Reinforcement
Stock bars & cost
How it's calculated
m = d² / 162.2 kg/m (d in mm, steel 7850 kg/m³)W = Σ (count × cut length × m)n = ⌈ span / spacing ⌉ + 1, span = dimension − 2 × coverL = n_x × (length − 2c) + n_y × (width − 2c), × layers × quantityk = ⌈ (L − ℓ) / (S − ℓ) ⌉, ℓ = lap = N × dL_total = L + (k − 1) × ℓwaste % = 1 − steel in place / steel boughtcost = steel bought × priceBar masses come from the standard tables (BS 4449 Table 4 for 6–40 mm, ASTM A615 Table 1 for #3–#18). The d²/162.2 formula only shows where they come from. In a mesh, the bars that run along the length are spaced across the width, and the number of bars is rounded up so the real spacing is never wider than the one you enter; the real spacing is shown in the working line. A bar longer than the stock length is cut in pieces and lapped; the laps add steel. Stock bars are then planned size by size: with "Reuse offcuts" the longest pieces are placed first and shorter ones fill the space left on the same bar (a first-fit-decreasing heuristic, not a guaranteed optimum). "Extra bars" rounds the order up per bar size and is not counted as cut waste.
Worked example: a slab mesh
A 5.0 × 4.0 m slab with 25 mm cover, 12 mm bars at 200 mm in both directions, one layer, bought as 12 m bars.
| Step | Working | Result |
|---|---|---|
| Bars along the length | ⌈(4.0 − 0.05) / 0.2⌉ + 1 | 21 × 4.95 m |
| Bars along the width | ⌈(5.0 − 0.05) / 0.2⌉ + 1 | 26 × 3.95 m |
| Length of steel | 21 × 4.95 + 26 × 3.95 | 206.65 m |
| Weight | 206.65 × 0.888 | 183.51 kg |
| 4.95 m pieces | 2 per 12 m bar: 10 bars, plus one bar for the 21st | 11 bars |
| 3.95 m pieces | 1 on the 11th bar, then 25 at 3 per bar | 9 bars |
| Steel bought | 20 × 12 × 0.888 | 213.12 kg |
| Cut waste | 1 − 183.51 / 213.12 | 13.9% |
Lap example: a single 30 m run of 16 mm bar with a lap of 40 × 16 = 640 mm needs ⌈(30 − 0.64) / (12 − 0.64)⌉ = 3 lengths and 2 laps. Steel in place is 30 + 2 × 0.64 = 31.28 m, which is 49.39 kg, bought as 3 bars.
Assumptions
- All bars are straight. Hooks, bends and BS 8666 shape codes are not included, so use cut lengths that already include them.
- The lap length is N times the bar diameter (40 by default). Real lap lengths depend on the code, concrete grade, bar position and cover: take them from the structural drawings.
- Laps are added only where a piece is longer than the stock bar. Laps that the drawings call for between shorter pieces are not added.
- Nominal masses are used. Delivered bars can differ: BS 4449 allows ±4.5% on mass per metre above 8 mm (±6% up to 8 mm), and ASTM A615 allows a bar to weigh up to 6% under nominal.
- Mesh bars end at the cover on every side and chairs, spacers, starter bars and tie wire are not counted.
- No allowance is made for saw kerf. Use "Extra bars" for losses, damage or site changes.
- Switching between metric and imperial maps each bar to the nearest diameter (for example 12 mm becomes #4). The sizes are close but not equivalent.
- Cost is steel bought times the price you enter. It does not include cutting, bending, delivery or fixing.
Frequently asked questions
How much does a 12 mm rebar weigh per metre?
0.888 kg/m (BS 4449). A 12 m bar weighs 10.66 kg. The same size in other diameters: 8 mm 0.395 kg/m, 10 mm 0.617 kg/m, 16 mm 1.579 kg/m, 20 mm 2.466 kg/m, 25 mm 3.854 kg/m. ASTM #4 (about 12.7 mm) weighs 0.668 lb/ft.
How do I work out the weight of rebar from the diameter?
Divide the square of the diameter in millimetres by 162.2 to get kg per metre: 16² ÷ 162.2 = 1.578 kg/m. This uses steel at 7850 kg/m³. The tables in the standard are slightly rounded, and this calculator uses the tables.
How many bars are in a slab mesh?
Divide the span by the spacing, round up, and add one. A 3.95 m span at 200 mm gives ⌈19.75⌉ + 1 = 21 bars, because both edge bars count.
Why do I buy more steel than the weight in the structure?
Stock bars come in fixed lengths (usually 12 m) and the offcuts that are too short to use are waste. Cut waste is shown as a share of the steel you buy. Combining different cut lengths on the same bar reduces it.
Does this include laps, hooks and bends?
It adds laps where a bar is longer than the stock length. Hooks and bends are not modelled; enter the full cut length from the bar schedule instead. For concrete quantities, see theconcrete volume calculator; for walls, see thebrick and block calculator.
Standards and references
- BS 4449:2005+A3:2016: steel for the reinforcement of concrete (weldable reinforcing steel), nominal sizes and masses.
- BS 8666:2020: scheduling, dimensioning, bending and cutting of steel reinforcement.
- BS EN 10080: steel for the reinforcement of concrete.
- ASTM A615/A615M: deformed and plain carbon-steel bars for concrete reinforcement, designations #3 to #18.
- Lap lengths: BS EN 1992-1-1 (Eurocode 2) and ACI 318. This calculator only uses a multiple of the bar diameter that you choose.
This calculator estimates quantities. It does not check cover, anchorage, lap lengths, detailing or any code requirement. Have the design checked by a qualified engineer.