Rebar Calculator
Work out a two-way rebar grid for a slab, driveway or footing — total length, weight, lap splices, ties and cost.
Your slab or grid area
Center to center, both directions.
Covers cutting waste and layout adjustments.
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Bar size profile
Lap splice planner
Grid summary
The short answer
Divide the effective width by the spacing to find how many bars run the length, and the effective length by the spacing for bars running the width, then multiply each by its run length and add them together. A 20 × 10 ft slab on a 12 inch grid with #4 rebar needs roughly 238 linear feet — about 12 sticks of 20 ft rebar, weighing around 159 lb.
Rebar sizes and weight per foot
| Size | Diameter | Weight per ft | Typical use |
|---|---|---|---|
| #3 | ⅜″ (9.5 mm) | 0.376 lb | Sidewalks, light slabs |
| #4 | ½″ (12.7 mm) | 0.668 lb | Driveways, standard slabs |
| #5 | ⅝″ (15.9 mm) | 1.043 lb | Footings, foundation walls |
| #6 | ¾″ (19.1 mm) | 1.502 lb | Structural footings, columns |
| #7 | ⅞″ (22.2 mm) | 2.044 lb | Heavy structural elements |
| #8 | 1″ (25.4 mm) | 2.670 lb | Major structural elements |
Rebar needed for common slab sizes
Figures below use #4 rebar on a 12 inch grid with 2 inch edge cover, before waste.
| Slab size | Linear ft | 20 ft sticks | Weight |
|---|---|---|---|
| 10 × 10 ft | 110 ft | 6 | 73 lb |
| 12 × 12 ft | 134 ft | 7 | 90 lb |
| 20 × 10 ft | 216 ft | 11 | 144 lb |
| 20 × 20 ft | 420 ft | 21 | 281 lb |
| 24 × 24 ft | 506 ft | 26 | 338 lb |
How to calculate rebar for a grid
- Find the effective area. Subtract the edge cover from both the length and width.
- Count bars each direction. Divide the effective width by spacing for bars running the length, and the effective length by spacing for bars running the width.
- Multiply by run length. Each bar running the length is as long as the effective length, and vice versa.
- Add lap splices. Any run longer than one stick needs an overlap — a common rule is about 40 times the bar’s diameter.
- Add waste, then convert. Add 5–10%, then divide by stick length for sticks, or multiply by weight per foot for total weight.
Worked example
A 20 × 10 ft slab, #4 rebar, 12″ grid, 2″ edge cover:
- Effective length20 − 2×(2÷12) = 19.67 ft
- Effective width10 − 2×(2÷12) = 9.67 ft
- Bars running the length (spaced across width)floor(9.67 ÷ 1) + 1 = 10, each 19.67 ft
- Bars running the width (spaced across length)floor(19.67 ÷ 1) + 1 = 20, each 9.67 ft
- Total length10×19.67 + 20×9.67 = 390.1 ft
- Plus 10% waste429.1 ft
- Sticks needed429.1 ÷ 20 = 22 sticks
Lap splices
Any single bar run longer than the stick length you’re buying needs a lap splice — two sticks overlapped and tied together to act as one continuous bar. A common rule of thumb for normal residential work is an overlap of roughly 40 times the bar’s diameter. For #4 rebar (½ inch), that’s about 20 inches of overlap at each splice. Longer runs on wider slabs, or jobs using shorter stock, need more splices — and more total footage than the raw span suggests.
Placement checklist
- Set out the gridMark spacing on the forms before placing any bar.
- Lift bars onto chairsRebar needs to sit mid-slab, not resting on the sub-base.
- Tie every intersectionOr at minimum every other one on a dense grid, to keep the grid from shifting during the pour.
- Keep edge cover consistent2–3 inches from every edge keeps the steel fully encased in concrete.
- Overlap splices correctlyTie, don’t just butt, overlapping bars — and keep the full lap length on longer runs.
- Check for high spotsRebar sitting too close to the surface can show through as rust stains later.
Common mistakes
- Leaving rebar on the groundReinforcement resting on the sub-base does almost nothing — it needs to be lifted mid-slab on chairs before the pour.
- Forgetting lap splicesA run longer than one stick needs overlap length added on top of the raw span, or the order comes up short.
- Using too small a bar for the load#3 rebar suits light slabs, but footings and structural elements usually need #4 or larger.
- Inconsistent edge coverSteel too close to the surface can corrode and cause unsightly rust staining as the slab weathers.
- Not tying enough intersectionsAn untied grid can shift out of place as concrete is placed and vibrated around it.
- Rounding the total instead of accounting for cut lengthsOrdering exact linear footage ignores how sticks actually get cut and laid out — always round up per stick, not just the grand total.
What this calculator assumes
Adjust your result if your project differs from these.
- Standard ASTM weight-per-foot values for each bar size
- A two-way grid with equal spacing in both directions
- Lap splice length of 40 times the bar’s diameter, a common rule of thumb for normal-weight concrete
- Rebar chairs spaced roughly every 3 feet along the total grid length
- One tie per intersection, with a roll of tie wire covering about 200 intersections
- Sticks purchased at the length selected (20 or 40 ft) and cut to size on site
- Prices are whatever you enter; they are not live market rates
Common questions
What size rebar is used for a slab?
#3 or #4 rebar is standard for residential slabs, driveways and sidewalks. #5 or larger is used for footings and structural elements carrying heavier loads.
How much does rebar weigh per foot?
#3 rebar weighs about 0.376 lb per foot, #4 about 0.668 lb per foot, and #5 about 1.043 lb per foot. Weight increases with bar diameter.
What spacing should rebar be in a slab?
12 to 18 inches on center is typical for residential slabs. Heavier structural slabs or footings often use 8 to 12 inch spacing for extra strength.
What is a lap splice?
Where two rebar sticks overlap and are tied together to act as one continuous bar across a run longer than a single stick. A common rule of thumb is an overlap of about 40 times the bar’s diameter.
How long is a standard rebar stick?
20 feet is the most common cut length sold at hardware and building supply stores, though mill lengths of 40 or 60 feet are available for larger commercial orders.
Does rebar need to be centered in the slab?
Yes. Rebar resists cracking most effectively sitting in the middle third of the slab’s depth, held up on chairs — not resting on the sub-base or sitting at the very top.
How far apart should rebar chairs be?
Roughly every 3 feet along each bar is a common spacing, close enough to prevent the grid from sagging under the weight of the wet concrete during the pour.
Should I use rebar or wire mesh?
Wire mesh is common for standard residential slabs and driveways. Rebar grids are used for heavier loads, structural elements, and anywhere a stronger, more precisely placed grid is needed.
How much edge cover does rebar need?
2 to 3 inches from the edge of the slab is typical for residential work, keeping the steel fully encased in concrete so it doesn’t corrode.
How many rolls of tie wire do I need?
It depends on the number of intersections in the grid. A standard roll of tie wire typically ties a few hundred intersections, so most residential slabs need only one roll.
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Project
| Dimensions | — |
| Bar size | — |
| Grid spacing | — |
| Edge cover | — |
| Waste allowance | — |
Grid
| Bars running length (X) | — |
| Bars running width (Y) | — |
| Total linear feet | — |
| Total weight | — |
Shopping list
| Rebar sticks | — |
| Tie wire | — |
| Chairs | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes a two-way grid with standard ASTM rebar weights and a 40x-diameter lap splice. Confirm quantities with your supplier and check local structural requirements before ordering.