When you mix concrete on site you proportion it by volume, for example 1 part cement to 2 of sand and 4 of gravel (M15). Because the sand and stone have voids that the cement paste fills, the dry ingredients shrink by about a third when mixed with water – so engineers multiply the wet volume by about 1.54 to get the dry volume of materials. Richer mixes (more cement) are stronger.
How to use the Cement Calculator
- Mix ratio (cement : sand : aggregate)
- Length
- Width
- Thickness
- Cement bag size
- Water–cement ratio
- Price per bag of cement
- Sand price
- Aggregate price
- Every result field is editable: type into Cement and Thickness plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the mix ratio, enter the slab dimensions, the cement bag size, the water–cement ratio and unit prices. The calculator converts the wet volume to dry volume, splits it by the ratio, converts the cement to bags and the water to litres and gallons.
Mix ratios and typical use
| Grade | Ratio | Typical use |
|---|---|---|
| M25 | 1 : 1 : 2 | Heavily loaded slabs, beams |
| M20 | 1 : 1.5 : 3 | Slabs, beams, columns |
| M15 | 1 : 2 : 4 | Footings, floors, PCC |
| M10 | 1 : 3 : 6 | Foundations, blinding |
| M7.5 | 1 : 4 : 8 | Mass concrete |
| M5 | 1 : 5 : 10 | Lean concrete, levelling |
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Mix ratio (cement : sand : aggregate) | Choose one: M25 – 1 : 1 : 2, M20 – 1 : 1.5 : 3, M15 – 1 : 2 : 4, M10 – 1 : 3 : 6 … |
| Length | Units: m, ft, cm, in, yd |
| Width | Units: m, ft, cm, in, yd |
| Thickness | Units: cm, in, m |
| Cement bag size | Choose one: 50 kg, 94 lb (US), 42.5 kg, 40 kg … |
| Water–cement ratio | A number |
| Price per bag of cement | Units: /piece |
| Sand price | Units: /m³, /yd³, /ft³ |
| Aggregate price | Units: /m³, /yd³, /ft³ |
What each result means
- Concrete volume (wet) = [Length]×[Width]×[Thickness]
- Dry volume of materials = [Concrete volume (wet)]×1.54
- Cement = [Dry volume of materials]÷(1 + [Sand parts] + [Aggregate parts])×1440
- Bags of cement = ceil([Cement]÷[Cement bag size])
- Sand = [Dry volume of materials]×[Sand parts]÷(1 + [Sand parts] + [Aggregate parts])
- Coarse aggregate = [Dry volume of materials]×[Aggregate parts]÷(1 + [Sand parts] + [Aggregate parts])
- Water = [Cement]×[Water–cement ratio]÷1000
- Material cost = [Bags of cement]×[Price per bag of cement] + [Sand]×[Sand price] + [Coarse aggregate]×[Aggregate price]
Formula
Dry volume = wet volume × 1.54Cement = dry volume × 1 ÷ (1 + sand + aggregate) × 1,440 kg/m³Sand = dry volume × sand ÷ sum; aggregate = dry volume × aggregate ÷ sumWater = cement mass × water–cement ratioWorked example
Inputs — Mix ratio (cement : sand : aggregate): M15 – 1 : 2 : 4; Length: 4 m; Width: 3 m; Thickness: 15 cm; Cement bag size: 50 kg; Water–cement ratio: 0.5; Price per bag of cement: $8 /piece; Sand price: $35 /m³; Aggregate price: $40 /m³.
Results — Concrete volume (wet) = 1.8 m³; Dry volume of materials = 2.772 m³; Cement = 570.2 kg; Bags of cement = 12; Sand = 0.792 m³; Coarse aggregate = 1.584 m³; Water = 285 L; Material cost = $187.08.
Key terms
- Nominal mix
- A concrete mix proportioned by volume ratios rather than by laboratory design.
Frequently asked questions
How many bags of cement are in 1 m³ of concrete?
About 8 bags of 50 kg for M15 (1:2:4), 8.1 for M20 (1:1.5:3) on a dry-volume basis of 1.54, and about 4.4 for M7.5.
Why multiply by 1.54?
Dry sand and stone contain voids; when water and cement are added the mix shrinks by roughly 54%, so you need more dry material than the finished volume.
What is a good water–cement ratio?
0.4–0.5 for durable structural concrete. More water makes it easier to place but weaker.
Which mix should I use?
M20 (1:1.5:3) for slabs, beams and columns; M15 (1:2:4) for footings and floors; M7.5–M10 for blinding and mass fill.
What formula does the Cement Calculator use?
Dry volume = wet volume × 1.54.