Concrete calculator
Count the volume of concrete for a slab, a strip footing, columns or steps — and how to buy it: by truck from a plant, mixed on site from cement, sand and crushed stone, or in bags of dry ready mix.
You need
5.25 m³ to order
Volume 4.80 m³ · C20/25 · with 5 % reserve
- Cement
- 65 bags of 25 kg1,613 kg
- Sand
- 3,780 kg
- Crushed stone
- 5,292 kg
- Water
- 907 l
- Or dry ready mix
- 531 bags of 20 kg
- Volume6.00 × 4.00 × 0.20 = 4.800 m³
- With 5 % reserve, ordered in steps of 0.25 m³4.800 × 1.05 = 5.040 → 5.25 m³
- Mixing on site: 320 kg of cement per 1 m³5.040 × 320 = 1,613 kg → 65 bags of 25 kg
- Sand, crushed stone and water per 1 m³750 · 1,050 kg · 180 l → 3,780 · 5,292 kg · 907 l
- Dry ready mix: one bag gives 9.5 l5.040 ÷ 0.01 → 531 bags
Consumption is the average from the «United Kingdom, Ireland, Australia and New Zealand» preset. Check it against your manufacturer's pack.
The calculator gives an estimate by the usual norms. Consumption depends on the product and the surface: check the manufacturer’s data before you buy.
How it’s calculated
- 1
Volume: slab = length × width × thickness; strip footing = total length × width × height; columns = area of the section × height × count; steps = each step as a block from the ground to its tread.
- 2
Add a reserve of a few percent for spillage and the uneven base.
- 3
A plant delivers in steps of 0.5 m³ (or the region’s step) — the order is rounded up.
- 4
Mixing on site: cement, sand, crushed stone and water per cubic metre come from the grade’s recipe.
- 5
Dry ready mix: the volume ÷ how much concrete one bag gives.
Worked example
A slab 6 × 4 m, 20 cm thick, of the region’s usual grade:
The input
- What we pour
- Slab
- Length, m
- 6
- Width, m
- 4
- Thickness, cm
- 20
- Grade
- C20/25
The result
5.25 m³ to order
Volume 4.80 m³ · C20/25 · with 5 % reserve
- Cement: 65 bags of 25 kg
- Sand: 3,780 kg
- Crushed stone: 5,292 kg
- Water: 907 l
- Or dry ready mix: 531 bags of 20 kg
The calculation
- Volume6.00 × 4.00 × 0.20 = 4.800 m³
- With 5 % reserve, ordered in steps of 0.25 m³4.800 × 1.05 = 5.040 → 5.25 m³
- Mixing on site: 320 kg of cement per 1 m³5.040 × 320 = 1,613 kg → 65 bags of 25 kg
- Sand, crushed stone and water per 1 m³750 · 1,050 kg · 180 l → 3,780 · 5,292 kg · 907 l
- Dry ready mix: one bag gives 9.5 l5.040 ÷ 0.01 → 531 bags
How many cubic metres of concrete for a slab?
Multiply the three sizes in metres: a 6 × 4 m slab 20 cm thick holds 4.8 m³. With a 3 % reserve it is 4.94 m³, and a plant that delivers in 0.5 m³ steps brings 5 m³. Order a little more rather than less — a cold joint in the middle of a slab is worse than leftover concrete.
How many bags of cement per cubic metre?
It depends on the grade: from about 250 kg of cement per m³ for lean concrete to 380 kg and more for structural grades — 5 to 8 bags of 50 kg. The calculator takes the recipe of the grade you choose.
How many bags of ready mix make a cubic metre?
A 40 kg bag of dry concrete mix gives about 19 litres of concrete, so a cubic metre takes about 53 bags. For more than a cubic metre a truck is usually cheaper and the concrete more uniform.
Questions
How much reserve to add?
Usually 3–5 %: some concrete stays in the mixer and the pump, the base is never perfectly flat, and the formwork bulges a little.
How is a strip footing counted?
The total length of all strips — outer walls plus the inner bearing walls — times the width and the height of the strip.
Which grade of concrete do I need?
It depends on the structure and the local building code: lean concrete for preparation, medium grades for slabs and footings of houses. Follow the project or ask the supplier.
The same calculator in the Kvadrino app
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