📊 Concrete mix per m³ by strength (f'c)
Typical dosages per cubic metre of concrete (cement bag = 42.5 kg). They vary by region and materials — the calculator keeps every value editable:
| f'c (kg/cm²) |
Cement (bags) |
Sand (m³) |
Gravel (m³) |
Water (L) |
| 180 (~C16/20) | 7.0 | 0.56 | 0.57 | 185 |
| 210 (~C20/25) | 8.4 | 0.54 | 0.55 | 186 |
| 250 (~C25/30) | 9.2 | 0.51 | 0.55 | 190 |
| 280 (~C28/35) | 9.7 | 0.50 | 0.54 | 193 |
🏗️ Rebar weight per metre (d² / 162)
| Diameter |
8 mm |
10 mm |
12 mm |
16 mm |
20 mm |
25 mm |
| kg/m | 0.395 | 0.617 | 0.889 | 1.580 | 2.469 | 3.858 |
📐 How it is calculated
Concrete: V = a × b × H, × (1 + waste), × the mix dosage. Longitudinal steel: length per bar = H + bottom anchorage (40·db) + top anchorage (12·db) + laps (40·db per splice above the 9 m commercial length); weight = bars × length × d²/162. Stirrups: confined ends at the tight spacing + central zone at the wide spacing; each closed tie = 2·((a−2·cover)+(b−2·cover)) + two 135° hooks; weight = positions × tie length × d²/162.
✍️ Worked example
Column 0.30 × 0.30 m, 3 m tall, f'c 210, 4 bars of 12 mm, 8 mm stirrups (confined 0.50 m each end at 10 cm, central at 20 cm), 5% waste. Concrete = 0.30 × 0.30 × 3 = 0.27 m³ → ≈ 2.4 cement bags, 0.15 m³ sand, 0.16 m³ gravel. Longitudinal = 4 × (3 + 0.48 + 0.144) = 14.5 m × 0.889 ≈ 13.5 kg. Stirrups: 6 + 6 at the ends + 9 central = 21 ties × 1.12 m × 0.395 ≈ 9.8 kg. Total steel ≈ 23 kg.
🌍 Naming around the world
Column = columna / pilar = Stütze / Pfeiler · Rebar = acero de refuerzo / fierro = Bewehrung / Bewehrungsstahl · Stirrup = estribo = Bügel · Confined zone = zona de confinamiento = Verbügelungsbereich · Cover = recubrimiento = Betondeckung · Lap splice = empalme / traslape = Übergreifungsstoß.
❓ Frequently Asked Questions
How do I calculate the concrete for a column?
Multiply the section by the height: volume = a × b × H. A 0.30 × 0.30 m column, 3 m tall, is 0.27 m³. Add a waste factor (typically 5%) and multiply by the mix dosage for your f'c to get cement bags, sand, gravel and water.
How is the weight of rebar calculated?
Steel weight per metre = d² / 162, with d in millimetres. A 12 mm bar weighs 0.89 kg/m; an 8 mm stirrup weighs 0.40 kg/m. Multiply by the total length of longitudinal bars (including anchorage and laps) and stirrups.
Why are stirrups closer together at the ends of a column?
The ends (the confined zones near beams and the base) take the largest seismic shear and need tighter confinement, so stirrups are spaced ~10 cm there and wider (~20 cm) in the central zone. The calculator counts each zone separately.