Where D²/162 comes from
Steel weighs about 7,850 kg per m³. A bar of diameter d mm has a cross-section of πd²/4 mm², so one metre weighs 7,850 × πd² ÷ 4,000,000 kg. That simplifies to d² ÷ 162.2, rounded to 162 for site use. It's the same figure BS 8666 schedules use, within a fraction of a percent.
Weight per metre by bar size
| Bar | kg per metre | Typical use |
|---|---|---|
| H6 | 0.222 | Links, mesh |
| H8 | 0.395 | Links and stirrups |
| H10 | 0.617 | Stirrups, light slabs |
| H12 | 0.889 | Slabs and light beams |
| H16 | 1.580 | Beam and column bars |
| H20 | 2.469 | Columns, foundations |
| H25 | 3.858 | Heavy beams, piles |
| H32 | 6.321 | Heavy columns, piles |
| H40 | 9.877 | Very heavy foundations |
Totalling a bar schedule
For each bar mark: number of bars × length of each × kg/m. Add the marks together, then add an allowance for laps if the schedule doesn't already include them.
| Mark | Bars | Each | Total length | kg/m | Weight |
|---|---|---|---|---|---|
| B1 (H12) | 40 | 6.0 m | 240 m | 0.889 | 213.3 kg |
| B2 (H10) | 60 | 4.0 m | 240 m | 0.617 | 148.1 kg |
| Total | 361.5 kg (0.36 t) |
That's the steel to order for the bottom mat of a small slab. Rebar is usually priced per tonne, so 0.36 t × your supplier's rate gives the material cost before cutting and bending.
UK notes
- UK bars are B500 high-yield steel, marked H on schedules (H12 = 12 mm), to BS 4449.
- Schedules, shape codes and cutting lengths follow BS 8666.
- Lap lengths and cover come from the engineer's design (Eurocode 2); don't shorten laps to save steel.