Rebar calculator for slabs, footings and walls
This rebar calculator works out how many reinforcing bars a slab, footing or wall needs each way, how long each bar is cut, where long runs must be lapped, and the total length and weight of steel to order. Enter the plan dimensions, clear cover, bar size and spacing to get bar counts, laps, kilograms or pounds, steel per square metre or square foot and a cost at your price per kg or lb.
It covers metric millimetre bars (IS 1786, AS/NZS 4671, BS 4449), US # bars to ASTM A615 and Canadian M bars to CSA G30.18, with the nominal mass of each size built in. Results update as you type and nothing leaves your browser.
These are quantity estimates for planning and ordering. Bar sizes, spacing, cover and laps come from the structural drawings; a qualified engineer must check any design.
How to use the Rebar Calculator
- Choose metric or imperial, then the element: slab mesh, footing grid or a wall with vertical and horizontal bars.
- Enter the length and width (or wall length and height), the clear cover at each edge and the bar standard and size.
- Set the spacing in each direction and the number of layers — one bottom mesh, or top and bottom (both faces for a wall).
- Pick the stock bar length and the lap length as a multiple of the bar diameter; add a hook allowance and a waste percentage if you want them.
- Enter the steel price and read the weight, bar counts, total length, stock bars to order and cost. Copy the summary or download the step table as CSV.
Rebar Calculator features
Bars each way from spacing and cover
Count = ⌊clear span ÷ spacing⌋ + 1 in each direction, so an edge bar always sits inside the cover line.
Laps for long runs
A run longer than the stock bar is split into pieces, each joint overlapped by the lap you set (50d by default), and the lap steel is added to the total.
Three bar standards
Metric mm sizes at IS 1786 / AS 4671 nominal mass (d² ÷ 162), ASTM A615 # bars in lb/ft and CSA G30.18 M bars in kg/m.
Steel per square foot and per cubic metre
Weight per m² or ft² of element, plus kg per m³ of concrete when you give the thickness — a quick check against typical ratios.
How the rebar quantity is calculated
For a 6 × 4 m slab with 25 mm cover and 12 mm bars at 150 mm both ways the clear spans are 5.95 m and 3.95 m. Bars running along the length are spaced across the width: ⌊3.95 ÷ 0.15⌋ + 1 = 27 bars of 5.95 m. Bars across the width: ⌊5.95 ÷ 0.15⌋ + 1 = 40 bars of 3.95 m. Total 318.65 m × 0.888 kg/m = 283 kg, or 11.8 kg per m² and about 79 kg per m³ at 150 mm thick. Make the slab 15 m long and each 14.95 m run needs two 12 m pieces with one 600 mm lap (50 × 12 mm), added automatically.
| Bar | Nominal mass | Standard |
|---|---|---|
| 8 · 10 · 12 · 16 mm | 0.395 · 0.617 · 0.888 · 1.58 kg/m | IS 1786 / AS/NZS 4671 |
| 20 · 25 · 32 mm | 2.47 · 3.85 · 6.31 kg/m | IS 1786 / AS/NZS 4671 |
| #3 · #4 · #5 · #6 | 0.376 · 0.668 · 1.043 · 1.502 lb/ft | ASTM A615 |
| #7 · #8 · #9 · #11 | 2.044 · 2.670 · 3.400 · 5.313 lb/ft | ASTM A615 |
| 10M · 15M · 20M · 25M | 0.785 · 1.570 · 2.355 · 3.925 kg/m | CSA G30.18 |
Cover, spacing and lap length: what the codes say
- Cover. IS 456:2000 Table 16 gives 20 mm nominal cover for mild exposure, rising to 45–75 mm for severe conditions, and clause 26.4.2.2 asks for at least 50 mm on footings cast against earth. ACI 318-19 Table 20.5.1.3.1 requires 3 in (75 mm) for concrete cast against ground and 3/4 in for interior slabs with #11 bars and smaller.
- Lap length. IS 456 clause 26.2.5.1 sets a flexural-tension lap of the development length or 30d, whichever is greater; with Fe500 steel in M20 concrete the development length is about 57d, so 50d is a planning figure only. ACI 318 Class B tension laps are 1.3 × ℓd, commonly 40–60 diameters. Use the value on your drawings.
- Spacing. IS 456 clause 26.3.3 limits main slab bars to 3 × effective depth or 300 mm, whichever is less; typical meshes are 8–12 mm at 100–200 mm in slabs, 12–16 mm at 100–200 mm in footings and 10–16 mm at 150–300 mm in walls.
Typical steel ratios and related estimates
Residential slabs usually carry 60–100 kg of steel per m³ of concrete, footings 50–80 kg/m³ and RCC walls 80–120 kg/m³; a figure far outside these ranges means a spacing or cover slip. Use the concrete calculator for the concrete around the bars, the beam calculator for shear, moment and deflection, and the brick calculator for the walls above.
Rebar Calculator FAQ
How many bars do I need for a slab?
Divide the clear span (length minus cover at both ends) by the spacing, round down and add one — in each direction. A 6 × 4 m slab at 150 mm spacing needs 27 bars along the length and 40 across the width per layer; the rebar calculator does this and converts the count to weight.
How much rebar is in a footing?
Same method as a slab but with more cover — 50 mm per IS 456 or 3 in per ACI 318 against earth — and usually 12–16 mm (#4–#5) bars at 150–200 mm. Choose “Footing” for matching defaults.
How is rebar calculated per square foot?
Divide the total weight by the plan area. A 20 × 20 ft footing with #4 bars at 12 in both ways and 3 in cover uses about 521 lb, or 1.3 lb per square foot.
What lap length should I use?
Use the value on the drawings. Otherwise 50 bar diameters is a reasonable allowance: IS 456 requires at least the development length (about 47d for Fe415 and 57d for Fe500 in M20) and never under 30d; ACI 318 Class B laps are 1.3 times the development length.
Does it include chairs, spacers and binding wire?
No. Chairs, binding wire, dowels, starter bars and extra bars at openings are not included — add a few percent to the waste figure, or take them off separately for a bar bending schedule.