Rebar calculator
Rebar calculator
and size chart
Work out the reinforcing grid for a slab — how many bars each way, how long each one runs, where they need lapping and what the steel weighs. The full #3 to #8 size chart is below the calculator.
Enter measurements ↓More in this series
Put these figures to work
Sizing one part of a pour is easier when the slab itself is settled. Start with the main estimate, then come back for the detail.
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Tools for the job
What makes this job easier
Tools and materials that suit this kind of job. These are affiliate links: if you buy through one, Renovimo may earn a commission at no extra cost to you. It never changes the figures on this page.
Garvee
Electric rebar cutter, up to #6
Cuts slab and footing bar to length in seconds, with no sparks.
Check price
Garvee
Electric rebar cutter, up to #7
The larger cutter, for footing and wall bar up to 7/8 in.
Check price
Garvee
Electric rebar bender, up to 5/8 in
Bends corner and hook bars for footings and walls in one stroke.
Check price
Garvee
20V cordless hammer drill
Drills masonry and concrete for anchors, and drives screws everywhere else.
Check price
Garvee
Two-wheel wheelbarrow
Stable on two wheels for bagged concrete, block and soil.
Check price
Garvee
Tool bag, 17 in
Keeps the tape, knife, screws and chalk line together on the job.
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Not the tool you were after? Browse the whole range at Garvee — the store these picks come from.
Based on your result
What you'll need
Quantities update after every calculation. Bars are sold in fixed stock lengths and cut on site, so the bar count is always rounded up.
- Reinforcing bar
- Grid bars at your chosen spacing
- Calculate above
- Tie wire
- One tie at every bar intersection
- —
- Bar chairs
- Hold the grid near mid-depth of the slab
- —
- Cutters and a bender
- Bolt cutters, a hickey bar and gloves
- As needed
The maths
How this rebar spacing calculator sizes a slab grid
Cover comes off both ends of every span first, because steel has to stay inside the concrete. What is left is divided by the spacing to give the gaps, and one bar is added for the far edge. Long runs are lapped, and the total is converted to weight, which is how steel is sold.
bars = ⌊ usable opposite span ÷ spacing ⌋ + 1
linear feet = bars each way × their bar length
lap = 40 × bar diameter, per splice
weight = linear feet × lb per foot
- 3 in
- cover at slab edges
- 12–18 in
- typical grid spacing
- 40 × d
- standard lap length
- 20 ft
- common stock bar
- 0.668
- lb per ft, #4 bar
- ft ↔ m
- converted automatically
-
01
Take cover off both ends
Steel inside a slab must stay about 3 inches clear of the edge, so every bar is shorter than the slab it sits in.
-
02
Space across the opposite span
Bars running the length of the slab are spaced across its width, which is why the two counts differ on a rectangular pour.
-
03
Add one
A grid at 18-inch centres across a 114-inch usable span has six full gaps and seven bars, because both edges carry one.
-
04
Splice the long runs
A bar longer than the stock length is lapped 40 diameters at each joint — 20 inches for a #4, 25 for a #5.
-
05
Weigh it
Mills and yards sell by weight, so the linear total is converted at the bar’s pounds per foot before you price it.
Reference
Rebar size chart: #3 to #8
Imperial bar sizes are named for the diameter in eighths of an inch, so a #4 bar is four eighths — half an inch. Areas and weights are the ASTM A615 nominal figures every supplier quotes from. These are the six sizes the calculator offers, which covers everything residential.
| Size | Diameter | Area | Weight | Typical use |
|---|---|---|---|---|
| #3 | 0.375 in / 9.5 mm | 0.11 sq in | 0.376 lb/ft · 0.560 kg/m | Light slabs, footpaths, stirrups |
| #4 | 0.500 in / 12.7 mm | 0.20 sq in | 0.668 lb/ft · 0.994 kg/m | Driveways, patios, most residential slabs |
| #5 | 0.625 in / 15.9 mm | 0.31 sq in | 1.043 lb/ft · 1.552 kg/m | Thicker slabs, strip footings, retaining walls |
| #6 | 0.750 in / 19.1 mm | 0.44 sq in | 1.502 lb/ft · 2.235 kg/m | Heavy footings and columns |
| #7 | 0.875 in / 22.2 mm | 0.60 sq in | 2.044 lb/ft · 3.042 kg/m | Structural beams and columns |
| #8 | 1.000 in / 25.4 mm | 0.79 sq in | 2.670 lb/ft · 3.973 kg/m | Heavy structural work |
Worked example
How much rebar do I need for a slab?
A 20 × 20 ft patio slab with #4 bar on an 18 inch grid and 3 inches of cover:
- 1. Usable span — 20 ft × 12 − 3 × 2 = 234 in, or 19.5 ft each way
- 2. Bars each way — ⌊ 234 ÷ 18 ⌋ + 1 = 14
- 3. Linear feet — 28 bars × 19.5 ft = 546 ft, no splices since each bar fits a 20 ft stock length
- 4. Weight — 546 ft × 0.668 lb = 365 lb of #4
- 5. Ties — 14 × 14 = 196 intersections
For a quick figure before you measure properly, a square grid uses 24 ÷ spacing linear feet of bar per square foot of slab: 1.33 ft at 18 inches, 1.5 ft at 16 and 2 ft at 12. Tighten the same patio to a 12 inch grid and it takes 780 ft — about 520 lb of #4.
Rebar for common slab sizes
#4 bar, 3 inches of cover, no laps. Slabs longer than a stock bar add a lap at every splice, which the calculator counts and this table does not.
| Slab | Typical use | 18 in grid | 12 in grid |
|---|---|---|---|
| 10 × 10 ft | Shed base, landing | 133 ft · 89 lb | 190 ft · 127 lb |
| 12 × 12 ft | Hot tub pad, small patio | 184 ft · 123 lb | 276 ft · 184 lb |
| 12 × 20 ft | Patio | 317 ft · 212 lb | 464 ft · 310 lb |
| 20 × 20 ft | Patio, one-car pad | 546 ft · 365 lb | 780 ft · 521 lb |
| 12 × 40 ft | Single driveway | 627 ft · 419 lb | 934 ft · 624 lb |
| 24 × 24 ft | Two-car garage floor | 752 ft · 502 lb | 1,128 ft · 754 lb |
The concrete for the same slab is a separate sum — the concrete calculator gives the cubic yards and bags for its area and thickness, and the two orders usually go in together. Piers and strip footings take their steel as cages and continuous bars rather than a grid; size those with the footing calculator.
Rebar spacing chart for 4-inch and 6-inch slabs
Residential flatwork is usually reinforced to control cracking rather than to carry a designed load, so spacing follows habit and the thickness of the slab. These are the common starting points; a slab that carries a structure, sits on poor soil or falls under a permit takes whatever the drawings or the local code call for.
| Slab | Typical use | Bar and spacing | Bar per sq ft |
|---|---|---|---|
| 4 in | Patios, walkways, shed floors | #3 or #4 at 18 in each way | 1.33 ft |
| 5 in | Garage floors, light vehicles | #4 at 16–18 in each way | 1.33–1.5 ft |
| 6 in | Driveways, RV pads, heavier loads | #4 at 12 in, or #5 at 16–18 in | 1.33–2 ft |
Height matters as much as spacing. The grid belongs in the middle of the slab, or just above it, so set it on chairs: 1½ to 2 inches for a 4 inch slab, 2½ to 3 inches for a 6 inch one. Steel lying on the ground at the bottom of the pour does next to nothing.
Rebar or wire mesh?
Welded wire mesh is the cheaper way to control shrinkage cracks in a thin patio or path, and it comes in rolls and sheets that go down quickly. Its weakness is height: it is hard to keep up in the middle of the slab while the concrete is placed, and a lot of it ends up trodden to the bottom. A rebar grid on chairs stays where it is set and holds cracked sections together under load, which is why driveways and garage floors use it. Fibre-reinforced mixes are a third option for crack control, but they are not a substitute for bar where the slab carries vehicles.
Diameters, areas and weights beyond the #3 to #8 range above — and the metric sizes they correspond to — are in rebar sizes and weights.
Good to know
Rebar questions, answered
#3 or #4 at 16 to 18 inch centres covers most residential 4-inch flatwork. #4 is the common default because it is stiff enough to stay put while concrete is placed, and the price difference over #3 is small. Move to #5 for a 6-inch slab or anything taking vehicles.
It is the diameter in eighths of an inch. #4 is 4/8 of an inch, or half an inch; #8 is a full inch. Above #8 the naming changes and the sizes jump, which is why residential work rarely goes past #6.
Twelve to eighteen inches on centre for most slabs, both directions. Wider than 24 inches and the steel stops controlling cracks between the bars; tighter than 12 inches costs more without adding much.
Forty bar diameters is the usual rule, so 20 inches for #4 and 25 inches for #5. Codes give tighter figures depending on concrete strength and bar position; 40 diameters is the safe default when you are not working to a drawing.
No. Steel needs concrete all around it or it rusts and spalls the slab. Bar chairs hold the grid near mid-depth, roughly 2 inches up in a 4-inch slab, and 3 inches of cover is kept at the edges.
For light-duty flatwork mesh is adequate and quicker. Rebar is stiffer, easier to keep at the right height and stronger in tension, which matters once the slab carries vehicles or spans soft ground.
#4 is 0.668 pounds per foot, so a 20-foot bar is about 13.4 pounds. The calculator totals the whole grid, which is the figure a supplier prices from.
With #4 on an 18 inch grid and 3 inches of cover, 14 bars each way at 19.5 feet: 546 linear feet, about 365 pounds, and 28 twenty-foot stock bars. On a 12 inch grid it is 20 bars each way, 780 feet and about 520 pounds.
Twenty-four divided by the spacing in inches, for a square grid: 1.33 feet per square foot at 18 inch centres, 1.5 at 16 and 2 at 12. It ignores cover, the extra bar at each edge and laps, so treat it as a first estimate: on a 20 by 20 foot slab it gives 533 feet against 546 counted bar by bar.