Cement, Sand & Aggregate Calculator

What goes into concrete

Concrete is cement, sand, stone chips and water in a fixed proportion. The grade — M15, M20, M25 — is just a shorthand for that proportion and the strength it reaches after 28 days. Order the wrong ratio and you either waste cement or pour something weaker than the drawing calls for.

Formula

• Dry volume = wet volume × 1.54 — dry materials pack down once water is added
• Each material = dry volume × its part ÷ the sum of the parts
• Cement bags = cement volume × 1,440 kg/m³ ÷ 50 kg
• Water = cement weight × water–cement ratio

Worked example

100 cft of M20 concrete (1:1.5:3) is 2.83 m³ wet and 4.36 m³ dry. That works out at 0.79 m³ of cement — about 23 bags — with 41.2 cft of sand, 82.4 cft of stone chips and roughly 568 litres of water at a 0.5 ratio.

How to use the concrete calculator

1. Enter the volume you need to pour, in cft or m³. For a slab, that is length × width × thickness.
2. Pick the grade from the drawing or from what your engineer specified.
3. Adjust the water–cement ratio if you have one; 0.45 to 0.55 covers most ordinary work.

Getting a good pour

• More water makes concrete easier to place and weaker once set. Stay near the ratio and use a plasticiser if it will not flow.
• The 1.54 factor is the standard allowance for voids between dry particles. Some engineers use 1.52 or 1.57; the difference is small.
• Add about 3–5% for spillage, and remember that steel, shuttering and labour are separate from these material quantities.

Frequently asked questions

How many cement bags are in 100 cft of M20 concrete?

About 23 bags, with roughly 41 cft of sand and 82 cft of stone chips, using the 1:1.5:3 ratio and the standard 1.54 dry-volume factor.

What does M20 mean?

It is concrete that reaches 20 newtons per square millimetre of compressive strength after 28 days. The usual nominal mix for it is 1 part cement, 1.5 parts sand and 3 parts stone chips.

How much water does concrete need?

Cement weight multiplied by the water–cement ratio, normally 0.45 to 0.55. For 1,150 kg of cement at 0.5, that is about 575 litres — including any water already in wet sand.