Concrete Mix Calculator

The formula

A ratio such as 1:2:4 means 1 part cement, 2 parts sand and 4 parts coarse aggregate by volume, 7 parts in all. Mixed with water, the dry materials shrink into a denser wet concrete, so more dry material than the finished volume is needed. This calculator uses a dry volume factor of 1.54: dry volume = wet volume × 1.54.

Cement volume = dry volume × cement part ÷ total parts. Sand and aggregate work the same way with their parts. Cement mass = cement volume × 1,440 kg per m³, an assumed loose bulk density, and bags = mass ÷ bag mass, rounded up. Wastage is added to the concrete volume before all of this.

Worked example

For 1 m³ of 1:2:4 concrete with no wastage, the dry volume is 1 × 1.54 = 1.54 m³. Cement is 1.54 × 1 ÷ 7 = 0.22 m³, which at 1,440 kg per m³ is 316.8 kg, so 7 bags of 50 kg (6.3 rounded up). Sand is 1.54 × 2 ÷ 7 = 0.44 m³ and aggregate is 1.54 × 4 ÷ 7 = 0.88 m³.

Common ratios

• 1:2:4 is the ratio set by default and is often used for general slabs and footings.
• 1:1.5:3 is a richer mix with more cement per cubic meter, and 1:3:6 is a leaner one for non-structural work.
• Ratios need not be whole numbers: 1:1.5:3 is accepted. Write them with colons, hyphens or the word "to".
• The ratio you need comes from your engineer, your project's specification or the supplier's mix design, not from this calculator.

Assumptions and limits

• The dry volume factor is fixed at 1.54, and the cement density at 1,440 kg per m³. A different factor or density gives different quantities.
• Sand and aggregate are given as volumes. Their weights depend on moisture and grading, so this calculator does not convert them to tonnes.
• It does not calculate water, admixtures or the water-cement ratio.
• Sand moisture bulking, in which damp sand takes up more space, is not allowed for.
• The bag is assumed to hold exactly its stated mass; check the bag you buy.
• To work out the volume from a slab, footing or column size first, use the Concrete Calculator.

Comparing common ratios for 1 m³

More worked examples

5 m³ of 1:1.5:3 with 5% wastage and 40 kg bags: the volume becomes 5.250 m³ and the dry volume 8.085 m³. Cement is 2,116.8 kg (53 bags), sand 2.205 m³ and aggregate 4.410 m³.

Imperial: 3 yd³ of 1:2:4 with 5% wastage and 94 lb bags. The volume with wastage is 3.150 yd³ (2.408 m³), the dry volume 4.851 yd³, the cement 763.0 kg (18 bags), the sand 1.386 yd³ and the aggregate 2.772 yd³.

With the default 5% wastage, 1 m³ of 1:2:4 becomes 1.050 m³ and needs 332.6 kg of cement (still 7 bags), 0.462 m³ of sand and 0.924 m³ of aggregate. For a half cubic meter with no wastage, 158.4 kg of cement rounds up to 4 bags.

Typing the ratio

• 1:2:4, 1-2-4 and 1 to 2 to 4 all give the same answer.
• Exactly three numbers are needed, each above zero. 1:2 or 1:0:4 shows "Enter the ratio as three positive numbers separated by colons, for example 1:2:4."
• A blank wastage box counts as 0%.