What this calculator does
This tool estimates the materials you need to mix concrete on site: bags of cement, cubic meters of sand, cubic meters of gravel, and an approximate amount of water in liters. Enter the dimensions of the element — length, width and thickness in meters — or type the volume in m³ directly if you already know it. Everything runs in your browser; nothing is sent to a server.
It answers the question every DIY builder and foreman asks before a pour: how many bags should I buy and how much aggregate should I order, with a waste margin so the mix does not run out halfway through a slab.
How to use it
- Pick the input mode: by dimensions (length × width × thickness) or direct volume in m³.
- Choose the mix ratio: 1:2:3 for structural elements, 1:2:4 for general work, or 1:3:5 for lean concrete.
- Select the bag size sold at your hardware store: 42.5 kg or 50 kg.
- Add a waste allowance (5–10 % is typical).
- Read the results: exact bags plus bags to buy (rounded up), sand, gravel and water. The copy button gives you a summary you can text to your supplier.
Material yields per m³ of concrete
The calculator relies on average field yields widely published in Latin American building handbooks. The ratio is by volume: 1 part cement to X parts sand and X parts gravel.
| Mix | Typical use | Cement (kg) | 50 kg bags | 42.5 kg bags | Sand (m³) | Gravel (m³) | Water (L) |
|---|---|---|---|---|---|---|---|
| 1:2:3 | Structural: slabs, beams, columns | 420 | 8.4 | 9.9 | 0.52 | 0.78 | 200 |
| 1:2:4 | General: floors, sidewalks, footings | 375 | 7.5 | 8.8 | 0.50 | 0.85 | 190 |
| 1:3:5 | Lean: blinding layers and fill | 280 | 5.6 | 6.6 | 0.53 | 0.88 | 170 |
The math is straightforward: element volume × (1 + waste) × yield per material. Bags come from dividing the cement weight by the bag size you selected.
Worked example
You are pouring a 4 × 5 meter floor slab, 0.10 m (10 cm) thick, with a 1:2:4 mix, 50 kg bags and 10 % waste.
- Volume: 4 × 5 × 0.10 = 2 m³ of concrete.
- With waste: 2 × 1.10 = 2.2 m³ to mix.
- Cement: 375 × 2.2 = 825 kg → 825 ÷ 50 = 16.5 bags, so buy 17 bags.
- Sand: 0.50 × 2.2 = 1.1 m³.
- Gravel: 0.85 × 2.2 = 1.87 m³.
- Water: 190 × 2.2 = roughly 418 liters.
A handy rule of thumb: one cubic meter of 1:2:4 concrete takes about 7.5–8 bags of 50 kg cement, which matches what most experienced builders quote.
Frequently asked questions
How many bags of cement per cubic meter of concrete?
It depends on the mix: about 8.4 bags of 50 kg for 1:2:3, about 7.5 bags for 1:2:4 and about 5.6 bags for lean 1:3:5. With 42.5 kg bags the figures become 9.9, 8.8 and 6.6 bags respectively.
Which mix ratio should I choose?
Use 1:2:3 for load-bearing elements such as roof slabs, beams and columns. 1:2:4 is the everyday mix for floors, sidewalks and light footings. 1:3:5 is lean concrete, only suitable for blinding layers under foundations and non-structural fill. If a drawing or an engineer specifies a strength, that specification always wins.
Why add a waste percentage?
Because real pours always lose material: mix left in the wheelbarrow and mixer drum, uneven ground, and actual thicknesses slightly larger than planned. Use 5 % under controlled conditions and 10 % for most jobs. Running out of concrete mid-slab creates cold joints that weaken the element, so it is cheaper to over-order slightly.
Is the water amount exact?
No — it is indicative. Actual water depends on how moist the sand is and on the workability you need. Crews adjust water at the mixer; keep in mind that excess water lowers the final strength, so start low and correct gradually.
Can I use this for structural work?
Use it to estimate the shopping list, not as a mix design. Required strength, aggregate size, admixtures and steel reinforcement must be defined by a civil engineer or qualified professional according to the intended use and local building codes.