Concrete Mix Calculator: Sand, Gravel & Cement

This concrete mix calculator works out the exact amounts of cement, sand, and gravel you need for a given volume of concrete, based on the mix ratio you choose. Enter the volume you want to fill (or the dimensions of your project), select a mix ratio, and the calculator returns the required dry volumes and weights of each material. The default below uses a standard 1:2:4 mix (cement:sand:gravel by volume), which is a common general-purpose ratio.

Formula

For a mix ratio of cement:sand:gravel = 1:x:y, the total parts are T = 1 + x + y. The volume of each material is the total concrete volume divided by T, then multiplied by the number of parts for that material:

• Cement volume = V / T
• Sand volume = (V / T) × x
• Gravel volume = (V / T) × y

Where V is the required concrete volume (m³), x is the sand parts (e.g., 2), y is the gravel parts (e.g., 4), and T is the sum of parts (e.g., 7). Water is typically added at about 0.5 parts by weight of cement, but a practical estimate is 200 liters per cubic meter of concrete, which is what the calculator uses.

Worked Example

Using the default values: V = 1 m³, mix ratio 1:2:4 (T = 7).

1. Cement = 1 / 7 = 0.1429 m³
2. Sand = (1 / 7) × 2 = 0.2857 m³
3. Gravel = (1 / 7) × 4 = 0.5714 m³
4. Water = 1 × 200 = 200 liters

The calculator rounds to 3 decimal places, giving cement 0.143 m³, sand 0.286 m³, gravel 0.571 m³, and water 200 L.

How to Use the Results

The volumes are dry, loose volumes. When you mix with water, the wet concrete occupies less volume than the sum of dry materials (shrinkage). In practice, for 1 m³ of concrete you need about 1.5 m³ of dry aggregate and cement combined. The calculator gives the theoretical dry proportions; add a 10–15% waste factor for uneven subgrades, spillage, or over-excavation.

Weights are often more useful than volumes for purchasing. Typical densities: cement ≈ 1,440 kg/m³, sand ≈ 1,600 kg/m³, gravel ≈ 1,680 kg/m³. Multiply the volume by these densities to get approximate weight. For the 1 m³ example: cement ≈ 206 kg, sand ≈ 457 kg, gravel ≈ 959 kg. Check local supplier densities, as moisture and compaction change them.

Choosing the Right Mix Ratio

The mix ratio determines the strength and workability. Common ratios:

• 1:2:4 (C20/25) – general purpose: footings, slabs, paths, driveways for light vehicles.
• 1:1.5:3 (C25/30) – stronger: structural beams, columns, heavy-duty foundations.
• 1:3:6 (C10/15) – lean mix: mass concrete, blinding layers, non-structural fill.
• 1:2:3 – a high-strength mix for precast or reinforced elements.

For most DIY projects, 1:2:4 is a safe default. If the concrete will bear heavy loads or be exposed to freeze-thaw, use a richer mix (more cement) and consider adding an air-entraining admixture. The water-cement ratio is more critical than the aggregate ratio: too much water weakens concrete. Keep water as low as possible while still being workable.

Common Mistakes and How to Avoid Them

• Mixing up volume and weight – Always use the same basis (volume or weight) for all materials. A 1:2:4 ratio by weight is different from by volume.
• Ignoring shrinkage – Dry materials bulk up when mixed with water. For 1 m³ of concrete, order about 1.3–1.5 m³ of dry materials.
• Adding too much water – Water reduces strength exponentially. Use a plasticizer if you need more workability.
• Forgetting the waste factor – Always add 10% extra for uneven surfaces, spillage, and rounding.
• Using damp sand – Wet sand takes up more volume than dry sand. If your sand is damp, reduce the measured volume by about 20%.

Limitations of the Calculator

This calculator assumes a standard mix and does not account for admixtures, reinforcement, or specific strength requirements. It gives theoretical volumes; actual yield depends on the aggregate shape, moisture content, and compaction. For structural work, consult an engineer and follow local building codes. The water estimate (200 L/m³) is a starting point – adjust based on the moisture of your sand and gravel.

Safety and Handling

Concrete is highly alkaline and can cause burns. Wear gloves, long sleeves, and eye protection. Avoid inhaling cement dust – use a dust mask when mixing dry materials. If cement gets on skin, rinse with clean water immediately. Never mix concrete indoors without ventilation. Dispose of wash water responsibly; it can contaminate soil and waterways.

Frequently Asked Questions

How many bags of cement do I need for 1 cubic meter of concrete?

For a 1:2:4 mix, you need about 0.143 m³ of cement. A standard 50 kg bag of cement is roughly 0.033 m³, so you’d need about 4.3 bags. In practice, round up to 5 bags to account for waste and spillage.

What is the best mix ratio for a concrete slab?

For a domestic slab (e.g., a garden path or shed base), a 1:2:4 mix is adequate. For a driveway that will support vehicles, use 1:1.5:3. Always check local building regulations for minimum strength requirements.

Can I use this calculator for a mix by weight?

No, the calculator works with volume ratios. If you want to mix by weight, convert the volumes to weights using the densities mentioned above, or use a weight-based mix design such as 1 part cement, 2 parts sand, 4 parts gravel by weight – but that will yield a different strength.

Why does the calculator give more gravel than sand?

Because the ratio 1:2:4 means 1 part cement, 2 parts sand, and 4 parts gravel. Gravel makes up the bulk of the mix to reduce shrinkage and cost. The exact proportion depends on the aggregate grading; this is a standard general-purpose ratio.

How much water should I add?

Start with about 200 liters per cubic meter of concrete (about 0.5 parts water by weight of cement). Add water gradually and mix until the concrete is workable but not soupy. The water-cement ratio should be around 0.5 for most mixes.

What if I need to fill an irregular shape?

Break the shape into rectangular or cylindrical sections, calculate each volume, and add them together. For a cylinder, use π × radius² × height. For a triangular prism, use 0.5 × base × height × length. Enter the total volume into the calculator.

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in v | Concrete volume | m³ | 1 | 0
sel r | Mix ratio | 1:2:4=7, 1:1.5:3=5.5, 1:3:6=10, 1:2:3=6 | 7
out cement | Cement | m³ | v / r | 3
out sand | Sand | m³ | (v / r) * 2 | 3
out gravel | Gravel | m³ | (v / r) * 4 | 3
out water | Water (approx) | L | v * 200 | 0