Rebar Calculator for Concrete Slabs
This rebar calculator lays out a two-way grid across your slab and totals the bars, length and weight so you can order the right amount.
For a 20 × 20 ft slab with #4 bars on a 16-inch grid you need 16 bars each way, about 640 ft of rebar. Add 10% for laps and cuts and the total is 704 ft: 36 twenty-foot sticks weighing about 470 lb. Closer spacing or heavier bars raise the quantity quickly.
Your numbers
Result
How much rebar does a slab need?
Slab reinforcement is usually a grid: one set of bars running each way, tied where they cross. The quantity depends only on the slab size and the spacing between bars. Closer spacing means more bars and more weight. Rebar controls cracks and holds a cracked slab together; it does not make a slab stronger against a poor base, so size and spacing should come from your plans or local code.
bars across the width = ⌊length ÷ spacing⌋ + 1
total feet = (bars × length + bars × width) × (1 + waste %)
weight = total feet × weight per foot
Example: a 20 × 20 ft slab with a 16 in (1.333 ft) grid has ⌊20 ÷ 1.333⌋ + 1 = 16 bars each way. That is 16 × 20 + 16 × 20 = 640 ft. With 10% for laps and cuts: 704 ft, which is 36 sticks of 20 ft. #4 bar weighs 0.668 lb per foot, so the order is about 470 lb.
Rebar sizes and weights
| Bar | Diameter | Weight per foot | Per 20 ft stick |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb |
| #4 | 1/2 in | 0.668 lb | 13.4 lb |
| #5 | 5/8 in | 1.043 lb | 20.9 lb |
| #6 | 3/4 in | 1.502 lb | 30.0 lb |
| #7 | 7/8 in | 2.044 lb | 40.9 lb |
| #8 | 1 in | 2.670 lb | 53.4 lb |
How spacing changes the order (20 × 20 ft slab, 10% waste)
| Spacing | Bars each way | Total feet |
|---|---|---|
| 12 in | 21 | 924 ft |
| 16 in | 16 | 704 ft |
| 18 in | 14 | 616 ft |
| 24 in | 11 | 484 ft |
Practical notes
- Lap bars where they end, typically a minimum multiple of the bar diameter; your plans give the figure.
- Keep bars off the ground on chairs or dobies so they sit in the middle of the slab.
- Match the bar weight to the job: see the rebar weight calculator for any bar length.
- Order the concrete with the concrete calculator and the base stone with the gravel calculator.
Frequently asked questions
How much rebar do I need for a slab?
Count the bars each way from the slab size and spacing, multiply by their lengths, and add 10% for laps and cuts. A 20 × 20 ft slab on a 16 in grid is about 704 ft.
What spacing should I use for rebar in a slab?
Patios, walks and garage floors commonly use 12 to 18 in grids, but your plans or local code decide. This calculator shows how each spacing changes the quantity.
What size rebar is used for a concrete slab?
Residential slabs commonly use #3 or #4 bar. Heavier loads and structural slabs use larger bars; follow the engineer's drawings.
How much does a stick of rebar weigh?
A 20 ft stick of #4 bar weighs about 13.4 lb (0.668 lb per foot). #3 is 7.5 lb and #5 is 20.9 lb.
Is wire mesh the same as rebar?
No. Welded wire mesh is a lighter reinforcement sold in sheets or rolls. This calculator is for individual rebar bars.
Sources and further reading
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
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Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.