📊 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 = b × h × L, × (1 + waste), × the mix dosage. Longitudinal steel: length per bar = L + one anchorage at each end (anchor·db) + laps (lap·db per splice above the 9 m commercial length); weight = bars × length × d²/162, computed separately for the top and bottom layers. Stirrups: confined zones next to both supports at the tight spacing + a central zone at the wide spacing; each closed tie = 2·((b−2·cover)+(h−2·cover)) + two 135° hooks; weight = positions × tie length × d²/162.
✍️ Worked example
Beam 0.25 × 0.50 m, 5 m span, f'c 210, 2 top bars + 3 bottom bars of 16 mm, 8 mm stirrups (confined 1.0 m each end at 10 cm, central at 20 cm), 5% waste. Concrete = 0.25 × 0.50 × 5 = 0.625 m³ → ≈ 6 cement bags, 0.35 m³ sand, 0.36 m³ gravel. Longitudinal = 5 bars × (5 + 2 × 0.64) = 5 × 6.28 = 31.4 m × 1.58 ≈ 52.1 kg. Stirrups: 11 + 11 at the ends + 14 central = 36 ties × 1.42 m × 0.395 ≈ 21.2 kg. Total steel ≈ 73.3 kg.
🌍 Naming around the world
Beam = viga = Balken / Unterzug (Träger) · Top/bottom steel = acero superior / inferior = obere / untere Bewehrung · Rebar = acero de refuerzo / fierro = Bewehrung / Bewehrungsstahl · Stirrup = estribo = Bügel · Span = luz / vano = Spannweite · Cover = recubrimiento = Betondeckung · Lap splice = empalme / traslape = Übergreifungsstoß.
❓ Frequently Asked Questions
How do I calculate the concrete for a beam?
Multiply the section by the length: volume = width × depth × length (b × h × L). A 0.25 × 0.50 m beam, 5 m long, is 0.625 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.
Why does a beam have top and bottom steel?
Bottom bars resist the positive (sagging) bending at midspan, while top bars resist the negative bending over the supports — so a beam is reinforced top and bottom. The calculator lets you set a different number and diameter for each layer and adds the anchorage into the supports.
How is the weight of the rebar and stirrups calculated?
Steel weight per metre = d² / 162, with d in millimetres. A 16 mm bar weighs 1.58 kg/m; an 8 mm stirrup weighs 0.40 kg/m. Stirrups are counted per zone — closer together near the supports (high shear) and wider in the middle — and each closed tie equals the core perimeter plus two 135° hooks.