📊 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 × L × t, × (1 + waste), × the mix dosage. Steel mesh: in each direction the number of bars = (perpendicular side − 2 × cover) ÷ spacing + 1, and each bar spans the other side minus cover (plus optional end hooks); weight = bars × length × d²/162, added for both directions (× the number of mats). Cast against soil, footings use ~75 mm cover.
✍️ Worked example
Footing 1.5 × 1.5 m, 0.30 m thick, f'c 210, Ø12 mesh at 15 cm both ways, 75 mm cover, 5% waste. Concrete = 1.5 × 1.5 × 0.30 = 0.675 m³ → ≈ 6 cement bags, 0.38 m³ sand, 0.39 m³ gravel. Mesh = 10 + 10 bars of 12 mm, each 1.35 m = 27 m × 0.889 ≈ 25.2 kg (with waste). Total steel ≈ 25 kg.
🌍 Naming around the world
Footing / isolated footing = zapata / cimiento aislado = Einzelfundament / Fundament · Reinforcing mat = malla / parrilla = Bewehrungsmatte · Rebar = acero / fierro = Bewehrung · Cover = recubrimiento = Betondeckung · Thickness = espesor = Dicke · Span = luz = Spannweite.
❓ Frequently Asked Questions
How do I calculate the concrete for a footing?
Multiply width × length × thickness: V = B × L × t. A 1.5 × 1.5 m footing, 0.30 m thick, is 0.675 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 many rebars does the mesh need?
In each direction the number of bars = (the perpendicular side − 2 × cover) ÷ the spacing + 1. For a 1.5 m side with 75 mm cover and bars every 15 cm that is (1.35 ÷ 0.15) + 1 = 10 bars, and the same the other way — a 10 × 10 mat.
What concrete cover should a footing have?
A footing cast directly against the soil normally uses about 75 mm (3 in) of clear cover on the bottom, more than a beam or column, to protect the steel. The calculator keeps the cover editable so you can match your code and detail.