How to use it
- Enter the volume of concrete you need. The concrete calculator can work this out for you.
- Choose the mix ratio of cement to sand to gravel.
- Set the water–cement ratio; 0.5 suits most site work.
Worked example
For 1 m³ of 1 : 2 : 4 concrete, the dry volume is 1 × 1.54 = 1.54 m³. The mix has 7 parts, so cement is 1.54 ÷ 7 = 0.22 m³, or about 317 kg, which is 7 bags of 50 kg. Sand is 0.44 m³, gravel is 0.88 m³, and water at a ratio of 0.5 is about 158 litres.
For 1 yd³ of the same mix, you need about 534 lb of cement, or 6 bags of 94 lb, with 0.44 yd³ of sand, 0.88 yd³ of gravel and about 32 gallons of water.
Frequently asked questions
What does a 1:2:4 concrete mix mean?
One part cement, two parts sand and four parts gravel, measured by volume. It is a common general-purpose mix, roughly C20 or 3,000 psi.
Why multiply by 1.54?
Loose sand, cement and gravel have air gaps that disappear when mixed with water, so you need about 54% more dry material than the finished volume.
What water–cement ratio should I use?
Between 0.45 and 0.55 suits most site work. Less water gives stronger, more durable concrete but is harder to place. Adding water to make concrete easier to work is the most common way to weaken it.
Is a nominal mix good enough for structural work?
Nominal mixes vary with the materials you use. For structural elements, use a tested design mix or ready-mix concrete ordered to a specified strength.