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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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 calculator sizes a 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.
| 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 |
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.