 0.668 lb = 521 lb, with 400 crossings to tie.
#4 rebar for common slabs
Bars 3 in from the edges, 20 ft sticks, laps of 40 bar diameters where runs are longer than a stick.
| Slab | Spacing | 20 ft sticks | Linear ft | Weight |
|---|---|---|---|---|
| 10 × 10 ft | 12 in | 10 | 190 | 127 lb |
| 12 × 12 ft | 12 in | 24 | 276 | 184 lb |
| 20 × 20 ft | 12 in | 40 | 780 | 521 lb |
| 20 × 20 ft | 18 in | 28 | 546 | 365 lb |
| 24 × 24 ft (garage) | 12 in | 96 | 1,208 | 807 lb |
| 30 × 12 ft (driveway) | 18 in | 36 | 479 | 320 lb |
Rebar sizes and weights
The bar number is its diameter in eighths of an inch. Weights are nominal, from ASTM A615:
- #3: 3/8 in, 0.376 lb per ft
- #4: 1/2 in, 0.668 lb per ft (the most common for slabs and footings)
- #5: 5/8 in, 1.043 lb per ft
- #6: 3/4 in, 1.502 lb per ft
Laps, chairs and ties
- Laps: where a run is longer than one stick, bars overlap and are tied together. 40 bar diameters is a common lap; your plans or code set the real figure. Stagger laps so they don't line up.
- Chairs: rebar must sit in the middle of the slab, not on the ground. Use plastic chairs or concrete dobies to hold it up.
- Ties: tie the grid at crossings with tie wire so it doesn't move during the pour.
- Follow your plans. Bar size and spacing for structural slabs, footings and driveways should come from your plans or an engineer.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
With #4 bars on a 12 in grid, 3 in from the edges: 20 bars each way, 780 linear ft, 40 sticks of 20 ft, about 521 lb.
How much does #4 rebar weigh?
0.668 lb per foot (ASTM A615), so a 20 ft stick weighs about 13.4 lb.
What spacing should rebar be in a slab?
It depends on the slab's thickness, load and soil. 12 to 18 in grids are common for residential slabs, but use the spacing on your plans.
How do I calculate the number of rebar bars?
Subtract the edge clearance from both sides of the slab dimension, divide by the spacing, round down and add one.
What size rebar for a driveway?
#3 or #4 bar is often used for residential driveways, but the right size and spacing depend on the design. Check your local code or plans.
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Sources and method
- ASTM A615 — Table 1, deformed bar nominal weights and dimensions (ANSI webstore) — #3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft
Last checked October 12, 2026. How we build and test every calculator: methodology.